PHP, Perl, Python, Ruby 语言特性的区别

Interpreted Languages: PHP, Perl, Python, Ruby (Sheet One)

a side-by-side reference sheet

	php (1995)	perl (1987)	python (1991)	ruby (1995)
versions used	5.3	5.12; 5.14	2.7; 3.2	1.8; 1.9
implicit prologue	none	use strict;	import os, re, sys	none
show version	$ php --version	$ perl --version	$ python -V	$ ruby --version
interpreter	$ php -f foo.php	$ perl foo.pl	$ python foo.py	$ ruby foo.rb
repl	$ php	$ perl -de 0	$ python	$ irb
command line script	$ php -r 'echo "hi\n";'	$ perl -e 'print("hi\n")'	$ python -c "print('hi')"	$ ruby -e 'puts "hi"'
statement separator	; statements must be semicolon terminated inside {}	;	newline or ; newlines not separators inside (), [], {}, triple quote literals, or after backslash: \	newline or ; newlines not separators inside (), [], {}, ``, '', "", or after binary operator or backslash: \
block delimiters	{}	{}	offside rule	{} do end
assignment	$v = 1;	$v = 1;	assignments can be chained but otherwise don't return values: v = 1	v = 1
parallel assignment	list($x, $y, $z) = array(1 ,2, 3); # 3 is discarded: list($x, $y) = array(1, 2, 3); # $z set to NULL: list($x, $y, $z) = array(1, 2);	($x, $y, $z) = (1, 2, 3); # 3 is discarded: ($x, $y) = (1, 2, 3); # $z set to undef: ($x, $y, $z) = (1, 2);	x, y, z = 1, 2, 3 # raises ValueError: x, y = 1, 2, 3 # raises ValueError: x, y, z = 1, 2	x, y, z = 1, 2, 3 # 3 is discarded: x, y = 1, 2, 3 # z set to nil: x, y, z = 1, 2
swap	list($x, $y) = array($y, $x);	($x, $y) = ($y, $x);	x, y = y, x	x, y = y, x
compound assignment operators: arithmetic, string, logical, bit	+= -= = none* /= %= *= .= none* &= \|= none <<= >>= &= \|= ^=	+= -= = none* /= %= **= .= x= &&= \|\|= ^= <<= >>= &= \|= ^=	# do not return values: += -= = /= //= %= = += = &= \|= ^= <<= >>= &= \|= ^=	+= -= = /= none* %= *= += = &&= \|\|= ^= <<= >>= &= \|= ^=
increment and decrement	$x = 1; $y = ++$x; $z = --$y;	my $x = 1; my $y = ++$x; my $z = --$y;	none	x = 1 # x and y not mutated: y = x.succ z = y.pred
local variable declarations	# in function body: $v = NULL; $a = array(); $d = array(); $x = 1; list($y, $z) = array(2, 3);	my $v; my (@a, %d); my $x = 1; my ($y, $z) = (2, 3);	# in function body: v = None a, d = [], {} x = 1 y, z = 2, 3	v = nil a, d = [], {} x = 1 y, z = 2, 3
regions which define local scope	top level: function or method body nestable (with use clause): anonymous function body	top level: file nestable: function body anonymous function body anonymous block	nestable (read only): function or method body	top level: file class block module block method body nestable: anonymous function block anonymous block
global variable	list($g1, $g2) = array(7, 8); function swap_globals() { global $g1, $g2; list($g1, $g2) = array($g2, $g1); }	our ($g1, $g2) = (7, 8); sub swap_globals { ($g1, $g2) = ($g2, $g1); }	g1, g2 = 7, 8 def swap_globals(): global g1, g2 g1, g2 = g2, g1	$g1, $g2 = 7, 8 def swap_globals $g1, $g2 = $g2, $g1 end
constant declaration	define("PI", 3.14);	use constant PI => 3.14;	# uppercase identifiers # constant by convention PI = 3.14	# warning if capitalized # identifier is reassigned PI = 3.14
to-end-of-line comment	// comment # comment	# comment	# comment	# comment
comment out multiple lines	/* comment line another line */	=for comment line another line =cut	use triple quote string literal: '''comment line another line'''	=begin comment line another line =end
null	NULL # case insensitive	undef	None	nil
null test	is_null($v) ! isset($v)	! defined $v	v == None v is None	v == nil v.nil?
undefined variable access	NULL	error under use strict; otherwise undef	raises NameError	raises NameError
undefined test	same as null test; no distinction between undefined variables and variables set to NULL	same as null test; no distinction between undefined variables and variables set to undef	not_defined = False try: v except NameError: not_defined = True	! defined?(v)
arithmetic and logic
	php	perl	python	ruby
true and false	TRUE FALSE # case insensitve	1 ""	True False	true false
falsehoods	FALSE NULL 0 0.0 "" "0" array()	undef 0 0.0 "" "0" ()	False None 0 0.0 '' [] {}	false nil
logical operators	&& \|\| ! lower precedence: and or xor	&& \|\| ! lower precedence: and or xor not	and or not	&& \|\| ! lower precedence: and or not
conditional expression	$x > 0 ? $x : -$x	$x > 0 ? $x : -$x	x if x > 0 else -x	x > 0 ? x : -x
comparison operators	== != or <> > < >= <= no conversion: === !==	numbers only: == != > < >= <= strings: eq ne gt lt ge le	comparison operators are chainable: == != > < >= <=	== != > < >= <=
three value comparison	none	0 <=> 1 "do" cmp "re"	removed from Python 3: cmp(0, 1) cmp('do', 're')	0 <=> 1 "do" <=> "re"
convert from string, to string	7 + "12" 73.9 + ".037" "value: " . 8	7 + "12" 73.9 + ".037" "value: " . 8	7 + int('12') 73.9 + float('.037') 'value: ' + str(8)	7 + "12".to_i 73.9 + ".037".to_f "value: " + "8".to_s
arithmetic operators	+ - * / none % pow(b,e)	+ - * / none % **	+ - * / // % **	+ - * x.fdiv(y) / % **
integer division and divmod	(int) (13 / 5) none	int ( 13 / 5 ) none	13 // 5 q, r = divmod(13, 5)	13 / 5 q, r = 13.divmod(5)
float division	13 / 5	13 / 5	float(13) / 5 # Python 3: 13 / 5	13.to_f / 5 or 13.fdiv(5)
arithmetic functions	sqrt exp log sin cos tan asin acos atan atan2	use Math::Trig qw( tan asin acos atan); sqrt exp log sin cos tan asin acos atan atan2	from math import sqrt, exp, log, \ sin, cos, tan, asin, acos, atan, atan2	include Math sqrt exp log sin cos tan asin acos atan atan2
arithmetic truncation	(int)$x round($x) ceil($x) floor($x) abs($x)	# cpan -i Number::Format use Number::Format 'round'; use POSIX qw(ceil floor); int($x) round($x, 0) ceil($x) floor($x) abs($x)	import math int(x) int(round(x)) math.ceil(x) math.floor(x) abs(x)	x.to_i x.round x.ceil x.floor x.abs
min and max	min(1,2,3) max(1,2,3) $a = array(1,2,3) min($a) max($a)	use List::Util qw(min max); min(1,2,3); max(1,2,3); @a = (1,2,3); min(@a); max(@a);	min(1,2,3) max(1,2,3) min([1,2,3]) max([1,2,3])	[1,2,3].min [1,2,3].max
division by zero	returns FALSE with warning	error	raises ZeroDivisionError	integer division raises ZeroDivisionError float division returns Infinity
integer overflow	converted to float	converted to float; use Math::BigInt to create arbitrary length integers	becomes arbitrary length integer of type long	becomes arbitrary length integer of type Bignum
float overflow	INF	inf	raises OverflowError	Infinity
sqrt -2	NaN	error unless use Math::Complex in effect	# raises ValueError: import math math.sqrt(-2) # returns complex float: import cmath cmath.sqrt(-2)	raises Errno::EDOM
rational numbers	none	use Math::BigRat; my $x = Math::BigRat->new("22/7"); $x->numerator(); $x->denominator();	from fractions import Fraction x = Fraction(22,7) x.numerator x.denominator	require 'rational' x = Rational(22,7) x.numerator x.denominator
complex numbers	none	use Math::Complex; my $z = 1 + 1.414 * i; Re($z); Im($z);	z = 1 + 1.414j z.real z.imag	require 'complex' z = 1 + 1.414.im z.real z.imag
random integer, uniform float, normal float	rand(0,99) lcg_value() none	int(rand() * 100) rand() none	import random random.randint(0,99) random.random() random.gauss(0,1)	rand(100) rand none
set random seed, get and restore seed	srand(17); none	srand 17; my $sd = srand; srand($sd);	import random random.seed(17) sd = random.getstate() random.setstate(sd)	srand(17) sd = srand srand(sd)
bit operators	<< >> & \| ^ ~	<< >> & \| ^ ~	<< >> & \| ^ ~	<< >> & \| ^ ~
binary, octal, and hex literals	none 052 0x2a	0b101010 052 0x2a	0b101010 052 0x2a	0b101010 052 0x2a
base conversion	base_convert("42", 10, 7); base_convert("60", 7, 10);	# cpan -i Math::BaseCalc use Math::BaseCalc; $c = new Math::BaseCalc(digits=> [0..6]); $c->to_base(42); $c->from_base("60");	none int("60", 7)	42.to_s(7) "60".to_i(7)
strings
	php	perl	python	ruby
string literal	"don't say \"no\"" 'don\'t say "no"'	"don't say \"no\"" 'don\'t say "no"'	'don\'t say "no"' "don't say \"no\"" "don't " 'say "no"' '''don't say "no"''' """don't say "no\""""	"don't say \"no\"" 'don\'t say "no"' "don't " 'say "no"'
newline in literal	yes	yes	triple quote literals only	yes
backslash escapes	double quoted: \f \n \r \t \v \xhh \$ \" \ooo single quoted: \' \\	double quoted: \a \b \cx \e \f \n \r \t \xhh \x{hhhh} \ooo single quoted: \' \\	single and double quoted: \newline \\ \' \" \a \b \f \n \r \t \v \ooo \xhh Python 3: \uhhhh \Uhhhhhhhh	double quoted: \a \b \cx \e \f \n \r \s \t \v \xhh \ooo Ruby 1.9 double quoted: \uhhhh \u{hhhhh} single quoted: \' \\
variable interpolation	$count = 3; $item = "ball"; echo "$count ${item}s\n";	my $count = 3; my $item = "ball"; print "$count ${item}s\n";	count = 3 item = 'ball' print('{count} {item}s'.format( **locals()))	count = 3 item = "ball" puts "#{count} #{item}s"
custom delimiters	none	my $s1 = q(lorem ipsum); my $s2 = qq($s1 dolor sit amet);	none	s1 = %q(lorem ipsum) s2 = %Q(#{s1} dolor sit amet)
sprintf	$fmt = "lorem %s %d %f"; sprintf($fmt, "ipsum", 13, 3.7);	my $fmt = "lorem %s %d %f"; sprintf($fmt, "ipsum", 13, 3.7)	'lorem %s %d %f' % ('ipsum', 13, 3.7) fmt = 'lorem {0} {1} {2}' fmt.format('ipsum', 13, 3.7)	"lorem %s %d %f" % ["ipsum",13,3.7]
here document	$word = "amet"; $s = <<<EOF lorem ipsum dolor sit $word EOF;	$word = "amet"; $s = <<EOF; lorem ipsum dolor sit $word EOF	none	word = "amet" s = <<EOF lorem ipsum dolor sit #{word} EOF
concatenate	$s = "Hello, "; $s2 = $s . "World!";	my $s = "Hello, "; my $s2 = $s . "World!";	s = 'Hello, ' s2 = s + 'World!' juxtaposition can be used to concatenate literals: s2 = 'Hello, ' "World!"	s = "Hello, " s2 = s + "World!" juxtaposition can be used to concatenate literals: s2 ="Hello, " 'World!'
replicate	$hbar = str_repeat("-", 80);	my $hbar = "-" x 80;	hbar = '-' * 80	hbar = "-" * 80
split, in two, with delimiters, into characters	explode(" ", "do re mi fa") preg_split('/\s+/', "do re mi fa", 2) preg_split('/(\s+)/', "do re mi fa", NULL, PREG_SPLIT_DELIM_CAPTURE); str_split("abcd")	split(/\s+/, "do re mi fa") split(/\s+/, "do re mi fa", 2) split(/(\s+)/, "do re mi fa"); split(//, "abcd")	'do re mi fa'.split() 'do re mi fa'.split(None, 1) re.split('(\s+)', 'do re mi fa') list('abcd')	"do re mi fa".split "do re mi fa".split(/\s+/, 2) "do re mi fa".split(/(\s+)/) "abcd".split("")
join	$a = array("do", "re", "mi", "fa"); implode(" ", $a)	join(" ", qw(do re mi fa))	' '.join(['do', 're', 'mi', 'fa'])	%w(do re mi fa).join(' ')
case manipulation	strtoupper("lorem") strtolower("LOREM") ucfirst("lorem")	uc("lorem") lc("LOREM") ucfirst("lorem")	'lorem'.upper() 'LOREM'.lower() 'lorem'.capitalize()	"lorem".upcase "LOREM".downcase "lorem".capitalize
strip	trim(" lorem ") ltrim(" lorem") rtrim("lorem ")	# cpan -i Text::Trim use Text::Trim; trim " lorem " ltrim " lorem" rtrim "lorem "	' lorem '.strip() ' lorem'.lstrip() 'lorem '.rstrip()	" lorem ".strip " lorem".lstrip "lorem ".rstrip
pad on right, on left	str_pad("lorem", 10) str_pad("lorem", 10, " ", STR_PAD_LEFT)	sprintf("%-10s", "lorem") sprintf("%10s", "lorem")	'lorem'.ljust(10) 'lorem'.rjust(10)	"lorem".ljust(10) "lorem".rjust(10)
length	strlen("lorem")	length("lorem")	len('lorem')	"lorem".length "lorem".size
index of substring	strpos("do re re", "re") strrpos("do re re", "re") return FALSE if not found	index("lorem ipsum", "ipsum") rindex("do re re", "re") return -1 if not found	'do re re'.index('re') 'do re re'.rindex('re') raise ValueError if not found	"do re re".index("re") "do re re".rindex("re") return nil if not found
extract substring	substr("lorem ipsum", 6, 5)	substr("lorem ipsum", 6, 5)	'lorem ipsum'[6:11]	"lorem ipsum"[6, 5]
extract character	syntax error to use index notation directly on string literal: $s = "lorem ipsum"; $s[6];	can't use index notation with strings: substr("lorem ipsum", 6, 1)	'lorem ipsum'[6]	"lorem ipsum"[6]
chr and ord	chr(65) ord("A")	chr(65) ord("A")	chr(65) ord('A')	65.chr "A"[0] Ruby 1.9: "A".ord
character translation	$ins = implode(range("a", "z")); $outs = substr($ins, 13, 13) . substr($ins, 0, 13); strtr("hello", $ins, $outs)	$s = "hello"; $s =~ tr/a-z/n-za-m/;	from string import lowercase as ins from string import maketrans outs = ins[13:] + ins[:13] 'hello'.translate(maketrans(ins,outs))	"hello".tr("a-z", "n-za-m")
regular expresions
	php	perl	python	ruby
literal, custom delimited literal	'/lorem\|ipsum/' '(/etc/hosts)'	/lorem\|ipsum/ qr(/etc/hosts)	re.compile('lorem\|ipsum') none	/lorem\|ipsum/ %r(/etc/hosts)
character class abbreviations and anchors	char class abbrevs: . \d \D \h \H \s \S \v \V \w \W anchors: ^ $ \A \b \B \z \Z	char class abbrevs: . \d \D \h \H \s \S \v \V \w \W anchors: ^ $ \A \b \B \z \Z	char class abbrevs: . \d \D \s \S \w \W anchors: ^ $ \A \b \B \Z	char class abbrevs: . \d \D \h \H \s \S \w \W anchors: ^ $ \A \b \B \z \Z
match test	if (preg_match('/1999/', $s)) { echo "party!\n"; }	if ($s =~ /1999/) { print "party!\n"; }	if re.search('1999', s): print('party!')	if /1999/.match(s) puts "party!" end
case insensitive match test	preg_match('/lorem/i', "Lorem")	"Lorem" =~ /lorem/i	re.search('lorem', 'Lorem', re.I)	/lorem/i.match("Lorem")
modifiers	e i m s x	i m s p x	re.I re.M re.S re.X	i o m x
substitution	$s = "do re mi mi mi"; $s = preg_replace('/mi/', "ma", $s);	my $s = "do re mi mi mi"; $s =~ s/mi/ma/g;	s = 'do re mi mi mi' s = re.compile('mi').sub('ma', s)	s = "do re mi mi mi" s.gsub!(/mi/, "ma")
match, prematch, postmatch	none	if ($s =~ /\d{4}/p) { $match = ${^MATCH}; $prematch = ${^PREMATCH}; $postmatch = ${^POSTMATCH}; }	m = re.search('\d{4}', s) if m: match = m.group() prematch = s[0:m.start(0)] postmatch = s[m.end(0):len(s)]	m = /\d{4}/.match(s) if m match = m[0] prematch = m.pre_match postmatch = m.post_match end
group capture	$s = "2010-06-03"; $rx = '/(\d{4})-(\d{2})-(\d{2})/'; preg_match($rx, $s, $m); list($_, $yr, $mo, $dy) = $m;	$rx = qr/(\d{4})-(\d{2})-(\d{2})/; "2010-06-03" =~ $rx; ($yr, $mo, $dy) = ($1, $2, $3);	rx = '(\d{4})-(\d{2})-(\d{2})' m = re.search(rx, '2010-06-03') yr, mo, dy = m.groups()	rx = /(\d{4})-(\d{2})-(\d{2})/ m = rx.match("2010-06-03") yr, mo, dy = m[1..3]
scan	$s = "dolor sit amet"; preg_match_all('/\w+/', $s, $m); $a = $m[0];	my $s = "dolor sit amet"; @a = $s =~ m/\w+/g;	s = 'dolor sit amet' a = re.findall('\w+', s)	a = "dolor sit amet".scan(/\w+/)
backreference in match and substitution	preg_match('/(\w+) \1/', "do do") $s = "do re"; $rx = '/(\w+) (\w+)/'; $s = preg_replace($rx, '\2 \1', $s);	"do do" =~ /(\w+) \1/ my $s = "do re"; $s =~ s/(\w+) (\w+)/$2 $1/;	none rx = re.compile('(\w+) (\w+)') rx.sub(r'\2 \1', 'do re')	/(\w+) \1/.match("do do") "do re".sub(/(\w+) (\w+)/, '\2 \1')
recursive regex	'/$([^()]*\|($R))$/'	/$([^()]*\|(?R))$/	none	Ruby 1.9: /(?<p>$([^()]\|\g<p>)$)/
dates and time
	php	perl	python	ruby
date/time type	DateTime	Time::Piece if use Time::Piece in effect, otherwise tm array	datetime.datetime	Time
current date/time	$t = new DateTime("now"); $utc_tmz = new DateTimeZone("UTC"); $utc = new DateTime("now", $utc_tmz);	use Time::Piece; my $t = localtime(time); my $utc = gmtime(time);	import datetime t = datetime.datetime.now() utc = datetime.datetime.utcnow()	t = Time.now utc = Time.now.utc
to unix epoch, from unix epoch	$epoch = $t->getTimestamp(); $t2 = new DateTime(); $t2->setTimestamp(1304442000);	use Time::Local; use Time::Piece; my $epoch = timelocal($t); my $t2 = localtime(1304442000);	from datetime import datetime as dt epoch = int(t.strftime("%s")) t2 = dt.fromtimestamp(1304442000)	epoch = t.to_i t2 = Time.at(1304442000)
current unix epoch	$epoch = time();	$epoch = time;	import datetime t = datetime.datetime.now() epoch = int(t.strftime("%s"))	epoch = Time.now.to_i
strftime	strftime("%Y-%m-%d %H:%M:%S", $epoch); date("Y-m-d H:i:s", $epoch); $t->format("Y-m-d H:i:s");	use Time::Piece; $t = localtime(time); $fmt = "%Y-%m-%d %H:%M:%S"; print $t->strftime($fmt);	t.strftime('%Y-%m-%d %H:%M:%S')	t.strftime("%Y-%m-%d %H:%M:%S")
default format example	no default string representation	Tue Aug 23 19:35:19 2011	2011-08-23 19:35:59.411135	2011-08-23 17:44:53 -0700
strptime	$fmt = "Y-m-d H:i:s"; $s = "2011-05-03 10:00:00"; $t = DateTime::createFromFormat($fmt, $s);	use Time::Local; use Time::Piece; $s = "2011-05-03 10:00:00"; $fmt = "%Y-%m-%d %H:%M:%S"; $t = Time::Piece->strptime($s,$fmt);	from datetime import datetime s = '2011-05-03 10:00:00' fmt = '%Y-%m-%d %H:%M:%S' t = datetime.strptime(s, fmt)	require 'date' s = "2011-05-03 10:00:00" fmt = "%Y-%m-%d %H:%M:%S" t = Date.strptime(s, fmt).to_time
parse date w/o format	$epoch = strtotime("July 7, 1999");	# cpan -i Date::Parse use Date::Parse; $epoch = str2time("July 7, 1999");	# pip install python-dateutil import dateutil.parser s = 'July 7, 1999' t = dateutil.parser.parse(s)	require 'date' s = "July 7, 1999" t = Date.parse(s).to_time
result of date subtraction	DateInterval object if diff method used: $fmt = "Y-m-d H:i:s"; $s = "2011-05-03 10:00:00"; $then = DateTime::createFromFormat($fmt, $s); $now = new DateTime("now"); $interval = $now->diff($then);	Time::Seconds object if use Time::Piece in effect; not meaningful to subtract tm arrays	datetime.timedelta object	Float containing time difference in seconds
add time duration	$now = new DateTime("now"); $now->add(new DateInterval("PT10M3S");	use Time::Seconds; $now = localtime(time); $now += 10 * ONE_MINUTE() + 3;	import datetime delta = datetime.timedelta( minutes=10, seconds=3) t = datetime.datetime.now() + delta	require 'date/delta' s = "10 min, 3 s" delta = Date::Delta.parse(s).in_secs t = Time.now + delta
local timezone	DateTime objects can be instantiated without specifying the timezone if a default is set: $s = "America/Los_Angeles"; date_default_timezone_set($s);	Time::Piece has local timezone if created with localtimeand UTC timezone if created with gmtime; tm arrays have no timezone or offset info	a datetime object has no timezone information unless atzinfo object is provided when it is created	if no timezone is specified the local timezone is used
timezone name; offset from UTC; is daylight savings?	$tmz = date_timezone_get($t); timezone_name_get($tmz); date_offset_get($t) / 3600; $t->format("I");	# cpan -i DateTime use DateTime; use DateTime::TimeZone; $dt = DateTime->now(); $tz = DateTime::TimeZone->new( name=>"local"); $tz->name; $tz->offset_for_datetime($dt) / 3600; $tz->is_dst_for_datetime($dt);	import time tm = time.localtime() time.tzname[tm.tm_isdst] (time.timezone / -3600) + tm.tm_isdst tm.tm_isdst	t.zone t.utc_offset / 3600 t.dst?
microseconds	list($frac, $sec) = explode(" ", microtime()); $usec = $frac * 1000 * 1000;	use Time::HiRes qw(gettimeofday); ($sec, $usec) = gettimeofday;	t.microsecond	t.usec
sleep	a float argument will be truncated to an integer: sleep(1);	a float argument will be truncated to an integer: sleep 1;	import time time.sleep(0.5)	sleep(0.5)
timeout	use set_time_limit to limit execution time of the entire script; use stream_set_timeout to limit time spent reading from a stream opened with fopen or fsockopen	eval { $SIG{ALRM}= sub {die "timeout!";}; alarm 5; sleep 10; }; alarm 0;	import signal, time class Timeout(Exception): pass def timeout_handler(signo, fm): raise Timeout() signal.signal(signal.SIGALRM, timeout_handler) try: signal.alarm(5) time.sleep(10) except Timeout: pass signal.alarm(0)	require 'timeout' begin Timeout.timeout(5) do sleep(10) end rescue Timeout::Error end
arrays
	php	perl	python	ruby
literal	$a = array(1, 2, 3, 4);	@a = (1, 2, 3, 4);	a = [1, 2, 3, 4]	a = [1, 2, 3, 4]
quote words	none	@a = qw(do re mi);	none	a = %w(do re mi)
size	count($a)	$#a + 1 or scalar(@a)	len(a)	a.size a.length # same as size
empty test	!$a	!@a	not a	NoMethodError if a is nil: a.empty?
lookup	$a[0]	$a[0]	a[0]	a[0]
update	$a[0] = "lorem";	$a[0] = "lorem";	a[0] = 'lorem'	a[0] = "lorem"
out-of-bounds behavior	$a = array(); evaluates as NULL: $a[10]; increases array size to one: $a[10] = "lorem";	@a = (); evaluates as undef: $a[10]; increases array size to 11: $a[10] = "lorem";	a = [] raises IndexError: a[10] raises IndexError: a[10] = 'lorem'	a = [] evaluates as nil: a[10] increases array size to 11: a[10] = "lorem"
index of array element	$a = array("x", "y", "z", "w"); $i = array_search("y", $a);	use List::Util 'first'; @a = qw(x y z w); $i = first {$a[$_] eq "y"} (0..$#a);	a = ['x', 'y', 'z', 'w'] i = a.index('y')	a = %w(x y z w) i = a.index("y")
slice by endpoints, by length	select 3rd and 4th elements: none array_slice($a, 2, 2)	select 3rd and 4th elements: @a[2..3] splice(@a, 2, 2)	select 3rd and 4th elements: a[2:4] none	select 3rd and 4th elements: a[2..3] a[2, 2]
slice to end	array_slice($a, 1)	@a[1..$#a]	a[1:]	a[1..-1]
manipulate back	$a = array(6,7,8); array_push($a, 9); $a[] = 9; # same as array_push array_pop($a);	@a = (6,7,8); push @a, 9; pop @a;	a = [6,7,8] a.append(9) a.pop()	a = [6,7,8] a.push(9) a << 9 # same as push a.pop
manipulate front	$a = array(6,7,8); array_unshift($a, 5); array_shift($a);	@a = (6,7,8); unshift @a, 5; shift @a;	a = [6,7,8] a.insert(0,5) a.pop(0)	a = [6,7,8] a.unshift(5) a.shift
concatenate	$a = array(1,2,3); $a2 = array_merge($a,array(4,5,6)); $a = array_merge($a,array(4,5,6));	@a = (1,2,3); @a2 = (@a,(4,5,6)); push @a, (4,5,6);	a = [1,2,3] a2 = a + [4,5,6] a.extend([4,5,6])	a = [1,2,3] a2 = a + [4,5,6] a.concat([4,5,6])
replicate		@a = (undef) x 10;	a = [None] * 10 a = [None for i in range(0, 10)]	a = [nil] * 10 a = Array.new(10, nil)
address copy, shallow copy, deep copy	$a = array(1,2,array(3,4)); $a2 =& $a; none $a4 = $a;	use Storable 'dclone' my @a = (1,2,[3,4]); my $a2 = \@a; my @a3 = @a; my @a4 = @{dclone(\@a)};	import copy a = [1,2,[3,4]] a2 = a a3 = list(a) a4 = copy.deepcopy(a)	a = [1,2,[3,4]] a2 = a a3 = a.dup a4 = Marshal.load(Marshal.dump(a))
arrays as function arguments	parameter contains deep copy	each element passed as separate argument; use reference to pass array as single argument	parameter contains address copy	parameter contains address copy
iteration	foreach (array(1,2,3) as $i) { echo "$i\n"; }	for $i (1, 2, 3) { print "$i\n" }	for i in [1,2,3]: print(i)	[1,2,3].each { \|i\| puts i }
indexed iteration	$a = array("do", "re", "mi" "fa"); foreach ($a as $i => $s) { echo "$s at index $i\n"; }	none; use range iteration from 0 to $#a and use index to look up value in the loop body	a = ['do', 're', 'mi', 'fa'] for i, s in enumerate(a): print('%s at index %d' % (s, i))	a = %w(do re mi fa) a.each_with_index do \|s,i\| puts "#{s} at index #{i}" end
iterate over range	not space efficient; use C-style for loop	for $i (1..1_000_000) { code }	range replaces xrange in Python 3: for i in xrange(1, 1000001): code	(1..1_000_000).each do \|i\| code end
instantiate range as array	$a = range(1, 10);	@a = 1..10;	a = range(1, 11) Python 3: a = list(range(1, 11))	a = (1..10).to_a
reverse	$a = array(1,2,3); array_reverse($a); $a = array_reverse($a);	@a = (1,2,3); reverse @a; @a = reverse @a;	a = [1,2,3] a[::-1] a.reverse()	a = [1,2,3] a.reverse a.reverse!
sort	$a = array("b", "A", "a", "B"); none sort($a); none, but usort sorts in place	@a = qw(b A a B); sort @a; @a = sort @a; sort { lc($a) cmp lc($b) } @a;	a = ['b', 'A', 'a', 'B'] sorted(a) a.sort() a.sort(key=str.lower)	a = %w(b A a B) a.sort a.sort! a.sort do \|x,y\| x.downcase <=> y.downcase end
dedupe	$a = array(1,2,2,3); $a2 = array_unique($a); $a = array_unique($a);	use List::MoreUtils 'uniq'; my @a = (1,2,2,3); my @a2 = uniq @a; @a = uniq @a;	a = [1,2,2,3] a2 = list(set(a)) a = list(set(a))	a = [1,2,2,3] a2 = a.uniq a.uniq!
membership	in_array(7, $a)	7 ~~ @a	7 in a	a.include?(7)
intersection	$a = array(1,2); $b = array(2,3,4) array_intersect($a, $b)		{1,2} & {2,3,4}	[1,2] & [2,3,4]
union	$a1 = array(1,2); $a2 = array(2,3,4); array_unique(array_merge($a1, $a2))		{1,2} \| {2,3,4}	[1,2] \| [2,3,4]
relative complement, symmetric difference	$a1 = array(1,2,3); $a2 = array(2); array_values(array_diff($a1, $a2)) none		{1,2,3} - {2} {1,2} ^ {2,3,4}	require 'set' [1,2,3] - [2] Set[1,2] ^ Set[2,3,4]
map	array_map(function ($x) { return $x*$x; }, array(1,2,3))	map { $_ * $_ } (1,2,3)	map(lambda x: x * x, [1,2,3]) # or use list comprehension: [x*x for x in [1,2,3]]	[1,2,3].map { \|o\| o*o }
filter	array_filter(array(1,2,3), function ($x) { return $x>1; })	grep { $_ > 1 } (1,2,3)	filter(lambda x: x > 1, [1,2,3]) # or use list comprehension: [x for x in [1,2,3] if x > 1]	[1,2,3].select { \|o\| o > 1 }
reduce	array_reduce(array(1,2,3), function($x,$y) { return $x+$y; }, 0)	use List::Util 'reduce'; reduce { $x + $y } 0, (1,2,3)	# import needed in Python 3 only from functools import reduce reduce(lambda x, y: x+y, [1,2,3], 0)	[1,2,3].inject(0) { \|m,o\| m+o }
universal and existential tests	use array_filter	# cpan -i List::MoreUtils use List::MoreUtils qw(all any); all { $_ % 2 == 0 } (1,2,3,4) any { $_ % 2 == 0 } (1,2,3,4)	all(i%2 == 0 for i in [1,2,3,4]) any(i%2 == 0 for i in [1,2,3,4])	[1,2,3,4].all? {\|i\| i.even? } [1,2,3,4].any? {\|i\| i.even? }
shuffle and sample	$a = array(1, 2, 3, 4); shuffle($a); array_rand($a, 2)	use List::Util 'shuffle'; @a = (1, 2, 3, 4); shuffle(@a); none	from random import shuffle, sample a = [1, 2, 3, 4] shuffle(a) sample(a, 2)	[1, 2, 3, 4].shuffle Ruby 1.9: [1, 2, 3, 4].sample(2)
zip	# array of 3 pairs: $a = array_map(NULL, array(1, 2, 3), array("a", "b", "c"));	# cpan -i List::MoreUtils use List::MoreUtils 'zip'; @nums = (1, 2, 3); @lets = qw(a b c); # flat array of 6 elements: @a = zip @nums, @lets;	# array of 3 pairs: a = zip([1,2,3], ['a', 'b', 'c'])	# array of 3 pairs: a = [1,2,3].zip(["a", "b", "c"])
dictionaries
	php	perl	python	ruby
literal	$d = array("t" => 1, "f" => 0);	%d = ( t => 1, f => 0 );	d = { 't':1, 'f':0 }	d = { "t" => 1, "f" => 0 }
size	count($d)	scalar(keys %d)	len(d)	d.size d.length # same as size
lookup	$d["t"]	$d{"t"}	d['t']	d["t"]
out-of-bounds behavior	$d = array(); evaluates as NULL: $d["lorem"]; adds key/value pair: $d["lorem"] = "ipsum";	%d = (); evaluates as undef: $d{"lorem"}; adds key/value pair: $d{"lorem"} = "ipsum";	d = {} raises KeyError: d['lorem'] adds key/value pair: d['lorem'] = 'ipsum'	d = {} evaluates as nil: d["lorem"] adds key/value pair: d["lorem"] = "ipsum"
is key present	array_key_exists("y", $d);	exists $d{"y"}	'y' in d	d.has_key?("y")
delete entry	$d = array(1 => "t", 0 => "f"); unset($d[1]);	%d = ( 1 => "t", 0 => "f" ); delete $d{1};	d = {1: True, 0: False} del d[1]	d = {1 => true, 0 => false} d.delete(1)
from array of pairs, from even length array		@a = (1,"a",2,"b",3,"c"); %d = @a;	a = [[1,'a'], [2,'b'], [3,'c']] d = dict(a) a = [1,'a',2,'b',3,'c'] d = dict(zip(a[::2], a[1::2]))	a = [[1,"a"], [2,"b"], [3,"c"]] d = Hash[a] a = [1,"a",2,"b",3,"c"] d = Hash[*a]
merge	$d1 = array("a"=>1, "b"=>2); $d2 = array("b"=>3, "c"=>4); $d1 = array_merge($d1, $d2);	%d1 = (a=>1, b=>2); %d2 = (b=>3, c=>4); %d1 = (%d1, %d2);	d1 = {'a':1, 'b':2} d2 = {'b':3, 'c':4} d1.update(d2)	d1 = {"a"=>1, "b"=>2} d2 = {"b"=>3, "c"=>4} d1.merge!(d2)
invert	$to_num = array("t"=>1, "f"=>0); $to_let = array_flip($to_num);	%to_num = (t=>1, f=>0); %to_let = reverse %to_num;	to_num = {'t':1, 'f':0} # dict comprehensions added in 2.7: to_let = {v:k for k, v in to_num.items()}	to_num = {"t"=>1, "f"=>0} to_let = to_num.invert
iteration	foreach ($d as $k => $v ) { code }	while ( ($k, $v) = each %d ) { code }	for k, v in d.iteritems(): code Python 3: for k, v in d.items(): code	d.each do \|k,v\| code end
keys and values as arrays	array_keys($d) array_values($d)	keys %d values %d	d.keys() d.values() Python 3: list(d.keys()) list(d.values())	d.keys d.values
default value, computed value	$counts = array(); $counts['foo'] += 1; extend ArrayObject for computed values and defaults other than zero or empty string.	my %counts; $counts{'foo'} += 1 define a tied hash for computed values and defaults other than zero or empty string	from collections import defaultdict counts = defaultdict(lambda: 0) counts['foo'] += 1 class Factorial(dict): def __missing__(self, k): if k > 1: return k * self[k-1] else: return 1 factorial = Factorial()	counts = Hash.new(0) counts['foo'] += 1 factorial = Hash.new do \|h,k\| k > 1 ? k * h[k-1] : 1 end
functions
	php	perl	python	ruby
function declaration	function add($a, $b) { return $a + $b; }	sub add { $_[0] + $_[1] } sub add { my ($a, $b) = @_; $a + $b; }	def add(a, b): return a+b	def add(a, b) a+b end parens are optional and customarily omitted when defining functions with no parameters
function invocation	add(1, 2); function names are case insensitive: ADD(1, 2);	add(1, 2); parens are optional: add 1, 2;	add(1, 2)	add(1, 2) parens are optional: add 1, 2
missing argument behavior	set to NULL with warning	set to undef	raises TypeError	raises ArgumentError
default value	function my_log($x, $base=10) { return log($x)/log($base); } my_log(42); my_log(42, M_E);	sub my_log { my $x = shift; my $base = shift // 10; log($x)/log($base); } my_log(42); my_log(42, exp(1));	import math def my_log(x, base=10): return math.log(x)/math.log(base) my_log(42) my_log(42, math.e)	def my_log(x, base=10) Math.log(x)/Math.log(base) end my_log(42) my_log(42, Math::E)
variable number of arguments	function foo() { $arg_cnt = func_num_args(); if ($arg_cnt >= 1) { $n = func_get_arg(0); echo "first: " . $n . "\n"; } if ($arg_cnt >= 2) { $a = func_get_args(); $n = $a[$arg_cnt-1]; echo "last: " . $n . "\n"; } }	sub foo { if ( @_ >= 1 ) { print "first: $_[0]\n"; } if ( @_ >= 2 ) { print "last: $_[-1]\n"; } }	def foo(*a): if len(a) >= 1: print('first: ' + str(a[0])) if len(a) >= 2: print('last: ' + str(a[-1]))	def foo(*a) if a.size >= 1 puts "first: #{a[0]}" end if a.size >= 2 puts "last: #{a[-1]}" end end
named parameters	none	none	def fequal(x, y, opts): eps = opts.get('eps') or 0.01 return abs(x - y) < eps fequal(1.0, 1.001) fequal(1.0, 1.001, eps=0.110)	def fequal(x, y, opts={}) eps = opts[:eps] \|\| 0.01 (x - y).abs < eps end fequal(1.0, 1.001) fequal(1.0, 1.001, :eps=>0.1**10)
pass number or string by reference	function foo(&$x, &$y) { $x += 1; $y .= "ly"; } $n = 7; $s = "hard"; foo($n, $s);	sub foo { $_[0] += 1; $_[1] .= "ly"; } my $n = 7; my $s = "hard"; foo($n, $s);	not possible	not possible
pass array or dictionary by reference	function foo(&$x, &$y) { $x[2] = 5; $y["f"] = -1; } $a = array(1,2,3); $d = array("t"=>1,"f"=>0); foo($a, $d);	sub foo { $_[0][2] = 5; $_[1]{"f"} = -1; } my @a = (1,2,3); my %d = ("t"=> 1, "f" => 0); foo(\@a, \%d);	def foo(x, y): x[2] = 5 y['f'] = -1 a = [1,2,3] d = {'t':1, 'f':0} foo(a, d)	def foo(x, y) x[2] = 5 y["f"] = -1 end a = [1,2,3] d = {"t"=> 1, "f" => 0 } foo(a, d)
return value	return arg or NULL	return arg or last expression evaluated	return arg or None	return arg or last expression evaluated
multiple return values	function first_and_second(&$a) { return array($a[0], $a[1]); } $a = array(1,2,3); list($x, $y) = first_and_second($a);	sub first_and_second { return ($_[0], $_[1]); } @a = (1,2,3); ($x, $y) = first_and_second(@a);	def first_and_second(a): return a[0], a[1] x, y = first_and_second([1,2,3])	def first_and_second(a) return a[0], a[1] end x, y = first_and_second([1,2,3])
lambda declaration	$sqr = function ($x) { return $x * $x; };	$sqr = sub { $_[0] * $_[0] }	body must be an expression: sqr = lambda x: x * x	sqr = lambda { \|x\| x * x }
lambda invocation	$sqr(2)	$sqr->(2)	sqr(2)	sqr.call(2) or sqr[2]
function reference	$func = "add";	my $func = \&add;	func = add	func = lambda {\|args\| add(args)}
function with private state	function counter() { static $i = 0; return ++$i; } echo counter();	use feature state; sub counter { state $i = 0; ++$i; } print counter() . "\n";	# state not private: def counter(): counter.i += 1 return counter.i counter.i = 0 print(counter())	none
closure	function make_counter() { $i = 0; return function () use (&$i) { return ++$i; }; } $nays = make_counter(); echo $nays();	sub make_counter { my $i = 0; return sub { ++$i }; } my $nays = make_counter; print $nays->() . "\n";	# Python 3: def make_counter(): i = 0 def counter(): nonlocal i i += 1 return i return counter nays = make_counter()	def make_counter i = 0 return lambda { i +=1; i } end nays = make_counter puts nays.call
generator	none	none	def make_counter(): i = 0 while True: i += 1 yield i nays = make_counter() print(nays.next())	# Ruby 1.9: def make_counter return Fiber.new do i = 0 while true i += 1 Fiber.yield i end end end nays = make_counter puts nays.resume
execution control
	php	perl	python	ruby
if	if ( 0 == $n ) { echo "no hits\n"; } elseif ( 1 == $n ) { echo "one hit\n"; } else { echo "$n hits\n"; }	if ( 0 == $n ) { print "no hits\n" } elsif ( 1 == $n ) { print "one hit\n" } else { print "$n hits\n" }	if 0 == n: print('no hits') elif 1 == n: print('one hit') else: print(str(n) + ' hits')	if n == 0 puts "no hits" elsif 1 == n puts "one hit" else puts "#{n} hits" end
switch	switch ($n) { case 0: echo "no hits\n"; break; case 1: echo "one hit\n"; break; default: echo "$n hits\n"; }	use feature 'switch'; given ($n) { when (0) { print "no hits\n"; } when (1) { print "one hit\n"; } default { print "$n hits\n"; } }	none	case n when 0 puts "no hits" when 1 puts "one hit" else puts "#{n} hits" end
while	while ( $i < 100 ) { $i++; }	while ( $i < 100 ) { $i++ }	while i < 100: i += 1	while i < 100 do i += 1 end
c-style for	for ($i = 1; $i <= 10; $i++) { echo "$i\n"; }	for ( $i=0; $i <= 10; $i++ ) { print "$i\n"; }	none	none
break, continue, redo	break continue none	last next redo	break continue none	break next redo
control structure keywords	case default do else elseif for foreach goto if switch while	do else elsif for foreach goto if unless until while	elif else for if while	case do else elsif end for loop when while unless until
what do does	starts body of a do-while loop, a loop which checks the condition after the body is executed	executes following block and returns value of last statement executed	raises NameError unless a value was assigned to it	starts an anonymous block. Also starts the body of aloop, while, or until loop
statement modifiers	none	print "positive\n" if $i > 0; print "nonzero\n" unless $i == 0;	none	puts "positive" if i > 0 puts "nonzero" unless i == 0
raise exception	throw new Exception("bad arg");	die "bad arg";	raise Exception('bad arg')	# raises RuntimeError raise "bad arg"
catch exception	try { risky(); } catch (Exception $e) { echo "risky failed: ", $e->getMessage(), "\n"; }	eval { risky }; if ($@) { print "risky failed: $@\n"; }	try: risky() except: print('risky failed')	# catches StandardError begin risky rescue print "risky failed: " puts $!.message end
global variable for last exception	none	$EVAL_ERROR: $@ $OS_ERROR: $! $CHILD_ERROR: $?	last exception: sys.exc_info()[1]	last exception: $! backtrace array of exc.: $@ exit status of child: $?
define exception	class Bam extends Exception { function __construct() { parent::__construct("bam!"); } }	none	class Bam(Exception): def __init__(self): super(Bam, self).__init__('bam!')	class Bam < Exception def initialize super("bam!") end end
catch exception by type	try { throw new Bam; } catch (Bam $e) { echo $e->getMessage(), "\n"; }	none	try: raise Bam() except Bam as e: print(e)	begin raise Bam.new rescue Bam => e puts e.message end
finally/ensure	none	none	acquire_resource() try: risky() finally: release_resource()	acquire_resource begin risky ensure release_resource end
start thread	none	use threads; $func = sub { sleep 10 }; $thr = threads->new($func);	class sleep10(threading.Thread): def run(self): time.sleep(10) thr = sleep10() thr.start()	thr = Thread.new { sleep 10 }
wait on thread	none	$thr->join;	thr.join()	thr.join
files
	php	perl	python	ruby
print to standard output	echo "Hello, World!\n";	print "Hello, World!\n";	print('Hello, World!')	puts "Hello, World!"
read from standard input	$line = fgets(STDIN);	$line = <STDIN>;	line = sys.stdin.readline()	line = gets
standard file handles	only set by CLI; not set when reading script from standard input: STDIN STDOUT STDERR	STDIN STDOUT STDERR	sys.stdin sys.stdout sys.stderr	$stdin $stdout $stderr
open file	$f = fopen("/etc/hosts", "r");	open my $f, "/etc/hosts"; or open FILE, "/etc/hosts";	f = open('/etc/hosts')	f = File.open("/etc/hosts") or File.open("/etc/hosts") { \|f\|
open file for writing	$f = fopen("/tmp/php_test", "w");	open my $f, ">/tmp/perl_test"; or open FILE, ">/tmp/perl_test";	f = open('/tmp/test', 'w')	f = File.open('/tmp/test', 'w') or File.open('/tmp/test', 'w') { \|f\|
close file	fclose($f);	close $f; or close FILE;	f.close()	f.close
read line	$line = fgets($f);	$line = <$f>; or $line = <FILE>;	f.readline()	f.gets
iterate over file by line	while (!feof($f)) { $line = fgets($f);	while ($line = <$f>) {	for line in f:	f.each do \|line\|
chomp	chop($line);	chomp $line;	line = line.rstrip('\r\n')	line.chomp!
read entire file into array or string	$a = file("/etc/hosts"); $s = file_get_contents("/etc/hosts");	@a = <$f>; $s = do { local $/; <$f> };	a = f.readlines() s = f.read()	a = f.lines.to_a s = f.read
write to file	fwrite($f, "lorem ipsum");	print $f "lorem ipsum";	f.write('lorem ipsum')	f.write("lorem ipsum")
flush file handle	note that CLI output isn't buffered fflush($f);	use IO::Handle; $f->flush();	f.flush()	f.flush
file test, regular file test	file_exists("/etc/hosts") is_file("/etc/hosts")	-e "/etc/hosts" -f "/etc/hosts"	os.path.exists('/etc/hosts') os.path.isfile('/etc/hosts')	File.exists?("/etc/hosts") File.file?("/etc/hosts")
copy file, remove file, rename file	copy("/tmp/foo", "/tmp/bar"); unlink("/tmp/foo"); rename("/tmp/bar", "/tmp/foo");	use File::Copy; copy("/tmp/foo", "/tmp/bar"); unlink "/tmp/foo"; move("/tmp/bar", "/tmp/foo");	import shutil shutil.copy('/tmp/foo', '/tmp/bar') os.remove('/tmp/foo') shutil.move('/tmp/bar', '/tmp/foo')	require 'fileutils' FileUtils.cp("/tmp/foo", "/tmp/bar") FileUtils.rm("/tmp/foo") FileUtils.mv("/tmp/bar", "/tmp/foo")
set file permissions	chmod("/tmp/foo", 0755);	chmod 0755, "/tmp/foo";	os.chmod('/tmp/foo', 0755)	File.chmod(0755, "/tmp/foo")
temporary file	$tmp = tempnam(sys_get_temp_dir(), "foo"); $f = fopen($tmp, "w"); fwrite($f, "lorem ipsum\n"); fclose($f); echo "tmp file: $tmp\n";	use File::Temp; $f = File::Temp->new(); print $f "lorem ipsum\n"; close $f; print "tmp file: "; print $f->filename . "\n";	import tempfile f = tempfile.NamedTemporaryFile( prefix='foo') f.write('lorem ipsum\n') f.close() print("tmp file: %s" % f.name)	require 'tempfile' f = Tempfile.new('foo') f.puts "lorem ipsum" f.close puts "tmp file: #{f.path}"
in memory file	$meg = 1024 * 1024; $mem = "php://temp/maxmemory:$meg"; $f = fopen($mem, "r+"); fputs($f, "lorem ipsum"); rewind($f); $s = fread($f, $meg);	my ($f, $s); open($f, ">", \$s); print $f "lorem ipsum\n"; $s;	from StringIOimport StringIO f = StringIO() f.write('lorem ipsum\n') s = f.getvalue() Python 3 moved StringIO to the io module	require 'stringio' f = StringIO.new f.puts("lorem ipsum") f.rewind s = f.read
directories
	php	perl	python	ruby
build pathname	"/etc" . DIRECTORY_SEPARATOR . "hosts"	use File::Spec; File::Spec->catfile("/etc", "hosts")	os.path.join('/etc', 'hosts')	File.join("/etc", "hosts")
dirname and basename	dirname("/etc/hosts") basename("/etc/hosts")	use File::Basename; print dirname("/etc/hosts"); print basename("/etc/hosts");	os.path.dirname('/etc/hosts') os.path.basename('/etc/hosts')	File.dirname("/etc/hosts") File.basename("/etc/hosts")
absolute pathname	realpath("..")	use Cwd; Cwd::abs_path("..")	os.path.abspath('..')	File.expand_path("..")
iterate over directory by file	if ($dir = opendir("/etc")) { while ($file = readdir($dir)) { echo "$file\n"; } closedir($dir); }	use File::Basename; while ( </etc/*> ) { print basename($_) . "\n"; }	for filename in os.listdir('/etc'): print(filename)	Dir.open("/etc").each do \|file\| puts file end
make directory	mkdir("/tmp/foo/bar", 0755, TRUE);	use File::Path 'make_path'; make_path "/tmp/foo/bar";	dirname = '/tmp/foo/bar' if not os.path.isdir(dirname): os.makedirs(dirname)	require 'fileutils' FileUtils.mkdir_p("/tmp/foo/bar")
recursive copy	none	# cpan -i File::Copy::Recursive use File::Copy::Recursive 'dircopy'; dircopy "/tmp/foodir", "/tmp/bardir";	import shutil shutil.copytree('/tmp/foodir', '/tmp/bardir')	require 'fileutils' FileUtils.cp_r("/tmp/foodir", "/tmp/bardir")
remove empty directory	rmdir("/tmp/foodir");	rmdir "/tmp/foodir";	os.rmdir('/tmp/foodir')	File.rmdir("/tmp/foodir")
remove directory and contents	none	use File::Path 'remove_tree'; remove_tree "/tmp/foodir";	import shutil shutil.rmtree('/tmp/foodir')	require 'fileutils' FileUtils.rm_rf("/tmp/foodir")
directory test	is_dir("/tmp")	-d "/tmp"	os.path.isdir('/tmp')	File.directory?("/tmp")
processes and environment
	php	perl	python	ruby
command line args, script name	count($argv) $argv[0] $argv[1] etc $_SERVER["SCRIPT_NAME"]	scalar(@ARGV) $ARGV[0] $ARGV[1] etc $0	len(sys.argv)-1 sys.argv[1] sys.argv[2] etc sys.argv[0]	ARGV.size ARGV[0] ARGV[1] etc $0
getopt	$opts = getopt("f:h", array("file:", "help")); if ( array_key_exists("h", $opts) \|\| array_key_exists("help", $opts) ) { $script = $_SERVER["SCRIPT_NAME"]; echo "usage: $script -f SRC -h\n"; exit -1; } $src = $opts["f"] ? $opts["f"] : $opts["file"];	use Getopt::Long; my ($src, $help); sub usage { print "usage: $0 --f SRC\n"; exit -1; } GetOptions("file=s" => \$src, "help" => \$help); usage if $help;	import argparse parser = argparse.ArgumentParser() parser.add_argument('--file', '-f', dest='file') args = parser.parse_args() src = args.file	require 'getoptlong' opts = GetoptLong.new( ['--help', '-h', GetoptLong::NO_ARGUMENT], ['--file', '-f', GetoptLong::REQUIRED_ARGUMENT] ) opts.each do \|opt, arg\| case opt when '--file' src = arg when '--help' puts "usage: #{$0} -f SRC" exit -1 end end
get and set environment variable	getenv("HOME") putenv("PATH=/bin");	$ENV{"HOME"} $ENV{"PATH") = "/bin";	os.getenv('HOME') os.environ['PATH'] = '/bin'	ENV["HOME"] ENV["PATH"] = "/bin"
exit	exit 0;	exit 0;	sys.exit(0)	exit(0)
set signal handller		$SIG{INT} = sub { die "exiting…\n"; };	import signal def handler(signo, frame): print('exiting…') exit -1 signal.signal(signal.SIGINT, handler)	Signal.trap("INT", lambda do \|signo\| puts "exiting…" exit end )
executable test	is_executable("/bin/ls")	-x "/bin/ls"	os.access('/bin/ls', os.X_OK)	File.executable?("/bin/ls")
external command	system("ls -l /tmp", $retval); if ($retval) { throw new Exception("ls failed"); }	system("ls -l /tmp") == 0 or die "ls failed";	if os.system('ls -l /tmp'): raise Exception('ls failed')	unless system("ls -l /tmp") raise "ls failed" end
escaped external command	$path = chop(fgets(STDIN)); $safe = escapeshellarg($path); system("ls -l " . $safe, $retval); if ($retval) { throw new Exception("ls failed"); }	$path = <>; chomp($path); system("ls", "-l", $path) == 0 or die "ls failed";	import subprocess cmd = ['ls', '-l', '/tmp'] if subprocess.call(cmd): raise Exception('ls failed')	path = gets path.chomp! unless system("ls", "-l", path) raise "ls failed" end
backticks	$files = `ls -l /tmp`;	my $files = `ls -l /tmp`; or my $files = qx(ls);	import subprocess cmd = ['ls', '-l', '/tmp'] files = subprocess.check_output(cmd)	files = `ls -l /tmp` unless $?.success? raise "ls failed" end files = %x(ls) unless $?.success? raise "ls failed" end
	__________________________________________	__________________________________________	__________________________________________	__________________________________________

General Footnotes

versions used

The versions used for testing code in the reference sheet.

implicit prologue

Code which examples in the sheet assume to have already been executed.

perl:

We adopt the convention that if an example uses a variable without declaring it, it should be taken to have been previously declared with my.

python:

To keep the examples short we assume that os, re, and sys are always imported.

show version

How to get the version.

php:

The function phpversion() will return the version number as a string.

perl:

Also available in the predefined variable $], or in a different format in $^V and $PERL_VERSION.

python:

The following function will return the version number as a string:

import platform
platform.python_version()

ruby:

Also available in the global constant VERSION (Ruby 1.8) or RUBY_VERSION (Ruby 1.9).

interpreter

The customary name of the interpreter and how to invoke it.

php:

php -f will only execute portions of the source file within a <?php php code ?> tag as php code. Portions of the source file outside of such tags is not treated as executable code and is echoed to standard out.

If short tags are enabled, then php code can also be placed inside <? php code ?> and <?= php code ?> tags.

<?= php code ?> is identical to <?php echo php code ?>.

repl

The customary name of the repl.

php:

The php REPL does not save or display the result of an expression.

php -a offers a different style of interactive mode. It collects input until EOF is encountered and then it executes it. Text inside <? code ?> and <?= code ?> is executed as PHP code. Text outside of PHP markup tags is echoed.

perl:

The Perl REPL perl -de 0 does not save or display the result of an expression. perl -d is the Perl debugger and perl -e runs code provided on the command line.

perl -de 0 does not by default have readline, but it can be added:

$ cpan -i Term::Readline::Perl

  <cpan output omitted>

$ perl -de 0

  DB<1> use Term::Readline::Perl;

  DB<2> print 1 + 1;
2

python:

The python repl saves the result of the last statement in _.

ruby:

irb saves the result of the last statement in _.

command line script

How to pass the code to be executed to the interpreter as a command line argument.

statement separator

How the parser determines the end of a statement.

php:

Inside braces statements must be terminated by a semicolon. The following causes a parse error:

<? if (true) { echo "true" } ?>

The last statement inside <?= ?> or <? ?> tags does not need to be semicolon terminated, however. The following code is legal:

<?= $a = 1 ?>
<? echo $a ?>

perl:

In a script statements are separated by semicolons and never by newlines. However, when using 'perl -de 0', a newline terminates the statement.

python:

Newline does not terminate a statement when:

inside parens
inside list [] or dictionary {} literals

Python single quote '' and double quote "" strings cannot contain newlines except as the two character escaped form \n. Putting a newline in these strings results in a syntax error. There is however a multi-line string literal which starts and ends with three single quotes ''' or three double quotes: """.

A newline that would normally terminate a statement can be escaped with a backslash.

ruby:

Newline does not terminate a statement when:

inside single quotes '', double quotes "", backticks ``, or parens ()
after an operator such as + or , that expects another argument

Ruby permits newlines in array [] or hash literals, but only after a comma , or associator =>. Putting a newline before the comma or associator results in a syntax error.

A newline that would normally terminate a statement can be escaped with a backslash.

block delimiters

How blocks are delimited.

perl:

Curly brackets {} delimit blocks. They are also used for:

hash literal syntax which returns a reference to the hash: $rh = { 'true' => 1, 'false' => 0 }
hash value lookup: $h{'true'}, $rh->{'true'}
variable name delimiter: $s = "hello"; print "${s}goodbye";

python:

Python blocks begin with a line that ends in a colon. The block ends with the first line that is not indented further than the initial line. Python raises an IndentationError if the statements in the block that are not in a nested block are not all indented the same. Using tabs in Python source code is unrecommended and many editors replace them automatically with spaces. If the Python interpreter encounters a tab, it is treated as 8 spaces.

The python repl switches from a >>> prompt to a … prompt inside a block. A blank line terminates the block.

ruby:

Curly brackets {} delimit blocks. A matched curly bracket pair can be replaced by the do and end keywords. By convention curly brackets are used for one line blocks.

The end keyword also terminates blocks started by def, class, or module.

Curly brackets are also used for hash literals, and the #{ } notation is used to interpolate expressions into strings.

assignment

How to assign a value to a variable.

perl:

Assignment operators have right precedence and evaluate to the right argument, so assignments can be chained:

$a = $b = 3;

python:

If the variable on the left has not previously been defined in the current scope, then it is created. This may hide a variable in a containing scope.

Assignment does not return a value and cannot be used in an expression. Thus, assignment cannot be used in a conditional test, removing the possibility of using assignment (=) in place of an equality test (==). Assignments can nevertheless be chained to assign a value to multiple variables:

a = b = 3

ruby:

Assignment operators have right precedence and evaluate to the right argument, so they can be chained. If the variable on the left does not exist, then it is created.

parallel assignment

How to assign values to variables in parallel.

python:

The r-value can be a list or tuple:

nums = [1,2,3]
a,b,c = nums
more_nums = (6,7,8)
d,e,f = more_nums

Nested sequences of expression can be assigned to a nested sequences of l-values, provided the nesting matches. This assignment will set a to 1, b to 2, and c to 3:

(a,[b,c]) = [1,(2,3)]

This assignment will raise a TypeError:

(a,(b,c)) = ((1,2),3)

In Python 3 the splat operator * can be used to collect the remaining right side elements in a list:

x, y, *z = 1, 2        # assigns [] to z
x, y, *z = 1, 2, 3     # assigns [3] to z
x, y, *z = 1, 2, 3, 4  # assigns [3, 4] to z

ruby:

The r-value can be an array:

nums = [1,2,3]
a,b,c = nums

swap

How to swap the values held by two variables.

compound assignment

Compound assignment operators mutate a variable, setting it to the value of an operation which takes the value of the variable as an argument.

First row: arithmetic operator assignment: addition, subtraction, multiplication, (float) division, integer division, modulus, and exponentiation.
Second row: string concatenation assignment and string replication assignment
Third row: logical operator assignment: and, or, xor
Fourth row: bit operator assignment: left shift, right shift, and, or, xor.

python:

Python compound assignment operators do not return a value and hence cannot be used in expressions.

increment and decrement

The C-style increment and decrement operators can be used to increment or decrement values. They return values and thus can be used in expressions. The prefix versions return the value in the variable after mutation, and the postfix version return the value before mutation.

Incrementing a value two or more times in an expression makes the order of evaluation significant:

x = 1;
foo(++x, ++x); // foo(2, 3) or foo(3, 2)?

x = 1;
y = ++x/++x;  // y = 2/3 or y = 3/2?

Python avoids the problem by not having an in-expression increment or decrement.

Ruby mostly avoids the problem by providing a non-mutating increment and decrement. However, here is a Ruby expression which is dependent on order of evaluation:

x = 1
y = (x += 1)/(x += 1)

php:

The increment and decrement operators also work on strings. There are postfix versions of these operators which evaluate to the value before mutation:

$x = 1;
$x++;
$x--;

perl:

The increment and decrement operators also work on strings. There are postfix versions of these operators which evaluate to the value before mutation:

$x = 1;
$x++;
$x--;

ruby:

The Integer class defines succ, pred, and next, which is a synonym for succ.

The String class defines succ, succ!, next, and next!. succ! and next! mutate the string.

local variable declarations

How to declare variables which are local to the scope defining region which immediately contain them.

php:

Variables do not need to be declared and there is no syntax for declaring a local variable. If a variable with no previous reference is accessed, its value is NULL.

perl:

Variables don't need to be declared unless use strict is in effect.

If not initialized, scalars are set to undef, arrays are set to an empty array, and hashes are set to an empty hash.

Perl can also declare variables with local. These replace the value of a global variable with the same name, if any, for the duration of the enclosing scope, after which the old value is restored. local declarations became obsolete with the introduction of the my declaration introduced in Perl 5.

python:

A variable is created by assignment if one does not already exist. If the variable is inside a function or method, then its scope is the body of the function or method. Otherwise it is a global.

ruby:

Variables are created by assignment. If the variable does not have a dollar sign ($) or ampersand (@) as its first character then its scope is scope defining region which most immediately contains it.

A lower case name can refer to a local variable or method. If both are defined, the local variable takes precedence. To invoke the method make the receiver explicit: e.g. self.name. However, outside of class and modules local variables hide functions because functions are private methods in the class Object. Assignment to name will create a local variable if one with that name does not exist, even if there is a method name.

regions which define local scope

A list of regions which define a scope for the local variables they contain.

Local variables defined inside the region are only in scope while code within the region is executing. If the language does not have closures, then code outside the region has no access to local variables defined inside the region. If the language does have closures, then code inside the region can make local variables accessible to code outside the region by returning a reference.

A region which is top level hides local variables in the scope which contains it from the code it contains. A region can also be top level if the syntax requirements of the language prohibit it from being placed inside another scope defining region.

A region is nestable if it can be placed inside another scope defining region, and if code in the inner region can access local variables in the outer region.

php:

Only function bodies and method bodies define scope. Function definitions can be nested, but when this is done lexical variables in the outer function are not visible to code in the body of the inner function.

Braces can be used to set off blocks of codes in a manner similar to the anonymous blocks of Perl. However, these braces do not define a scope. Local variables created inside the braces will be visible to subsequent code outside of the braces.

Local variables cannot be created in class bodies.

perl:

A local variable can be defined outside of any function definition or anonymous block, in which case the scope of the variable is the file containing the source code. In this way Perl resembles Ruby and contrasts with PHP and Python. In PHP and Python, any variable defined outside a function definition is global.

In Perl, when a region which defines a scope is nested inside another, then the inner region has read and write access to local variables defined in the outer region.

Note that the blocks associated with the keywords if, unless, while, until, for, and foreach are anonymous blocks, and thus any my declarations in them create variables local to the block.

python:

Only functions and methods define scope. Function definitions can be nested. When this is done, inner scopes have read access to variables defined in outer scopes. Attempting to write (i.e. assign) to a variable defined in an outer scope will instead result in a variable getting created in the inner scope. Python trivia question: what would happen if the following code were executed?

def foo():
    v = 1
    def bar():
        print(v)
        v = 2
        print(v)
    bar()

foo()

ruby:

Note that though the keywords if, unless, case, while, and until each define a block which is terminated by an end keyword, none of these blocks have their own scope.

Anonymous functions can be created with the lambda keyword. Ruby anonymous blocks can be provided after a function invocation and are bounded by curly brackets { } or the do and end keywords. Both anonymous functions and anonymous blocks can have parameters which are specified at the start of the block within pipes. Here are some examples:

id = lambda { |x| x }

[3,1,2,4].sort { |a,b| a <=> b }

10.times do |i|
  print "#{i}..." 
end

In Ruby 1.8, the scope of the parameter of an anonymous block or function or block is local to the block or function body if the name is not already bound to a variable in the containing scope. However, if it is, then the variable in the containing scope will be used. This behavior was changed in Ruby 1.9 so that parameters are always local to function body or block. Here is an example of code which behaves differently under Ruby 1.8 and Ruby 1.9:

x = 3
id = lambda { |x| x }
id.call(7)
puts x # 1.8 prints 7; 1.9 prints 3

Ruby 1.9 also adds the ability mark variables as local, even when they are already defined in the containing scope. All such variables are listed inside the parameter pipes, separated from the parameters by a semicolon:

x = 3
noop = lambda { |; x| x = 15 } # bad syntax under 1.8
noop.call
# x is still 3

global variable

How to declare and access a variable with global scope.

php:

A variable is global if it is used at the top level (i.e. outside any function definition) or if it is declared inside a function with the global keyword. A function must use theglobal keyword to access the global variable.

perl:

Undeclared variables, which are permitted unless use strict is in effect, are global. If use strict is in effect, a global can be declared at the top level of a package (i.e. outside any blocks or functions) with the our keyword. A variable declared with my inside a function will hide a global with the same name, if there is one.

python:

A variable is global if it is defined at the top level of a file (i.e. outside any function definition). Although the variable is global, it must be imported individually or be prefixed with the module name prefix to be accessed from another file. To be accessed from inside a function or method it must be declared with the global keyword.

ruby:

A variable is global if it starts with a dollar sign: $.

constant declaration

How to declare a constant.

php:

A constant can be declared inside a class:

class Math {
  const pi = 3.14;
}

Refer to a class constant like this:

Math::pi

ruby:

Capitalized variables contain constants and class/module names. By convention, constants are all caps and class/module names are camel case. The ruby interpreter does not prevent modification of constants, it only gives a warning. Capitalized variables are globally visible, but a full or relative namespace name must be used to reach them: e.g. Math::PI.

to-end-of-line comment

How to create a comment that ends at the next newline.

comment out multiple lines

How to comment out multiple lines.

python:

The triple single quote ''' and triple double quote """ syntax is a syntax for string literals.

null

The null literal.

null test

How to test if a variable contains null.

php:

$v == NULL does not imply that $v is NULL, since any comparison between NULL and a falsehood will return true. In particular, the following comparisons are true:

$v = NULL;
if ($v == NULL) { echo "true"; }

$v = 0;
if ($v == NULL) { echo "sadly true"; }

$v = '';
if ($v == NULL) { echo "sadly true"; }

perl:

$v == undef does not imply that $v is undef. Any comparison between undef and a falsehood will return true. The following comparisons are true:

$v = undef;
if ($v == undef) { print "true"; }

$v = 0;
if ($v == undef) { print "sadly true"; }

$v = '';
if ($v == undef) { print "sadly true"; }

undefined variable access

The result of attempting to access an undefined variable.

undefined test

php:

A test showing that isset is the logical negation of is_null.

perl:

Perl does not distinguish between unset variables and variables that have been set to undef. In perl, calling defined($a) does not result in a error if $a is undefined, even with the strict pragma.

Arithmetic and Logic Footnotes

true and false

Literals for the booleans.

These are the return values of the comparison operators.

php:

Any identifier which matches TRUE case-insensitive can be used for the TRUE boolean. Similarly for FALSE.

In general, PHP variable names are case-sensitive, but function names are case-insensitive.

When converted to a string for display purposes, TRUE renders as "1" and FALSE as "". The equality tests TRUE == 1 and FALSE == "" evaluate as TRUE but the equality tests TRUE === 1 and FALSE === "" evaluate as FALSE.

falsehoods

Values which behave like the false boolean in a conditional context.

Examples of conditional contexts are the conditional clause of an if statement and the test of a while loop.

python:

Whether a object evaluates to True or False in a boolean context can be customized by implementing a __nonzero__ (Python 2) or __bool__ (Python 3) instance method for the class.

logical operators

Logical and, or, and not.

php, perl, ruby:

&& and || have higher precedence than assignment, compound assignment, and the ternary operator (?:), which have higher precedence than and and or.

conditional expression

How to write a conditional expression. A ternary operator is an operator which takes three arguments. Since

condition ? true value : false value

is the only ternary operator in C, it is unambiguous to refer to it as the ternary operator.

python:

The Python conditional expression comes from Algol.

ruby:

The Ruby if statement is also an expression:

x = if x > 0
  x
else
  -x
end

comparison operators

Equality, inequality, greater than, less than, greater than or equal, less than or equal.

Also known as the relational operators.

php:

Most of the comparison operators will convert a string to a number if the other operand is a number. Thus 0 == "0" is true. The operators === and !== do not perform this conversion, so 0 === "0" is false.

perl:

The operators: == != > < >= <= convert strings to numbers before performing a comparison. Many string evaluate as zero in a numeric context and are equal according to the == operator. To perform a lexicographic string comparison, use: eq, ne, gt, lt, ge, le.

python:

Comparison operators can be chained. The following expressions evaluate to true:

1 < 2 < 3
1 == 1 != 2

In general if A_i are expressions and op_i are comparison operators, then

A₁ op₁ A₂ op₂ A₃ … A_n op_n A_n+1

is true if and only if each of the following is true

    A₁ op₁ A₂
    A₂ op₂ A₃
    …
    A_n op_n A_n+1

three value comparison

Binary comparison operators which return -1, 0, or 1 depending upon whether the left argument is less than, equal to, or greater than the right argument.

The <=> symbol is called the spaceship operator.

convert from string, to string

How to convert string data to numeric data and vice versa.

php:

PHP converts a scalar to the desired type automatically and does not raise an error if the string contains non-numeric data. If the start of the string is not numeric, the string evaluates to zero in a numeric context.

perl:

Perl converts a scalar to the desired type automatically and does not raise an error if the string contains non-numeric data. If the start of the string is not numeric, the string evaluates to zero in a numeric context.

python:

float and int raise an error if called on a string and any part of the string is not numeric.

ruby:

to_i and to_f always succeed on a string, returning the numeric value of the digits at the start of the string, or zero if there are no initial digits.

arithmetic operators

The operators for addition, subtraction, multiplication, float division, integer division, modulus, and exponentiation.

integer division

How to get the integer quotient of two integers. How to get the integer quotient and remainder.

perl:

The integer pragma makes all arithmetic operations integer operations. Floating point numbers are truncated before they are used. Hence integer division could be performed with:

use integer;
my $a = 7 / 3;
no integer;

float division

How to perform floating point division, even if the operands might be integers.

arithmetic functions

Some arithmetic functions. Trigonometric functions are in radians unless otherwise noted. Logarithms are natural unless otherwise noted.

python:

Python also has math.log10. To compute the log of x for base b, use:

math.log(x)/math.log(b)

ruby:

Ruby also has Math.log2, Math.log10. To compute the log of x for base b, use

Math.log(x)/Math.log(b)

arithmetic truncation

How to truncate a float to the nearest integer towards zero; how to round a float to the nearest integer; how to find the nearest integer above a float; how to find the nearest integer below a float; how to take the absolute value.

perl:

The CPAN module Number::Format provides a round function. The 2nd argument specifies the number of digits to keep to the right of the radix. The default is 2.

use Number::Format 'round';

round(3.14, 0);

min and max

How to get the min and max.

division by zero

What happens when division by zero is performed.

integer overflow

What happens when the largest representable integer is exceeded.

float overflow

What happens when the largest representable float is exceeded.

sqrt -2

The result of taking the square root of negative two.

rational numbers

How to create rational numbers and get the numerator and denominator.

ruby:

Require the library mathn and integer division will yield rationals instead of truncated integers.

complex numbers

python:

Most of the functions in math have analogues in cmath which will work correctly on complex numbers.

random integer, uniform float, normal float

How to generate a random integer between 0 and 99, include, float between zero and one in a uniform distribution, or a float in a normal distribution with mean zero and standard deviation one.

set random seed, get and restore seed

How to set the random seed; how to get the current random seed and later restore it.

All the languages in the sheet set the seed automatically to a value that is difficult to predict. The Ruby 1.9 MRI interpreter uses the current time and process ID, for example. As a result there is usually no need to set the seed.

Setting the seed to a hardcoded value yields a random but repeatable sequence of numbers. This can be used to ensure that unit tests which cover code using random numbers doesn't intermittently fail.

The seed is global state. If multiple functions are generating random numbers then saving and restoring the seed may be necessary to produce a repeatable sequence.

bit operators

The bit operators for left shift, right shift, and, inclusive or, exclusive or, and negation.

binary, octal, and hex literals

Binary, octal, and hex integer literals

base conversion

How to convert integers to strings of digits of a given base. How to convert such strings into integers.

perl

Perl has the functions oct and hex which convert strings encoded in octal and hex and return the corresponding integer. The oct function will handle binary or hex encoded strings if they have "0b" or "0x" prefixes.

oct("60")
oct("060")
oct("0b101010")
oct("0x2a")

hex("2a")
hex("0x2a")

python

Python has the functions bin, oct, and hex which take an integer and return a string encoding the integer in base 2, 8, and 16.

bin(42)
oct(42)
hex(42)

String Footnotes

string literal

The syntax for string literals.

perl:

When use strict is not in effect bareword strings are permitted.

Barewords are strings without quote delimiters. They are a feature of shells. Barewords cannot contain whitespace or any other character used by the tokenizer to distinguish words.

Before Perl 5 subroutines were invoked with an ampersand prefix & or the older do keyword. With Perl 5 neither is required, but this made it impossible to distinguish a bareword string from a subroutine without knowing all the subroutines which are in scope.

The following code illustrates the bareword ambiguity:

no strict;

print rich . "\n";  # prints "rich"; rich is a bareword string

sub rich { return "poor" }

print rich . "\n";  # prints "poor"; rich is now a subroutine

newline in literal

Whether newlines are permitted in string literals.

python:

Newlines are not permitted in single quote and double quote string literals. A string can continue onto the following line if the last character on the line is a backslash. In this case, neither the backslash nor the newline are taken to be part of the string.

Triple quote literals, which are string literals terminated by three single quotes or three double quotes, can contain newlines:

'''This is
two lines'''

"""This is also
two lines"""

backslash escapes

Backslash escape sequences for inserting special characters into string literals.

python:

When string literals have an r or R prefix there are no backslash escape sequences and any backslashes thus appear in the created string. The delimiter can be inserted into a string if it is preceded by a backslash, but the backslash is also inserted. It is thus not possible to create a string with an r or R prefix that ends in a backslash. The r and R prefixes can be used with single or double quotes:

r'C:\Documents and Settings\Admin'
r"C:\Windows\System32"

The \uhhhh escapes are also available inside Python 2 Unicode literals. Unicode literals have a u prefiix:

u'lambda: \u03bb'

variable interpolation

How to interpolate variables into strings.

python:

str.format will take named or positional parameters. When used with named parameters str.format can mimic the variable interpolation feature of the other languages.

A selection of variables in scope can be passed explicitly:

count = 3
item = 'ball'
print('{count} {item}s'.format(
  count=count,
  item=item))

Python 3 has format_map which accepts a dict as an argument:

count = 3
item = 'ball'
print('{count} {item}s'.format_map(locals()))

custom delimiters

How to specify custom delimiters for single and double quoted strings. These can be used to avoid backslash escaping. If the left delimiter is (, [, or { the right delimiter must be ), ], or }, respectively.

sprintf

How to create a string using a printf style format.

python:

The % operator will interpolate arguments into printf-style format strings.

The str.format with positional parameters provides an alternative format using curly braces {0}, {1}, … for replacement fields.

The curly braces are escaped by doubling:

'to insert parameter {0} into a format, use {{{0}}}'.format(3)

If the replacement fields appear in sequential order and aren't repeated, the numbers can be omitted:

'lorem {} {} {}'.format('ipsum', 13, 3.7)

here document

Here documents are strings terminated by a custom identifier. They perform variable substitution and honor the same backslash escapes as double quoted strings.

perl:

Put the custom identifer in single quotes to prevent variable interpolation and backslash escape interpretation:

s = <<'EOF';
Perl code uses variables with dollar
signs, e.g. $var
EOF

python:

Python lacks variable interpolation in strings. Triple quotes honor the same backslash escape sequences as regular quotes, so triple quotes can otherwise be used like here documents:

s = '''here document
there computer
'''

ruby:

Put the customer identifier in single quotes to prevent variable interpolation and backslash escape interpretation:

s = <<'EOF'
Ruby code uses #{var} type syntax
to interpolate variables into strings.
EOF

concatenate

The string concatenation operator.

replicate

The string replication operator.

split, in two, with delimiters, into characters

How to split a string containing a separator into an array of substrings; how to split a string in two; how to split a string with the delimiters preserved as separate elements; how to split a string into an array of single character strings.

python:

str.split() takes simple strings as delimiters; use re.split() to split on a regular expression:

re.split('\s+', 'do re mi fa')
re.split('\s+', 'do re mi fa', 1)

join

How to concatenate the elements of an array into a string with a separator.

case manipulation

How to put a string into all caps or all lower case letters. How to capitalize the first letter of a string.

strip

How to remove whitespace from the ends of a string.

pad on right, on left

How to pad the edge of a string with spaces so that it is a prescribed length.

length

How to get the length in characters of a string.

index of substring

How to find the index of the leftmost occurrence of a substring in a string; how to find the index of the rightmost occurrence.

python:

Methods for splitting a string into three parts using the first or last occurrence of a substring:

'do re re mi'.partition('re')     # returns ('do ', 're', ' re mi')
'do re re mi'.rpartition('re')    # returns ('do re ', 're', ' mi')

extract substring

How to extract a substring from a string by index.

extract character

How to extract a character from a string by its index.

chr and ord

Converting characters to ASCII codes and back.

The languages in this reference sheet do not have character literals, so characters are represented by strings of length one.

character translation

How to apply a character mapping to a string.

Regular Expressions

PHP PCRE Regexes
perlre and perlreref
Python re library: 2.7, 3.1
Ruby Regexp

Regular expressions or regexes are a way of specifying sets of strings. If a string belongs to the set, the string and regex "match". Regexes can also be used to parse strings.

The modern notation for regexes was introduced by Unix command line tools in the 1970s. POSIX standardized the notation into two types: extended regexes and the more archaic basic regexes. Perl regexes are extended regexes augmented by new character class abbreviations and a few other features introduced by the Perl interpreter in the 1990s. All the languages in this sheet use Perl regexes.

Any string that doesn't contain regex metacharacters is a regex which matches itself. The regex metacharacters are: [ ] . | ( ) * + ? { } ^ $ \

character classes: [ ] .

A character class is a set of characters in brackets: [ ]. When used in a regex it matches any character it contains.

Character classes have their own set of metacharacters: ^ - \ ]

The ^ is only special when it the first character in the character class. Such a character class matches its complement; that is, any character not inside the brackets. When not the first character the ^ refers to itself.

The hypen is used to specify character ranges: e.g. 0-9 or A-Z. When the hyphen is first or last inside the brackets it matches itself.

The backslash can be used to escape the above characters or the terminal character class delimiter: ]. It can be used in character class abbreviations or string backslash escapes.

The period . is a character class abbreviation which matches any character except for newline. In all languages the period can be made to match all characters. In PHP and Perl use the m modifer. In Python use the re.M flag. In Ruby use the s modifer.

character class abbreviations:

abbrev	name	character class
\d	digit	[0-9]
\D	nondigit	[^0-9]
\h	PHP, Perl: horizontal whitespace character Ruby: hex digit	PHP, Perl: [ \t] Ruby: [0-9a-fA-F]
\H	PHP, Perl: not a horizontal whitespace character Ruby: not a hex digit	PHP, Perl: [^ \t] Ruby: [^0-9a-fA-F]
\s	whitespace character	[ \t\r\n\f]
\S	non whitespace character	[^ \t\r\n\f]
\v	vertical whitespace character	[\r\n\f]
\V	not a vertical whitespace character	[^\r\n\f]
\w	word character	[A-Za-z0-9_]
\W	non word character	[^A-Za-z0-9_]

alternation and grouping: | ( )

The vertical pipe | is used for alternation and parens () for grouping.

A vertical pipe takes as its arguments everything up to the next vertical pipe, enclosing paren, or end of string.

Parentheses control the scope of alternation and the quantifiers described below. The are also used for capturing groups, which are the substrings which matched parenthesized parts of the regular expression. Each language numbers the groups and provides a mechanism for extracting when a match is made. A parenthesized subexpression can be removed from the groups with this syntax: (?:expr)

quantifiers: * + ? { }

As an argument quantifiers take the preceding regular character, character class, or group. The argument can itself be quantified, so that ^a{4}*$ matches strings with the letter a in multiples of 4.

quantifier	# of occurrences of argument matched
*	zero or more, greedy
+	one or more, greedy
?	zero or one, greedy
{m,n}	m to n, greedy
{n}	exactly n
{m,}	m or more, greedy
{,n}	zero to n, greedy
*?	zero or more, lazy
+?	one or more, lazy
{m,n}?	m to n, lazy
{m,}?	m or more, lazy
{,n}?	zero to n, lazy

When there is a choice, greedy quantifiers will match the maximum possible number of occurrences of the argument. Lazy quantifiers match the minimum possible number.

anchors: ^ $

anchor	matches
^	beginning of a string. In Ruby or when m modifier is used also matches right side of a newline
$	end of a string. In Ruby or when m modifier is used also matches left side of a newline
\A	beginning of the string
\b	word boundary. In between a \w and a \W character or in between a \w character and the edge of the string
\B	not a word boundary. In between two \w characters or two \W characters
\z	end of the string
\Z	end of the string unless it is a newline, in which case it matches the left side of the terminal newline

escaping: \

To match a metacharacter, put a backslash in front of it. To match a backslash use two backslashes.

php:

PHP 5.3 still supports the EREG engine, though the functions which use it are deprecated. These include the split function and functions which start with ereg. The preferred functions are preg_split and the other functions with a preg prefix.

literal, custom delimited literal

The literal for a regular expression; the literal for a regular expression with a custom delimiter.

php:

PHP regex literals are strings. The first character is the delimiter and it must also be the last character. If the start delimiter is (, {, or [ the end delimiter must be ), }, or ], respectively.

Here are the signatures from the PHP manual for the preg functions used in this sheet:

array preg_split ( string $pattern , string $subject [, int $limit = -1 [, int $flags = 0 ]] )

int preg_match ( string $pattern , string $subject [, array &$matches [, int $flags = 0 [, int $offset = 0 ]]] )

mixed preg_replace ( mixed $pattern , mixed $replacement , mixed $subject [, int $limit = -1 [, int &$count ]] )

int preg_match_all ( string $pattern , string $subject [, array &$matches [, int $flags = PREG_PATTERN_ORDER [, int $offset = 0 ]]] )

python:

Python does not have a regex literal, but the re.compile function can be used to create regex objects.

Compiling regexes can always be avoided:

re.compile('\d{4}').search('1999')
re.search('\d{4}', '1999')

re.compile('foo').sub('bar', 'foo bar')
re.sub('foo', 'bar', 'foo bar')

re.compile('\w+').findall('do re me')
re.findall('\w+', 'do re me')

character class abbreviations and anchors

The supported character class abbreviations and anchors.

Note that \h refers to horizontal whitespace (i.e. a space or tab) in PHP and Perl and a hex digit in Ruby. Similarly \H refers to something that isn't horizontal whitespace in PHP and Perl and isn't a hex digit in Ruby.

match test

How to test whether a string matches a regular expression.

python:

The re.match function is like the re.search function, except that it is only true if the regular expression matches the entire string.

ruby:

match is a method of both Regexp and String so can match with both

/1999/.match("1999")

and

"1999".match(/1999/)

When variables are involved it is safer to invoke the Regexp method because string variables are more likely to contain nil.

case insensitive match test

How to perform a case insensitive match test.

modifiers

Modifiers that can be used to adjust the behavior of a regular expression.

The lists are not comprehensive. For all languages except Ruby there are additional modifiers.

modifier	behavior
e	PHP: when used with preg_replace, the replacement string, after backreferences are substituted, is eval'ed as PHP code and the result is used as the replacement.
i, re.I	all: ignores case. Upper case letters match lower case letters and vice versa.
m, re.M	PHP, Perl, Python: makes the ^ and $ match the right and left edge of newlines in addition to the beginning and end of the string. Ruby: makes the period . match newline characters.
o	Ruby: performs variable interpolation #{ } only once per execution of the program.
p	Perl: sets ${^MATCH} ${^PREMATCH} and ${^POSTMATCH}
s, re.S	PHP, Perl, Python: makes the period . match newline characters.
x, re.X	all: ignores whitespace in the regex which permits it to be used for formatting.

Python modifiers are bit flags. To use more than one flag at the same time, join them with bit or: |

substitution

How to replace all occurrences of a matching pattern in a string with the provided substitution string.

php:

The number of occurrences replaced can be controlled with a 4th argument to preg_replace:

$s = "foo bar bar";
preg_replace('/bar/', "baz", $s, 1);

If no 4th argument is provided, all occurrences are replaced.

perl:

The =~ operator performs the substitution in place on the string and returns the number of substitutions performed.

The g modifiers specifies that all occurrences should be replaced. If omitted, only the first occurrence is replaced.

python:

The 3rd argument to sub controls the number of occurrences which are replaced.

s = 'foo bar bar'
re.compile('bar').sub('baz', s, 1)

If there is no 3rd argument, all occurrences are replaced.

ruby:

The gsub operator returns a copy of the string with the substitution made, if any. The gsub! performs the substitution on the original string and returns the modified string.

The sub and sub! operators only replace the first occurrence of the match pattern.

match, prematch, postmatch

How to get the substring that matched the regular expression, as well as the part of the string before and after the matching substring.

perl:

The special variables $&, $`, and $' also contain the match, prematch, and postmatch.

ruby:

The special variables $&, $`, and $' also contain the match, prematch, and postmatch.

group capture

How to get the substrings which matched the parenthesized parts of a regular expression.

ruby:

Ruby has syntax for extracting a group from a match in a single expression. The following evaluates to "1999":

"1999-07-08"[/(\d{4})-(\d{2})-(\d{2})/, 1]

scan

How to return all non-overlapping substrings which match a regular expression as an array.

backreference in match and substitution

How to use backreferences in a regex; how to use backreferences in the replacement string of substitution.

recursive regex

Examples of recursive regexes.

The examples match substrings containing balanced parens.

Date and Time Footnotes

In ISO 8601 terminology, a date specifies a day in the Gregorian calendar and a time does not contain date information; it merely specifies a time of day. A data type which combines both date and time information is probably more useful than one which contains just date information or just time information; it is unfortunate that ISO 8601 doesn't provide a name for this entity. The word timestamp often gets used to denote a combined date and time. PHP and Python use the compound noundatetime for combined date and time values.

An useful property of ISO 8601 dates, times, and date/time combinations is that they are correctly ordered by a lexical sort on their string representations. This is because they are big-endian (the year is the leftmost element) and they used fixed-length fields for each term in the string representation.

The C standard library provides two methods for representing dates. The first is the UNIX epoch, which is the seconds since January 1, 1970 in UTC. If such a time were stored in a 32-bit signed integer, the rollover would happen on January 18, 2038.

The other method of representing dates is the tm struct, a definition of which can be found on Unix systems in /usr/include/time.h:

struct tm {
        int     tm_sec;         /* seconds after the minute [0-60] */
        int     tm_min;         /* minutes after the hour [0-59] */
        int     tm_hour;        /* hours since midnight [0-23] */
        int     tm_mday;        /* day of the month [1-31] */
        int     tm_mon;         /* months since January [0-11] */
        int     tm_year;        /* years since 1900 */
        int     tm_wday;        /* days since Sunday [0-6] */
        int     tm_yday;        /* days since January 1 [0-365] */
        int     tm_isdst;       /* Daylight Savings Time flag */
        long    tm_gmtoff;      /* offset from CUT in seconds */
        char    *tm_zone;       /* timezone abbreviation */
};

Perl and Python both use and expose the tm struct of the standard library. In the case of Perl, the first nine values of the struct (up to the member tm_isdst) are put into an array. Python, meanwhile, has a module called time which is a thin wrapper to the standard library functions which operate on this struct. Here is how get a tmstruct in Python:

import time

utc = time.gmtime(time.time())
t = time.localtime(time.time())

The tm struct is a low level entity, and interacting with it directly should be avoided. In the case of Python it is usually sufficient to use the datetime module instead. For Perl, one can use the Time::Piece module to wrap the tm struct in an object.

date/time type

The data type used to hold a combined date and time.

perl

Built in Perl functions work with either (1) scalars containing the Unix epoch as an integer or (2) arrays containing the first nine values of the standard C library tm struct. When use Time::Piece is in effect functions which work with tm arrays are replaced with variant that work with the Time::Piece wrapper.

The modules Time::Local and Date::Parse can create scalars containing the Unix epoch.

CPAN provides the DateTime module which provides objects with functionality comparable to the DateTime objects of PHP and Python.

current date/time

How to get the combined date and time for the present moment in both local time and UTC.

to unix epoch, from unix epoch

How to convert the native date/time type to the Unix epoch which is the number of seconds since the start of January 1, 1970 UTC.

current unix epoch

How to get the current time as a Unix epoch timestamp.

strftime

How to format a date/time as a string using the format notation of the strftime function from the standard C library. This same format notation is used by the Unix datecommand.

php:

PHP supports strftime but it also has its own time formatting system used by date, DateTime::format, and DateTime::createFromFormat. The letters used in the PHP time formatting system are described here.

default format example

Examples of how a date/time object appears when treated as a string such as when it is printed to standard out.

The formats are in all likelihood locale dependent. The provided examples come from a machine running Mac OS X in the Pacific time zone of the USA.

php:

It is a fatal error to treat a DateTime object as a string.

strptime

How to parse a date/time using the format notation of the strptime function from the standard C library.

parse date w/o format

How to parse a date without providing a format string.

result date subtraction

The data type that results when subtraction is performed on two combined date and time values.

add time duration

How to add a time duration to a date/time.

A time duration can easily be added to a date/time value when the value is a Unix epoch value.

ISO 8601 distinguishes between a time interval, which is defined by two date/time endpoints, and a duration, which is the length of a time interval and can be defined by a unit of time such as '10 minutes'. A time interval can also be defined by date and time representing the start of the interval and a duration.

ISO 8601 defines notation for durations. This notation starts with a 'P' and uses a 'T' to separate the day and larger units from the hour and smaller units. Observing the location relative to the 'T' is important for interpreting the letter 'M', which is used for both months and minutes.

local timezone

Do date/time values include timezone information. When a date/time value for the local time is created, how the local timezone is determined.

A date/time value can represent a local time but not have any timezone information associated with it.

On Unix systems processes determine the local timezone by inspecting the file /etc/localtime.

php:

The default timezone can also be set in the php.ini file.

date.timezone = "America/Los_Angeles"

Here is the list of timezones supported by PHP.

timezone name, offset from UTC, is daylight savings?

How to get time zone information: the name of the timezone, the offset in hours from UTC, and whether the timezone is currently in daylight savings.

Timezones are often identified by three or four letter abbreviations. As can be seen from the list, many of the abbreviations do not uniquely identify a timezone. Furthermore many of the timezones have been altered in the past. The Olson database (aka Tz database) decomposes the world into zones in which the local clocks have all been set to the same time since 1970 and it gives these zones unique names.

perl:

It is not necessary to create a DateTime object to get the local timezone offset:

use Time::Piece;

$t = localtime(time);
$offset_hrs = $t->tzoffset / 3600;

ruby:

The Time class has a zone method which returns the time zone abbreviation for the object. There is a tzinfo gem which can be used to create timezone objects using the Olson database name. This can in turn be used to convert between UTC times and local times which are daylight saving aware.

microseconds

How to get the microseconds component of a combined date and time value. The SI abbreviations for milliseconds and microseconds are ms and μs, respectively. The C standard library uses the letter u as an abbreviation for micro. Here is a struct defined in /usr/include/sys/time.h:

struct timeval {
  time_t       tv_sec;   /* seconds since Jan. 1, 1970 */
  suseconds_t  tv_usec;  /* and microseconds */
};

sleep

How to put the process to sleep for a specified number of seconds. In Python and Ruby the default version of sleep supports a fractional number of seconds.

php:

PHP provides usleep which takes an argument in microseconds:

usleep(500000);

perl:

The Perl standard library includes a version of sleep which supports fractional seconds:

use Time::HiRes qw(sleep);

sleep 0.5;

timeout

How to cause a process to timeout if it takes too long.

Techniques relying on SIGALRM only work on Unix systems.

Array Footnotes

What the languages call their basic container types:

	php	perl	python	ruby
array	array	array, list	list, tuple, sequence	Array, Enumerable
dictionary	array	hash	dict, mapping	Hash

php:

PHP uses the same data structure for arrays and dictionaries.

perl:

array refers to a data type. list refers to a context.

python:

Python has the mutable list and the immutable tuple. Both are sequences. To be a sequence, a class must implement __getitem__, __setitem__, __delitem__, __len__, __contains__, __iter__, __add__, __mul__, __radd__, and __rmul__.

ruby:

Ruby provides an Array datatype. If a class defines an each iterator and a comparison operator <=>, then it can mix in the Enumerable module.

literal

Array literal syntax.

perl:

Square brackets create an array and return a reference to it:

$a = [1,2,3]

quote words

The quote words operator, which is a literal for arrays of strings where each string contains a single word.

size

How to get the number of elements in an array.

empty test

How to test whether an array is empty.

lookup

How to access a value in an array by index.

perl:

A negative index refers to the length - index element.

python:

A negative index refers to the length - index element.

>>> a = [1,2,3]
>>> a[-1]
3

ruby:

A negative index refers to to the length - index element.

update

How to update the value at an index.

out-of-bounds behavior

What happens when the value at an out-of-bounds index is refererenced.

index of array element

perl:

Some techniques for getting the index of an array element.

slice by endpoints, by length

How to slice a subarray from an array by specifying a start index and an end index; how to slice a subarray from an array by specifying an offset index and a length index.

perl:

Perl arrays can take an array of indices as the index value. The range of values selected can be discontinuous and the order of the values can be manipulated:

@nums = (1,2,3,4,5,6);                                                                                                                                                  
@nums[(1,3,2,4)];

python:

Slices can leave the first or last index unspecified, in which case the first or last index of the sequence is used:

>>> a=[1,2,3,4,5]
>>> a[:3]
[1, 2, 3]

Python has notation for taking every nth element:

>>> a=[1,2,3,4,5]
>>> a[::2] 
[1, 3, 5]

The third argument in the colon-delimited slice argument can be negative, which reverses the order of the result:

>>> a = [1,2,3,4]
>>> a[::-1]
[4, 3, 2, 1]

slice to end

How to slice to the end of an array.

The examples take all but the first element of the array.

manipulate back

How to add and remove elements from the back or high index end of an array.

These operations can be used to use the array as a stack.

manipulate front

How to add and remove elements from the front or low index end of an array.

These operations can be used to use the array as a stack. They can be used with the operations that manipulate the back of the array to use the array as a queue.

concatenate

How to create an array by concatenating two arrays; how to modify an array by concatenating another array to the end of it.

replicate

How to create an array containing the same value replicated n times.

address copy, shallow copy, deep copy

How to make an address copy, a shallow copy, and a deep copy of an array.

After an address copy is made, modifications to the copy also modify the original array.

After a shallow copy is made, the addition, removal, or replacement of elements in the copy does not modify of the original array. However, if elements in the copy are modified, those elements are also modified in the original array.

A deep copy is a recursive copy. The original array is copied and a deep copy is performed on all elements of the array. No change to the contents of the copy will modify the contents of the original array.

perl:

Taking a reference is customary way to make an address copy in Perl, but the Perl example is not equivalent to the other languages in that different syntax has to be used to access the original array and the address copy: @a and @$a1. To make @a1 and @a refer to the same array, use typeglobs:

*a1 = *a;

python:

The slice operator can be used to make a shallow copy:

a2 = a[:]

list(v) always returns a list, but v[:] returns a value of the same as v. The slice operator can be used in this manner on strings and tuples but there is little incentive to do so since both are immutable.

copy.copy can be used to make a shallow copy on types that don't support the slice operator such as a dictionary. Like the slice operator copy.copy returns a value with the same type as the argument.

arrays as function arguments

How arrays are passed as arguments.

iteration

How to iterate through the elements of an array.

perl:

for and foreach are synonyms. Some use for exclusively for C-style for loops and foreach for array iteration.

indexed iteration

How to iterate through the elements of an array while keeping track of the index of each element.

iterate over range

Iterate over a range without instantiating it as a list.

perl:

With Perl 5.005 the for and foreach operators were optimized to not instantiate a range argument as a list.

instantiate range as array

How to convert a range to an array.

Python 3 ranges and Ruby ranges implement some of the functionality of arrays without allocating space to hold all the elements.

python:

In Python 2 range() returns a list.

In Python 3 range() returns an object which implements the immutable sequence API.

ruby:

The Range class includes the Enumerable module.

reverse

How to create a reversed copy of an array, and how to reverse an array in place.

python:

reversed returns an iterator which can be used in a for/in construct:

print("counting down:")
for i in reversed([1,2,3]):
  print(i)

reversed can be used to create a reversed list:

a = list(reversed([1,2,3]))

sort

How to create a sorted copy of an array, and how to sort an array in place. Also, how to set the comparison function when sorting.

php:

usort sorts an array in place and accepts a comparison function as a 2nd argument:

function cmp($x, $y) {
  $lx = strtolower($x);
  $ly = strtolower($y);
  if ( $lx < $ly ) { return -1; }
  if ( $lx == $ly ) { return 0; }
  return 1;
}

$a = array("b", "A", "a", "B");

usort($a, "cmp");

dedupe

How to remove extra occurrences of elements from an array.

python:

Python sets support the len, in, and for operators. It may be more efficient to work with the result of the set constructor directly rather than convert it back to a list.

membership

How to test for membership in an array.

intersection

How to compute an intersection.

python:

Python has literal notation for sets:

{1,2,3}

Use set and list to convert lists to sets and vice versa:

a = list({1,2,3})
ensemble = set([1,2,3])

ruby:

The intersect operator & always produces an array with no duplicates.

union

ruby:

The union operator | always produces an array with no duplicates.

relative complement, symmetric difference

How to compute the relative complement of two arrays or sets; how to compute the symmetric difference.

ruby:

If an element is in the right argument, then it will not be in the return value even if it is contained in the left argument multiple times.

map

Create an array by applying a function to each element of a source array.

ruby:

The map! method applies the function to the elements of the array in place.

collect and collect! are synonyms for map and map!.

filter

Create an array containing the elements of a source array which match a predicate.

ruby:

The in place version is select!.

reject returns the complement of select. reject! is the in place version.

reduce

Return the result of applying a binary operator to all the elements of the array.

python:

reduce is not needed to sum a list of numbers:

sum([1,2,3])

ruby:

The code for the reduction step can be provided by name. The name can be a symbol or a string:

[1,2,3].inject(:+)

[1,2,3].inject("+")

[1,2,3].inject(0, :+)

[1,2,3].inject(0, "+")

universal and existential tests

How to test whether a condition holds for all members of an array; how to test whether a condition holds for at least one member of any array.

A universal test is always true for an empty array. An existential test is always false for an empty array.

A existential test can readily be implemented with a filter. A universal test can also be implemented with a filter, but it is more work: one must set the condition of the filter to the negation of the predicate and test whether the result is empty.

shuffle and sample

How to shuffle an array. How to extract a random sample from an array.

php:

The array_rand function returns a random sample of the indices of an array. The result can easily be converted to a random sample of array values:

$a = array(1, 2, 3, 4);
$sample = array();
foreach (array_rand($a, 2) as $i) { array_push($sample, $a[$i]); }

zip

How to interleave arrays. In the case of two arrays the result is an array of pairs or an associative list.

perl:

zip expects arrays as arguments, which makes it difficult to define the arrays to be zipped on the same line as the invocation. It can be done like this:

@a = zip @{[1,2,3]}, @{['a','b','c']};

Dictionary Footnotes

literal

perl:

Curly brackets create a hash and return a reference to it:

$h = { 'hello' => 5, 'goodbye' => 7 }

size

How to get the number of dictionary keys in a dictionary.

lookup

How to lookup a dictionary value using a dictionary key.

perl:

Use the ampersand prefix @ to slice a Perl hash. The index is a list of keys.

%nums = ('b'=>1, 't'=>2, 'a'=>3);
@nums{('b','t')}

out-of-bounds behavior

What happens when a lookup is performed on a key that is not in a dictionary.

python:

Use dict.get() to avoid handling KeyError exceptions:

d = {}
d.get('lorem')      # returns None
d.get('lorem', '')  # returns ''

is key present

How to check for the presence of a key in a dictionary without raising an exception. Distinguishes from the case where the key is present but mapped to null or a value which evaluates to false.

delete entry

How to remove a key/value pair from a dictionary.

from array of pairs, from even length array

How to create a dictionary from an array of pairs; how to create a dictionary from an even length array.

merge

How to merge the values of two dictionaries.

In the examples, if the dictionaries d1 and d2 share keys then the values from d2 will be used in the merged dictionary.

invert

How to turn a dictionary into its inverse. If a key 'foo' is mapped to value 'bar' by a dictionary, then its inverse will map the key 'bar' to the value 'foo'. However, if multiple keys are mapped to the same value in the original dictionary, then some of the keys will be discarded in the inverse.

iteration

How to iterate through the key/value pairs in a dictionary.

python:

In Python 2.7 dict.items() returns a list of pairs and dict.iteritems() returns an iterator on the list of pairs.

In Python 3 dict.items() returns an iterator and dict.iteritems() has been removed.

keys and values as arrays

How to convert the keys of a dictionary to an array; how to convert the values of a dictionary to an array.

python:

In Python 3 dict.keys() and dict.values() return read-only views into the dict. The following code illustrates the change in behavior:

d = {}
keys = d.keys()
d['foo'] = 'bar'

if 'foo' in keys:
  print('running Python 3')
else:
  print('running Python 2')

default value, computed value

How to create a dictionary with a default value for missing keys; how to compute and store the value on lookup.

php:

Extend ArrayObject to compute values on lookup:

class Factorial extends ArrayObject {

  public function offsetExists($i) {
    return true;
  }

  public function offsetGet($i) {
    if(!parent::offsetExists($i)) {
      if ( $i < 2 ) {
        parent::offsetSet($i, 1);
      }
      else {
        $n = $this->offsetGet($i-1);
        parent::offsetSet($i, $i*$n);
      }
    }
    return parent::offsetGet($i);
  }
}

$factorial = new Factorial();

perl:

How to use a tied hash. If the CPAN module Tie::ExtraHash is installed there is a shorter way.

Function Footnotes

Python has both functions and methods. Ruby only has methods: functions defined at the t