Perl Learning - 18 (unless, until, elsif, for)

unless expression: when condition is false execute the block.

 

unless($fred=~/^[A-Z_]\w*$/i){
 print "The value of \$fred doesn't look like a perl identifier name.\n";
}

 

It's the same as written like this:

 

if($fred=~/^[A-Z_]\w*$/i){
 # do nothing
}else{
 print "The value of \$fred doesn't look like a perl identifier name.\n";
}

 

Or:

 

if(!($fred=~/^[A-Z_]\w*$/i)){
 print "The value of \$fred doesn't look like a perl identifier name.\n";
}

 

until expression: when condition is false execute the loop.

 

until($j>$i){
 $j *= 2;
}

 

if expression can be written without a block of {}

 

print "$n is a negative number.\n" if $n < 0;

 

Perl execute 'if $n < 0' first, if it's true then execute the print expression, if false do nothing.

 

Those styles are all popular in Perl:

 

&error("Invalid input") unless &valid($input);
$i *=2 until $i > $j;
print " ", ($n += 2) while $n < 10;
&greet($_) foreach @person;

 

If there are more than one expression, better to write them in {} blocks.

 

elsif expression:

 

if(condition){
 do something;
}elsif(condition){
 do something;
}elsif(condition){
 do something;
}else{
 do something;
}

 

my $bedrock=42;
$bedrock++;

 

my @people=qw{ Fred barney Fred wilma dino barney Fred pebbles };
my %count;
$count{$_}++ foreach @people;

 

++ means +1 byself, undef++ gets 1

 

my $n=++$m;
$m add 1, then give value to $n.

 

my $n=$m++;
give value to $n, then $m add 1.

 

for (init;test;increment){
 body;
 body;
}

 

for(i=1;$i<=10;$i++){
 print "I can count to $i!\n";
}

 

Any of init, test, increment can be empty, but the symbol ';' should be there.

 

for($_="bedrock";s/(.)//;){ # whenever s/// successes, loop continue
 print "One character is: $1\n";
}

 

for(1..10){
 print "I can count to $_!\n":
}

 

Here 'for' is the same as foreach. for and foreach are the same in Perl, Perl will judge by the contents in (). If there're ';' in () then it's for, otherwise it's foreach.