Python 函数化编程

python提供了itertools用于高效循环的迭代函数集合
本文将依据官方文档，原封不动抄过来，先记录下，有时间再润色。

1 无限迭代器

迭代器	参数	描述	结果	例子
count	start, [stop]	等差数列	start, start+step, start+2*step	count(10) --> 10 11 12 13 14 15 ......
cycle()	p	反复+重复	p0, p1, ...,plast, p0, p1,...	cycle('ABCD') --> A B C D A B C D ...
repeat()	elem [,n]	重复	elem, elem, elem,...到无穷 o或n倍	repeat(10, 3) -> 10, 10, 10

2 处理输入序列迭代器

迭代器	参数	描述	结果	例子
chain()	p, q, ...	连接	p0, p1, ... plast, q0, q1, ...	chain('ABC', 'DEF') --> A B C D E F
compress()	data, selectors	掩码	(d[0] if s[0]), (d[1] if s[1]), ...	compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F
dropwhile()	pred, seq	条件舍弃	seq[n], seq[n+1], starting when pred fails	dropwhile(lambda x: x<5, [1,4,6,4,1]) --> 6 4 1
groupby()	iterable [, keyfunc]	条件分组	sub-iterators grouped by value of keyfunc(v)
ifilter()	pred, seq	条件过滤	elements of seq where pred(elem) is True	ifilter(lambda x: x%2, range(10)) --> 1 3 5 7 9
ifilterfalse()	pred, seq	反向条件过滤	elements of seq where pred(elem) is False	ifilterfalse(lambda x: x%2, range(10)) --> 0 2 4 6 8
islice()	seq, [start,] stop	迭代切片	[, step] elements from seq[start:stop:step]	islice('ABCDEFG', 2, None) --> C D E F G
imap()	func, p, q, ...	迭代映射	func(p0, q0), func(p1, q1), ...	imap(pow, (2,3,10), (5,2,3)) --> 32 9 1000
starmap()	func, seq	带参的迭代映射	func(seq[0]), func(seq[1]), ...	starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000
tee()	it, n	分割迭代器	it1, it2 , ... itn splits one iterator into n
takewhile()	pred, seq	条件取值	seq[0], seq[1], until pred fails	takewhile(lambda x: x<5, [1,4,6,4,1]) --> 1 4
izip()	p, q, ...	二元配对	(p[0], q[0]), (p[1], q[1]), ...	izip('ABCD', 'xy') --> Ax By
izip_longest()	p, q, ...	最大长度的二元配对	(p[0], q[0]), (p[1], q[1]), ...	izip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-

3 组合生成器

迭代器	参数	描述	结果	例子
product()	p, q, ... [repeat=1]	笛卡尔乘积	cartesian product, equivalent to a nested for-loop	product('ABCD', repeat=2) AA AB AC AD BA BB BC BD CA CB CC CD DA DB DC DD
permutations()	p[, r]	排列	r-length tuples, all possible orderings, no repeated elements	permutations('ABCD', 2) AB AC AD BA BC BD CA CB CD DA DB DC
combinations()	p, r	组合	r-length tuples, in sorted order, no repeated elements	combinations('ABCD', 2) AB AC AD BC BD CD
combinations_with_replacement()	p, r	带重复组合	r-length tuples, in sorted order, with repeated elements	combinations_with_replacement('ABCD', 2) AA AB AC AD BB BC BD CC CD DD