标签：python
Python有很多有用有趣的内置函数，比如reduce，map，filter，lambda，zip等。已经写过了lambda和zip相关的博客。继续写关于reduce，map，filter。
Map
首先用help方法看一下map的具体用法。
help(map)
# result
Help on built-in function map in module __builtin__:
map(...)
map(function, sequence[, sequence, ...]) -> list
Return a list of the results of applying the function to the items of
the argument sequence(s). If more than one sequence is given, the
function is called with an argument list consisting of the corresponding
item of each sequence, substituting None for missing values when not all
sequences have the same length. If the function is None, return a list of
the items of the sequence (or a list of tuples if more than one sequence).
l1 = [1,2,3,4,5]
def func(x):
return x*x
print map(func, l1)
# result
[1, 4, 9, 16, 25]l1 = [1,2,3,4,5]
l2 = [6,7,8,9,10]
def func(x,y):
return x+y
print map(func, l1, l2)
# result
[7, 9, 11, 13, 15]对于不等长的sequences
l1 = [1,2,3,4,5,6]
l2 = [6,7,8,9,10]
def func(x,y):
return y
print map(func, l1, l2)
# result
[6, 7, 8, 9, 10, None]map可以和lambda一起用
l1 = [1,2,3,4,5] print map((lambda x: x **2), l1) # result [1, 4, 9, 16, 25]
def square(x):
return (x**2)
def cube(x):
return (x**3)
funcs = [square, cube]
for r in range(5):
value = map(lambda x: x(r), funcs)
print value
# result
[0, 0]
[1, 1]
[4, 8]
[9, 27]
[16, 64]
如果function是None的话
l1 = [1,2,3,4,5] l2 = [6,7,8,9,10] print map(None, l1, l2) # result [(1, 6), (2, 7), (3, 8), (4, 9), (5, 10)]
from math import sqrt def mymap(aFunc, aSeq): result = [] for x in aSeq: result.append(aFunc(x)) return result print list(map(sqrt, [1, 2, 3])) print mymap(sqrt, [1, 2, 3]) # result [1.0, 1.4142135623730951, 1.7320508075688772] [1.0, 1.4142135623730951, 1.7320508075688772]
Filter
用help看一下filter的用法
help(filter)
# result
Help on built-in function filter in module __builtin__:
filter(...)
filter(function or None, sequence) -> list, tuple, or string
Return those items of sequence for which function(item) is true. If
function is None, return the items that are true. If sequence is a tuple
or string, return the same type, else return a list.
print list(range(-5,5)) print list(filter((lambda x: x<0), range(-5,5))) # result [-5, -4, -3, -2, -1, 0, 1, 2, 3, 4] [-5, -4, -3, -2, -1]
print list(filter(None, range(-5,5))) # result [-5, -4, -3, -2, -1, 1, 2, 3, 4]
其他类型
def f(x):
return x != 'a'
print filter(f, 'abcdef')
# result
bcdef
Reduce
用help看一下reduce的用法
help(reduce)
# result
Help on built-in function reduce in module __builtin__:
reduce(...)
reduce(function, sequence[, initial]) -> value
Apply a function of two arguments cumulatively to the items of a sequence,
from left to right, so as to reduce the sequence to a single value.
For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates
((((1+2)+3)+4)+5). If initial is present, it is placed before the items
of the sequence in the calculation, and serves as a default when the
sequence is empty.
def add(x,y): return x+y print reduce(add, range(1,11)) print reduce(add, range(1,11), 20) print reduce(add, [],20) # result 55 (1+2+3+4+5+6+7+8+9+10) 75 (20+1+2+3+4+5+6+7+8+9+10) 20 (default)<span style="font-family: Arial, Helvetica, sans-serif;"></span>
print reduce((lambda x, y: x * y), [1, 2, 3, 4]) # result 24 (1*2*3*4)
Python interview - reduce & map & filter
标签：python
原文地址：http://blog.csdn.net/bearkino/article/details/41141495
