内置方法mapreducefilter

Posted 2020-09-17

map：

     map(func, *iterables) --> map object

    Make an iterator that computes the function using arguments from each of the iterables.Stops when the shortest iterable is exhausted.

l1 = [1,2,3,4,5,6,7,8,9]
def func(a):
    return a*10

l2 = map(func,l1)
print(l2)   # <map object at 0x0000000000A7CB00>  也是一个迭代器
for i in l2:
    print(i)

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.

from functools import reduce   #内置函数reduce需要导入后才能使用。

l1 = [1,2,3,4,5,6,7,8,9]

def func(a,b):
    return a+b

l2 = reduce(func,l1)
print(l2)

filter：

    filter(function or None, iterable) --> filter object

‘‘‘
Return an iterator yielding those items of iterable for which function(item) is true.
If function is None, return the items that are true.‘‘‘

# 一行代码实现对列表a中的偶数位置的元素进行加3后求和
a = [1,2,3,4,5]
l = sum([j+3 for j in  list(filter(lambda i:a.index(i)%2 == 0, a))])    #用到了过滤器和匿名函数
print(l)