python的map和zip操作
python的map和zip操作
如果要将一个string list转换成int list (list里每个string都转成int)，比如[python]
['0','1','2'] -> [0,1,2]
可以使用:
[python]
[int(x) for x in list]
或者使用map操作: map(func, list) 对list里的每个元素apply func.
[python]
map(int, list)
假设有一个2维数组（用list实现）:
[python]
list = [[0,1,2],[3,1,4]]
如果要得到每行之和，可以用以下两种方式：
[python]
>>> list = [[0,1,2],[3,1,4]]
>>> [sum(x) for x in list]
[3, 8]
>>> map(sum,list)
[3, 8]
如果要得到每列之和，需要用zip(*list)先unzip list,得到一个元组list，其中第i个元组包含了每行的第i个元素:
[python]
>>> list = [[0,1,2],[3,1,4]]
>>> zip(*list)
[(0, 3), (1, 1), (2, 4)]
>>> [sum(x) for x in zip(*list)]
[3, 2, 6]
>>> map(sum,zip(*list))
[3, 2, 6]
下面的例子是关于zip和unzip(其实是zip和*一起用)如何work的：
[python]
>>> x=[1,2,3]
>>> y=[4,5,6]
>>> zipped = zip(x,y)
>>> zipped
[(1, 4), (2, 5), (3, 6)]
>>> x2,y2=zip(*zipped)
>>> x2
(1, 2, 3)
>>> y2
(4, 5, 6)
>>> x3,y3=map(list,zip(*zipped))
>>> x3
[1, 2, 3]
>>> y3
[4, 5, 6]
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