Python的reduce

今天在搜用python求階乘的時候, 搜出來的最簡單的是用reduce這個built-in function, 但是我在用reduce的時候, 卻報NameError: name 'reduce' is not defined. 於是又搜了一下,發現在python 3.0.0.0以後, reduce已經不在built-in function裡了, 要用它就得from functools import reduce.

詳見The fate of reduce() in Python 3000

 

reduce的用法

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.

 

意思就是對sequence連續使用function, 如果不給出initial, 則第一次調用傳遞sequence的兩個元素, 以後把前一次調用的結果和sequence的下一個元素傳遞給function. 如果給出initial, 則第一次傳遞initial和sequence的第一個元素給function.

 

>>> from functools import reduce<br />>>> reduce(lambda x,y: x+y, [1, 2, 3])<br />6<br />>>> reduce(lambda x, y: x+y, [1,2,3], 9)<br />15<br />>>> reduce(lambda x,y: x+y, [1, 2, 3], 7)<br />13<br />>>>