PythonNote027---嵌套list偏平化操作

Intro

  嵌套list扁平化，把每個子元素取出來，再拉平，放到一個list中。R中有unlist方法，Scala中有flatMap方法，python中也可類似實作。直接看case。

方法一

x_list = [[0,1],[2,3],[4,5,6]]      

flat_list = [item for sublist in x_list for item in sublist]      

[0, 1, 2, 3, 4, 5, 6]      

這個用法也會加上if判斷，舉個例子，把偶數元素提取出來

y_list = [1,2,3,4,5]
even_list = [i for i in y_list if i%2==0]      

[2, 4]      

方法二

import functools
import operator
functools.reduce(operator.iconcat, x_list, [])      

[0, 1, 2, 3, 4, 5, 6]      

方法三

functools.reduce(lambda x,y: x+y,x_list)      

[0, 1, 2, 3, 4, 5, 6]      

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.      

functools.reduce(lambda x,y: x+y,list(range(5)))      

10      

方法二、三都是利用了reduce函數，傳入兩個參數(function, sequence)。

• 第一次計算時，向function中傳入sequence的第一個和第二個元素，得出計算結果res1
• 後面每次計算，傳入上一步計算結果和sequence中最近一個未參與計算的元素

方法四

from pandas.core.common import      

list(flatten(x_list))      

[0, 1, 2, 3, 4, 5, 6]      

Ref