列表推导式

列表推导式语法：

[表达式 for 变量 in 序列或迭代对象]
列表推导式在逻辑上相当于一个循环

alist = [x * x for x in range(10)]
相当于：
alist = []
for x in range(10):
    alist.append(x*x)
也等价于：
alist=list(map(lambda x: x*x,range(10)))

使用列表推导式实现嵌套列表的平铺

vec=[[1,2,3],[4,5,6],[7,8,9]]
[num for elem in vec for num in elem]
[1,2,3,4,5,6,7,8,9]
等价于：
vec=[[1,2,3],[4,5,6],[7,8,9]]
result=[]
for elem in vec:
    for num in elem:
        result.append(num)
result
[1,2,3,4,5,6,7,8,9]

过滤不符合条件的元素

列出当前文件夹下所有的 .py 文件
import os
[filename for filename in os.listdir(‘.’) if filename.endswith(‘.py’)]
从列表中选择符合条件的元素组成新的列表：
alist = [ -1, -4,6, 7.5, -2.3, 9, -11 ]
[i for i in alist if i>0]
[ 6, 7.5, 9]
scores = {'zhang shan':45 , 'li si':78 , 'wang wu':40}
highest = max(scores.values()) #求最高分
lowest = min(scores.values()) #求最低分
average = sum(scores.values())/len(scores) #求平均分
highestPerson = [name for name, score in scores.items() if score == highest]
highestPerson
[‘li si’]

实现多序列元素的任意组合

[(x,y) for x in (1,2,3) for y in (3,1,4) if x!=y]
[(1,3), (1,4), (2,3), (2,1), (2,4), (3,1), (3,4)]

实现矩阵转置

matrix = [[1,2,3,4], [5,6,7,8], [9,10,11,12]]
[[row[i] for row in matrix] for i in range(4)]
[[1,5,9], [2,6,10], [3,7,11], [4,8,12]]
相当于：
list(map(list,zip(*matrix)))
[[1,5,9], [2,6,10], [3,7,11], [4,8,12]]

列表推导式中可以使用函数或复杂表达式

def f(v):
    if v%2 == 0:
        v = v**2
    else:
        v = v+1
    return v
print([f(v) for v in [2,3,4,-1] if v>0])
[4,4,16]
print([v**2 if v%2==0 else v+1 for v in [2,3,4,-1] if v>0])
[4,4,16]

列表推导式支持文件对象的迭代

fp = open(‘c:\install.log’,’r’)
print([line for line in fp])
fp.close()

列表推导式生成 100 以内的素数

import math
list1=[ p for p in range(2,100) if 0 not in [p%d for d in range(2,p)]]
print(list1)
for p in range(2,100):
    for d in range(2, p):
        if 0==p%d:
            break
    else:
        print(p)