Python的列表推导式是什么

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语法

1.普通

[expression for target in iterable]

2.带条件

[expression for target in iterable if condition]

3.嵌套

[expression for target1 in iterable1 if condition1                        for target2 in iterable2 if condition2                                                ...                        for targetN in iterableN if conditionN]

实例

x = [1, 2, 3, 4, 5]x = [i * 2 for i in x]print(x)# [2, 4, 6, 8, 10]

y = [i for i in range(10)]print(y)# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

# 获取矩阵第二列matrix = [    [1, 2, 3],    [3, 5, 6],    [7, 8, 9]]col2 = [row[1] for row in matrix]print(col2)# [2, 5, 8]

# 获取矩阵主对角线元素matrix = [    [1, 2, 3],    [3, 5, 6],    [7, 8, 9]]diag = [matrix[i][i] for i in range(len(matrix))]print(diag)diag2 = [matrix[i][len(matrix) - i - 1] for i in range(len(matrix))]print(diag2)# [1, 5, 9]# [3, 5, 7]

# 创建内嵌列表a = [[0] * 3 for i in range(3)]print(a)a[1][1] = 5print(a)# [[0, 0, 0], [0, 0, 0], [0, 0, 0]]# [[0, 0, 0], [0, 5, 0], [0, 0, 0]]

# 20以内偶数列表b = [i for i in range(20) if i % 2 == 0]print(b)# [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

# 筛选F开头的单词words = ['Great', 'FishC', 'Brilliant', 'Excellent', 'Fantistic']res = [word for word in words if word.startswith('F')]print(res)# ['FishC', 'Fantistic']

# 展开二维列表matrix = [    [1, 2, 3],    [4, 5, 6],    [7, 8, 9]]flatten = [col for row in matrix for col in row ]print(flatten)# [1, 2, 3, 4, 5, 6, 7, 8, 9]

等价于

flatten = []for row in matrix:        for col in row:                flatten.append(col)

# 笛卡尔积fulljoin = [x + y for x in '12345' for y in 'abcde']print(fulljoin)# ['1a', '1b', '1c', '1d', '1e', '2a', '2b', '2c', '2d', '2e', '3a', '3b', '3c', '3d', '3e', '4a', '4b', '4c', '4d', '4e', '5a', '5b', '5c', '5d', '5e']

ans = [[x, y] for x in range(10) if x % 2 == 0 for y in range(10) if y % 3 == 0]print(ans)# [[0, 0], [0, 3], [0, 6], [0, 9], [2, 0], [2, 3], [2, 6], [2, 9], [4, 0], [4, 3], [4, 6], [4, 9], [6, 0], [6, 3], [6, 6], [6, 9], [8, 0], [8, 3], [8, 6], [8, 9]]

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