巢狀列表理解
與巢狀迴圈的列表推導不同,巢狀列表推導是列表推導中的列表推導。初始表示式可以是任意表示式,包括另一個列表理解。
#List Comprehension with nested loop
[x + y for x in [1, 2, 3] for y in [3, 4, 5]]
#Out: [4, 5, 6, 5, 6, 7, 6, 7, 8]
#Nested List Comprehension
[[x + y for x in [1, 2, 3]] for y in [3, 4, 5]]
#Out: [[4, 5, 6], [5, 6, 7], [6, 7, 8]]
巢狀示例等同於
l = []
for y in [3, 4, 5]:
temp = []
for x in [1, 2, 3]:
temp.append(x + y)
l.append(temp)
可以使用巢狀理解來轉置矩陣的一個示例。
matrix = [[1,2,3],
[4,5,6],
[7,8,9]]
[[row[i] for row in matrix] for i in range(len(matrix))]
# [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
就像巢狀的 for
迴圈一樣,巢狀的深度理解也沒有限制。
[[[i + j + k for k in 'cd'] for j in 'ab'] for i in '12']
# Out: [[['1ac', '1ad'], ['1bc', '1bd']], [['2ac', '2ad'], ['2bc', '2bd']]]