>>> 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]
在这个列表推导式中有2个循环,其中第一个循环可以看作是外循环,执行的慢;而第二个循环可以看作是内循环,执行的快。上面代码的执行过程等价于下面的写法:
>>> 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]