python中for...if...构建List

1.简单的for...[if]...语句

>>> a=[12, 3, 4, 6, 7, 13, 21]
>>> newList = [x for x in a]
>>> newList
[12, 3, 4, 6, 7, 13, 21]
>>> newList2 = [x for x in a if x%2==0]
>>> newList2
[12, 4, 6]

newList构建了一个与a具有相同元素的List。但是，newList和a是不同的List。执行b=a，b和newList是不同的。

newList2是从a中选取满足x%2==0的元素组成的List。

2.嵌套的for...[if]...语句

嵌套的for...[if]...语句可以从多个List中选择满足if条件的元素组成新的List。下面也举几个例子。

>>>a=[12, 3, 4, 6, 7, 13, 21]
>>>b=[‘a‘, ‘b‘, ‘x‘]
>>>newList=[(x, y) for x in a for y in b]
>>>newList
[(12, ‘a‘), (12, ‘b‘), (12, ‘x‘), (3, ‘a‘), (3, ‘b‘), (3, ‘x‘), (4, ‘a‘), (4, ‘b‘), (4, ‘x‘), (6, ‘a‘), (6, ‘b‘), (6, ‘x‘), (7, ‘a‘), (7, ‘b‘), (7, ‘x‘), (13, ‘a‘), (13, ‘b‘), (13, ‘x‘), (21, ‘a‘), (21, ‘b‘), (21, ‘x‘)]
>>>newList2=[(x, y) for x in a for y in b if x%2==0 and y<‘x‘]
>>>newList2
[(12, ‘a‘), (12, ‘b‘), (4, ‘a‘), (4, ‘b‘), (6, ‘a‘), (6, ‘b‘)]