列表理解与条件

Question

I have a simple list. 我有一个简单的清单。

>>> a = [0, 1, 2]

I want to make a new list from it using a list comprehension. 我想使用列表理解从它创建一个新列表。

>>> b = [x*2 for x in a]
>>> b
[0, 2, 4]

Pretty simple, but what if I want to operate only over nonzero elements? 很简单，但是如果我只想操作非零元素呢？

'if' needs 'else' in list comprehensions, so I came up with this. 'if'在列表推导中需要'else'，所以我想出了这个。

>>> b = [x*2 if x != 0 else None for x in a]
>>> b
[None, 2, 4]

But the desirable result is. 但理想的结果是。

>>> b
[2, 4]

I can do that this way 我可以这样做

>>> a = [0, 1, 2]
>>> def f(arg):
...     for x in arg:
...         if x != 0:
...             yield x*2
... 
>>> list(f(a))
[2, 4]

or using 'filter' and a lambda 或使用'过滤器'和lambda

>>> a = [0, 1, 2]
>>> list(filter(lambda x: x != 0, a))
[1, 2]

How do I get this result using a list comprehension? 如何使用列表推导获得此结果？

Answer 1

b = [x*2 for x in a if x != 0]

如果你把条件放在最后你不需要别的（事实上不能有其他的）

Answer 2

Following the pattern: 遵循模式：

[ <item_expression>
  for <item_variables> in <iterator>
  if <filtering_condition>
]

we can solve it like: 我们可以解决它：

>>> lst = [0, 1, 2]
>>> [num
... for num in lst
... if num != 0]
[1, 2]

It is all about forming an if condition testing "nonzero" value. 这是关于形成if条件测试“非零”值。