Python if else 在列表理解中
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if else in a list comprehension
提问by user225312
I have a list l:
我有一个清单l：
l = [22, 13, 45, 50, 98, 69, 43, 44, 1]
For numbers above 45 inclusive, I would like to add 1; and for numbers less than it, 5.
对于大于等于 45 的数字，我想加 1；对于小于它的数字，5。
I tried
我试过
[x+1 for x in l if x >= 45 else x+5]
But it gives me a syntax error. How can I achieve an if– elselike this in a list comprehension?
但它给了我一个语法错误。我怎样才能在列表理解中实现if-else这样的？
采纳答案by user225312
>>> l = [22, 13, 45, 50, 98, 69, 43, 44, 1]
>>> [x+1 if x >= 45 else x+5 for x in l]
[27, 18, 46, 51, 99, 70, 48, 49, 6]
Do-something if <condition>, else do-something else.
如果做某事<condition>，否则做其他事。
回答by AndiDog
You must put the expression at the beginning of the list comprehension, an if statement at the end filters elements!
您必须将表达式放在列表推导式的开头，最后的 if 语句过滤元素！
[x+1 if x >= 45 else x+5 for x in l]
回答by Dan D.
[x+1 if x >= 45 else x+5 for x in l]
And for a reward, here is the comment, I wrote to remember this the first time I did this error:
为了奖励，这是我第一次犯这个错误时写的评论：
Python's conditional expression is
a if C else band can't be used as:[a for i in items if C else b]The right form is:
[a if C else b for i in items]Even though there is a valid form:
[a for i in items if C]But that isn't the same as that is how you filter by
C, but they can be combined:[a if tC else b for i in items if fC]
Python 的条件表达式是
a if C else b并且不能用作：[a for i in items if C else b]正确的形式是：
[a if C else b for i in items]即使有一个有效的形式：
[a for i in items if C]但这与您过滤的方式不同
C，但它们可以组合：[a if tC else b for i in items if fC]
回答by Jeet
You could move the conditional to:
您可以将条件移动到：
v = [22, 13, 45, 50, 98, 69, 43, 44, 1]
[ (x+1 if x >=45 else x+5) for x in v ]
But it's starting to look a little ugly, so you might be better off using a normal loop. Note that I used vinstead of lfor the list variable to reduce confusion with the number 1 (I think land Oshould be avoided as variable names under any circumstances, even in quick-and-dirty example code).
但它开始看起来有点难看，所以你最好使用普通循环。请注意，我使用v而不是l作为列表变量来减少与数字 1 的混淆（我认为l并且O在任何情况下都应该避免将其作为变量名，即使在快速而肮脏的示例代码中）。
回答by arboc7
The reason you're getting this error has to do with how the list comprehension is performed.
您收到此错误的原因与列表理解的执行方式有关。
Keep in mind the following:
请记住以下几点：
[ expression for item in list if conditional ]
Is equivalent to:
相当于：
for item in list:
if conditional:
expression
Where the expressionis in a slightly different format (think switching the subject and verb order in a sentence).
其中 the 的expression格式略有不同（想想在句子中切换主语和动词顺序）。
Therefore, your code [x+1 for x in l if x >= 45]does this:
因此，您的代码[x+1 for x in l if x >= 45]执行以下操作：
for x in l:
if x >= 45:
x+1
However, this code [x+1 if x >= 45 else x+5 for x in l]does this (after rearranging the expression):
但是，此代码[x+1 if x >= 45 else x+5 for x in l]执行此操作（重新排列 后expression）：
for x in l:
if x>=45: x+1
else: x+5
回答by Stefan Gruenwald
You can also put the conditional expression in brackets inside the list comprehension:
您还可以将条件表达式放在列表推导式中的括号中：
l = [22, 13, 45, 50, 98, 69, 43, 44, 1]
print [[x+5,x+1][x >= 45] for x in l]
[false,true][condition] is the syntax
[false,true][condition] 是语法
回答by szeitlin
I just had a similar problem, and found this question and the answers really useful. Here's the part I was confused about. I'm writing it explicitly because no one actually stated it simply in English:
我刚刚遇到了类似的问题，发现这个问题和答案非常有用。这是我感到困惑的部分。我写得很明确，因为实际上没有人用英语简单地表达过：
The iteration goes at the end.
迭代进行到最后。
Normally, a loop goes
通常，一个循环
for this many times:
if conditional:
do this thing
else:
do something else
Everyone states the list comprehension part simply as the first answer did,
每个人都像第一个答案一样简单地陈述了列表理解部分，
[ expression for item in list if conditional ]
but that's actually not what you do in this case. (I was trying to do it that way)
但这实际上不是你在这种情况下所做的。（我试图这样做）
In this case, it's more like this:
在这种情况下，它更像是这样：
[ expression if conditional else other thing for this many times ]
回答by Xiaojun Chen
Like in [a if condition1 else b for i in list1 if condition2], the two ifs with condition1and condition2doing two different things. The part (a if condition1 else b)is from a lambda expression:
就像在[a if condition1 else b for i in list1 if condition2]，两个ifscondition1和condition2做两件不同的事情。该部分(a if condition1 else b)来自 lambda 表达式：
lambda x: a if condition1 else b
while the other condition2is another lambda:
而另一个condition2是另一个 lambda：
lambda x: condition2
Whole list comprehension can be regard as combination of mapand filter:
整个列表可以看作组合map和filter：
map(lambda x: a if condition1 else b, filter(lambda x: condition2, list1))
