if else in a list comprehension


Question

I have a list 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.

I tried

[x+1 for x in l if x >= 45 else x+5]

But it gives me a syntax error. How can I achieve an ifelse like this in a list comprehension?

1
372
12/26/2011 3:32:02 PM

Accepted Answer

>>> 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.

420
4/17/2015 8:33:41 PM

The reason you're getting this error has to do with how the list comprehension is performed.

Keep in mind the following:

[ expression for item in list if conditional ]

Is equivalent to:

for item in list:
    if conditional:
        expression

Where the expression is in a slightly different format (think switching the subject and verb order in a sentence).

Therefore, your code [x+1 for x in l if x >= 45] does this:

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):

for x in l:
    if x>=45: x+1
    else: x+5

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