Question

I have a list and I want to use a certain function only on those entries of it that fulfills a certain condition – leaving the other entries unmodified.

Example: Say I want to multiply by 2 only those elements who are even.

a_list = [1, 2, 3, 4, 5]

Wanted result:

a_list => [1, 4, 3, 8, 5]

But [elem * 2 for elem in a_list if elem %2 == 0] yields [4, 8] (it acted as a filter in addition).

What is the correct way to go about it?

Answer 1

Use a conditional expression:

[x * 2 if x % 2 == 0 else x
 for x in a_list]

(Math geek’s note: you can also solve this particular case with

[x * (2 - x % 2) for x in a_list]

but I’d prefer the first option anyway 😉