modify list element with list comprehension in python

Question

folks,

I want to modify list element with list comprehension. For example, if the element is negative, add 4 to it.

Thus the list

a = [1, -2 ,          


        

Answer 1

This version is older, it would work on Python 2.4

>>> [x < 0 and x + 4 or x for x in [1, -2, 2]]
0: [1, 2, 2]

For newer versions of Python use conditional expressions as in Adam Wagner or BenH answers

Answer 2

If you want to change the list in-place, this is almost the best way. List comprehension will create a new list. You could also use enumerate, and assignment must be done to a[i]:

for i, x in enumerate(a):
  if x < 0:
    a[i] = x + 4

Answer 3

Try this:

 b = [x + 4 if x < 0 else x for x in a]

Or if you like map more than a list comprehension:

 b = map(lambda x: x + 4 if x < 0 else x, a)

Answer 4

Why mutate, when you can just return a new list that looks like you want it to?

[4 + x if x < 0 else x for x in [1, -2, 2]]

Answer 5

a = [b + 4 if b < 0 else b for b in a]