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 , 2]

will be converted to

a = [1, 2, 2]

The following code works, but i am wondering if there is a better way to do it?

Thanks.

for i in range(len(a)):
    if a[i]<0:
        a[i] += 4

Answer 1

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

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

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 4

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)