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Changing Lists: range

Another common place where you may use the range and for combination is in loops that change a list as it is being traversed. Suppose, for example, that you need to add 1 to every item in a list. You can try this with a simple for loop, but the result probably won’t be exactly what you want:

>>> L = [1, 2, 3, 4, 5]

>>> for x in L:
...     x += 1
...
>>> L
[1, 2, 3, 4, 5]
>>> x
6

This doesn’t quite work—it changes the loop variable x, not the list L. The reason is somewhat subtle. Each time through the loop, x refers to the next integer already pulled out of the list. In the first iteration, for example, x is integer 1. In the next iteration, the loop body sets x to a different object, integer 2, but it does not update the list where 1 originally came from.

To really change the list as we march across it, we need to use indexes so we can assign an updated value to each position as we go. The range/len combination can produce the required indexes for us:

>>> L = [1, 2, 3, 4, 5]

>>> for i in range(len(L)):          # Add one to each item in L
...     L[i] += 1                    # Or L[i] = L[i] + 1
...
>>> L
[2, 3, 4, 5, 6]

When coded this way, the list is changed as we proceed through the loop. There is no way to do the same with a simple for x in L:-style loop, because such a loop iterates through actual items, not list positions. But what about the equivalent while loop? Such a loop requires a bit more work on our part, and likely runs more slowly:

>>> i = 0
>>> while i < len(L):
...     L[i] += 1
...     i += 1
...
>>> L
[3, 4, 5, 6, 7]

Here again, though, the range solution may not be ideal either. A list comprehension expression of the form:

[x+1 for x in L]

would do similar work, albeit without changing the original list in-place (we could assign the expression’s new list object result back to L, but this would not update any other references to the original list). Because this is such a central looping concept, we’ll save a complete exploration of list comprehensions for the next chapter.