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lists and for loops

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  lists and for loops Emily Anne Moravec <moravec@stolaf.edu> - 2011-08-17 20:08 -0700
    Re: lists and for loops alex23 <wuwei23@gmail.com> - 2011-08-17 20:23 -0700
      Re: lists and for loops Mark Niemczyk <prahamark@gmail.com> - 2011-08-18 05:22 -0700
        Re: lists and for loops Tim Chase <python.list@tim.thechases.com> - 2011-08-18 07:45 -0500
    Re: lists and for loops Peter Pearson <ppearson@nowhere.invalid> - 2011-08-18 16:43 +0000

#11733 — lists and for loops

I want to add 5 to each element of a list by using a for loop.

Why doesn't this work?

numbers = [1, 2, 3, 4, 5]
for n in numbers:
     n = n + 5
print numbers

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#11736

On Aug 18, 1:08 pm, Emily Anne Moravec <mora...@stolaf.edu> wrote:
> I want to add 5 to each element of a list by using a for loop.
>
> Why doesn't this work?
>
> numbers = [1, 2, 3, 4, 5]
> for n in numbers:
>      n = n + 5
> print numbers

As the for loop steps through numbers, it assigns each integer value
to the label n. What n holds is a number, _not_ a reference to the
number in the list. So when you reassign n to hold n+5, you're
pointing n at a new number, not modifying the original number being
referred to.

So what you need is a reference to the position of the number in the
list, so you can reassign the value that's held there. The common
pattern is to use enumerate, which lets you step through a list giving
you both an reference (index) & a value:

numbers = [1, 2, 3, 4, 5]
for i, n in enumerate(numbers):
    numbers[i] = n + 5

Here you're reassigning each list element to hold its old value plus
5.

Hope this helps.

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#11745

Or, using list comprehension.

>>> numbers = [1, 2, 3, 4, 5]
>>> numbers = [n + 5 for n in numbers]
>>> numbers
[6, 7, 8, 9, 10]

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#11746

On 08/18/2011 07:22 AM, Mark Niemczyk wrote:
> Or, using list comprehension.
>
>>>> numbers = [1, 2, 3, 4, 5]
>>>> numbers = [n + 5 for n in numbers]
>>>> numbers
> [6, 7, 8, 9, 10]

Or, if you want it in-place:

   numbers[:] = [n+5 for n in numbers]

which makes a difference if you have another reference to numbers:

 >>> numbers = [1,2,3,4,5]
 >>> digits = numbers
 >>> numbers = [n+5 for n in numbers]
 >>> numbers, digits
([6, 7, 8, 9, 10], [1, 2, 3, 4, 5])
 >>> numbers = [1,2,3,4,5]
 >>> digits = numbers
 >>> numbers[:] = [n+5 for n in numbers]
 >>> numbers, digits
([6, 7, 8, 9, 10], [6, 7, 8, 9, 10])

-tkc

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#11780

On Wed, 17 Aug 2011 20:08:23 -0700 (PDT), Emily Anne Moravec wrote:
> I want to add 5 to each element of a list by using a for loop.
>
> Why doesn't this work?
>
> numbers = [1, 2, 3, 4, 5]
> for n in numbers:
>      n = n + 5
> print numbers

Because integers are immutable.  You cannot turn 1 into 6.
Contrast this behavior with lists, which *are* mutable:

>>> numbers = [[1],[2],[3],[4],[5]]
>>> for n in numbers:
...   n[0]= n[0] + 5
... 
>>> numbers
[[6], [7], [8], [9], [10]]

For practical purposes, I'm sure you'll find other responders'
excellent posts to be of more immediate use, but keeping
mutability in mind helps.

-- 
To email me, substitute nowhere->spamcop, invalid->net.

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