Modifying values in a list

[tron.thomas@verizon.net]

The following code:

numbers = [1, 2, 3]
for value in numbers:
value *= 2
print numbers

results in the following output:
[1, 2, 3]

The intent of the code was to produce this output:
[2, 4, 6]

What is the reason for the output produced?
What code should be used to obtain the desired output?

[Björn Lindström]

Re: Modifying values in a list

tron.thomas@ver izon.net writes:
[color=blue]
> The following code:
>
> numbers = [1, 2, 3]
> for value in numbers:
> value *= 2
> print numbers
>
> results in the following output:
> [1, 2, 3]
>
> The intent of the code was to produce this output:
> [2, 4, 6]
>
> What is the reason for the output produced?
> What code should be used to obtain the desired output?[/color]

How about this?

numbers = [1, 2, 3]
print [x * 2 for x in numbers]

--
Björn Lindström <bkhl@stp.lingf il.uu.se>
Student of computational linguistics, Uppsala University, Sweden

[Brian van den Broek]

Re: Modifying values in a list

tron.thomas@ver izon.net said unto the world upon 29/12/05 10:43 AM:[color=blue]
> The following code:
>
> numbers = [1, 2, 3]
> for value in numbers:
> value *= 2
> print numbers
>
> results in the following output:
> [1, 2, 3]
>
> The intent of the code was to produce this output:
> [2, 4, 6]
>
> What is the reason for the output produced?
> What code should be used to obtain the desired output?[/color]

value is bound in the for loop, but none of that affects the list to
which numbers is bound.

One way:

IDLE 1.1.2[color=blue][color=green][color=darkred]
>>> numbers = [1, 2, 3]
>>> numbers = [x * 2 for x in numbers]
>>> print numbers[/color][/color][/color]
[2, 4, 6][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

HTH,

Brian vdB

[Carsten Haese]

Re: Modifying values in a list

On Thu, 2005-12-29 at 11:43, tron.thomas@ver izon.net wrote:[color=blue]
> The following code:
>
> numbers = [1, 2, 3]
> for value in numbers:
> value *= 2
> print numbers
>
> results in the following output:
> [1, 2, 3]
>
> The intent of the code was to produce this output:
> [2, 4, 6]
>
> What is the reason for the output produced?
> What code should be used to obtain the desired output?[/color]

1) Read http://www.effbot.org/zone/python-objects.htm and reread it
until you understand why your code doesn't work.

2) Use a list comprehension:
numbers = [ value*2 for value in numbers ]

-Carsten

[Paul McGuire]

Re: Modifying values in a list

As others have already posted, changing the value of 'value' has
nothing to do with the list variable 'numbers'. To modify the list in
place, you need to access its members by index. The following code
does this:

numbers = [1,2,3]
for i in range(len(numbe rs)):
numbers[i] *= 2
print numbers

But this is how a C or Java programmer would approach this task, and is
not very Pythonic.

A bit better is to use enumerate (avoids the ugly range(len(blah) )
looping):

numbers = [1,2,3]
for i,val in enumerate(numbe rs):
numbers[i] = val*2

This also modifies the list in place, so that if you are modifying a
very long (and I mean really quite long - long enough to question why
you are doing this in the first place) list, then this approach avoids
making two lists, one with the old value and one with the new.

But the most Pythonic is to use one of the list comprehension forms
already posted, such as:

numbers = [ x*2 for x in numbers ]

-- Paul