python for loop bug

[Mars8082686]

I'm trying to get a list to be numbered [0-3][0-3]. The following should work:

>>> for x in range(4):
... for y in range(4):
... img[x][y]=str(x)+str(y)
...
>>> print img


but it gives me:
[['30', '31', '32', '33'], ['30', '31', '32', '33'], ['30', '31', '32', '33'], ['30', '31', '32', '33']]


Why is x not incrementing? When I just print out x and y it gives me what I want:

>>> for x in range(4):
... for y in range(4):
... print x,y
...
0 0
0 1
0 2
0 3
1 0
1 1
1 2
1 3
2 0
2 1
2 2
2 3
3 0
3 1
3 2
3 3

I'm probably missing something basic in python. (I'm somewhat new to it).

Thanks.

[Wybiral]

Are you posting all of your code?

[Mars8082686]

Yea, It's not part of a larger program.

Here's my exact test session:



~ mars$ python
Python 2.5 (r25:51918, Sep 19 2006, 08:49:13)
[GCC 4.0.1 (Apple Computer, Inc. build 5341)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> img = [['']*4]*4
>>> for x in range(4):
... for y in range(4):
... img[x][y]=str(x)+str(y)
...
>>> print img
[['30', '31', '32', '33'], ['30', '31', '32', '33'], ['30', '31', '32', '33'], ['30', '31', '32', '33']]
>>> quit()
~ mars

[nvteighen]

This may be related to Python call-by-reference policy, that is, that you pass everywhere the location to some object that's being incremented subsequently in all slots... but it's really weird... it should work that way, at least all other similar programming languages would yield that correctly...

[unutbu]

img = [['']*4]*4

is not the same as


img = [['', '', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]


The first version makes a list of the form


[A,A,A,A]

each A = ['', '', '', ''] and each A is the exact same object.

The second version makes a list of the form


[A,B,C,D]

with each A,B,C,D being distinct objects.

So in the first version, when you alter elements of A, you alter the element in four places.



#!/usr/bin/env python

img = [['']*4]*4
print img
for x in range(4):
for y in range(4):
img[x][y]=str(x)+str(y)
print img

# [['', '', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '', '', ''], ['00', '', '', ''], ['00', '', '', ''], ['00', '', '', '']]
# [['00', '01', '', ''], ['00', '01', '', ''], ['00', '01', '', ''], ['00', '01', '', '']]
# [['00', '01', '02', ''], ['00', '01', '02', ''], ['00', '01', '02', ''], ['00', '01', '02', '']]
# [['00', '01', '02', '03'], ['00', '01', '02', '03'], ['00', '01', '02', '03'], ['00', '01', '02', '03']]
# [['10', '01', '02', '03'], ['10', '01', '02', '03'], ['10', '01', '02', '03'], ['10', '01', '02', '03']]
# [['10', '11', '02', '03'], ['10', '11', '02', '03'], ['10', '11', '02', '03'], ['10', '11', '02', '03']]
# [['10', '11', '12', '03'], ['10', '11', '12', '03'], ['10', '11', '12', '03'], ['10', '11', '12', '03']]
# [['10', '11', '12', '13'], ['10', '11', '12', '13'], ['10', '11', '12', '13'], ['10', '11', '12', '13']]
# [['20', '11', '12', '13'], ['20', '11', '12', '13'], ['20', '11', '12', '13'], ['20', '11', '12', '13']]
# [['20', '21', '12', '13'], ['20', '21', '12', '13'], ['20', '21', '12', '13'], ['20', '21', '12', '13']]
# [['20', '21', '22', '13'], ['20', '21', '22', '13'], ['20', '21', '22', '13'], ['20', '21', '22', '13']]
# [['20', '21', '22', '23'], ['20', '21', '22', '23'], ['20', '21', '22', '23'], ['20', '21', '22', '23']]
# [['30', '21', '22', '23'], ['30', '21', '22', '23'], ['30', '21', '22', '23'], ['30', '21', '22', '23']]
# [['30', '31', '22', '23'], ['30', '31', '22', '23'], ['30', '31', '22', '23'], ['30', '31', '22', '23']]
# [['30', '31', '32', '23'], ['30', '31', '32', '23'], ['30', '31', '32', '23'], ['30', '31', '32', '23']]
# [['30', '31', '32', '33'], ['30', '31', '32', '33'], ['30', '31', '32', '33'], ['30', '31', '32', '33']]


img = [['', '', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
print img
for x in range(4):
for y in range(4):
img[x][y]=str(x)+str(y)
print img

# [['', '', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '01', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '01', '02', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '', '', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', ''], ['', '', '', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['', '', '', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['20', '', '', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['20', '21', '', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['20', '21', '22', ''], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['20', '21', '22', '23'], ['', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['20', '21', '22', '23'], ['30', '', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['20', '21', '22', '23'], ['30', '31', '', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['20', '21', '22', '23'], ['30', '31', '32', '']]
# [['00', '01', '02', '03'], ['10', '11', '12', '13'], ['20', '21', '22', '23'], ['30', '31', '32', '33']]

[Mars8082686]

Makes sense. Thanks.

Is there an easier way to do this, though?
img = [['', '', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
That seems like it could bet somewhat cumbersome.

[Wybiral]

List comprehension:


lst = [[str(x) + str(y) for x in range(4)] for y in range(4)]

[wmcbrine]

img = [[''] * 4, [''] * 4, [''] * 4, [''] * 4]

works to initialize the list. Or better:


img = [[''] * 4 for i in range(4)]

But Wybrial's solution is best, replacing the entire loop. Although I think you were going more for:


img = [[str(x) + str(y) for y in range(4)] for x in range(4)]

[mssever]

Makes sense. Thanks.

Is there an easier way to do this, though?
img = [['', '', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
That seems like it could bet somewhat cumbersome.
Not that I know of, but I can't think of any situations where you'd need something like this in Python. You could easily append() to the list as needed. No need to declare it ahead of time.

[pmasiar]

... and use a dictionary instead of a list of lists (where key is a tuple): img[x,y] = '...'

[mssever]

... and use a dictionary instead of a list of lists (where key is a tuple): img[x,y] = '...'
Why do I never remember that you can do that? :)

[nvteighen]

Why do I never remember that you can do that? :)
I always forget dictionaries too... :p

[Mars8082686]

... and use a dictionary instead of a list of lists (where key is a tuple): img[x,y] = '...'

It should be a 4 x 4 list. The img var isn't a list of pixels, it's a 4 dimensional list of pygame surfaces (images).

[mssever]

It should be a 4 x 4 list. The img var isn't a list of pixels, it's a 4 dimensional list of pygame surfaces (images).
But what's the practical difference between these two:

img[w][x][y][z] = foo # img is a list of lists of lists...
# and
img[w,x,y,z] = foo #img is a single dictionaryEither way, you can store and retrieve the same data, but the second is easier to handle in Python.

[Mars8082686]

But what's the practical difference between these two:

img[w][x][y][z] = foo # img is a list of lists of lists...
# and
img[w,x,y,z] = foo #img is a single dictionaryEither way, you can store and retrieve the same data, but the second is easier to handle in Python.

Maybe I'm missing something. Right now it's a two dimensional list: [['sureface00', 'sureface01', 'sureface02', 'sureface03'], ['sureface10', 'sureface11', 'sureface12', 'sureface13'], ['sureface20', 'sureface21', 'sureface22', 'sureface23'], ['sureface30', 'sureface31', 'sureface32', 'sureface33']]

I'm writing a game where the sprite is a character and it's convenient for my coding for it to be a 2 dimensional list. When I return the image to display on screen, I can just say img[direction][steps] where direction is the direction of the sprite (up, down, left, right) and the steps is the number that it is in in his 4 part walking sequence.

In this case wouldn't a list be more applicable then a dictionary? I'm new to python so I may be wrong.

[mssever]

Maybe I'm missing something. Right now it's a two dimensional list: [['sureface00', 'sureface01', 'sureface02', 'sureface03'], ['sureface10', 'sureface11', 'sureface12', 'sureface13'], ['sureface20', 'sureface21', 'sureface22', 'sureface23'], ['sureface30', 'sureface31', 'sureface32', 'sureface33']]

I'm writing a game where the sprite is a character and it's convenient for my coding for it to be a 2 dimensional list. When I return the image to display on screen, I can just say img[direction][steps] where direction is the direction of the sprite (up, down, left, right) and the steps is the number that it is in in his 4 part walking sequence.

In this case wouldn't a list be more applicable then a dictionary? I'm new to python so I may be wrong.
The point is, the practical difference between the two is only syntactical. If you said img[direction,steps] it would work just as well (provided, of course, that you set up img that way in the first place). Try it.

[Wybiral]

The point is, the practical difference between the two is only syntactical. If you said img[direction,steps] it would work just as well (provided, of course, that you set up img that way in the first place). Try it.

And since you're using tuples as the key, you can pass a pre-existing tuple right into it, such as a position vector:



some_dict[position]
# instead of
some_list[position[0]][position[1]]