Help debugging some basic python.

[Beauxesprits13]

why is this:

    #where gap = -2
    for x in range(1, len(matrix[0])):
        matrix[0][x] = matrix[0][x-1] + gap

    print matrix 

returning:

[0, -2, -4, -6, -8],
[0, -2, -4, -6, -8],
[0, -2, -4, -6, -8]

instead of:

[0, -2, -4, -6, -8],
[0, 0, 0, 0, 0],
[0, 0, 0, 0, 0]...ect

It has been frustrating me for some time as I have been trying to iterate over x items in the zero-th nested list yet it iterates over x items in all lists.

[ofnuts]

It has been frustrating me for some time as I have been trying to iterate over x items in the zero-th nested list yet it iterates over x items in all lists.

Works for me:

>>> gap=-2
>>> matrix=[[0,0,0,0,0],[0,0,0,0,0],[0,0,0,0,0]]
>>> 
>>> for x in range(1, len(matrix[0])):
...     matrix[0][x] = matrix[0][x-1] + gap
... 
>>> print matrix
[[0, -2, -4, -6, -8], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]

But since your "matrix" has a two dimensions, the output should have two levels of brackets, so I suspect that you code is not really doing "print matrix" and prints the first line repeatedly.

[nvteighen]

All I can say is that probably, print matrix is misindented...

EDIT: Anyway, that's a quite weird way to iterate over the list; you could use xrange(len(matrix[0]) and not substracting 1 to x for accessing the matrix's item.

[Beauxesprits13]

Thanks for helping out. i think the problem was this line:

[[0]*(len(X)+1)]*(len(Y)+1) 

that i used to build my original matrix and then substitute values into. does this mean that python remembers the origanal equality of the rows in the matrix from [[0]*x]*y?

Ffrom memory i was printing with: for row in matrix: print row

[Tony Flury]

Thanks for helping out. i think the problem was this line:

[[0]*(len(X)+1)]*(len(Y)+1) 

Ffrom memory i was printing with: for row in matrix: print row

Yes what that does is create multiple references to a single object namely [0,0,0,0,0] (assuming len(X) = 3)

so when you change anything in matrix[1] you are changing the same object as matrix[0] since each row in your matrix is the same.

you need to populate your matrix explicitly (like ofnuts did) or do it another way - but using the * operator to create n copies of an object will actually create n copies of the referece to the object - and you will see the behaviour you see here.

Look up the difference between shallow and deepcopy for instance.