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Asked: 2026-06-13T03:08:59+00:00

Below is my script, which basically creates a zero matrix of 12×8 filled with

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Below is my script, which basically creates a zero matrix of 12×8 filled with 0. Then I want to fill it in, one by one. So lets say column 2 row 0 needs to be 5. How do I do that? The example below shows how I did it and the wrong (for my needs) output:

list_MatrixRow = []
list_Matrix = [] #Not to be confused by what the book calls, optimal alignment score matrix

int_NumbOfColumns = 12
int_NumbOfRows = 8

for i in range (0, int_NumbOfColumns): # Puts Zeros across the first Row
    list_AlignMatrixRow.append(0)
for i in range (0, int_NumbOfRows):
    list_AlignMatrix.append(list_AlignMatrixRow) 
#add the list in another list to make matrix of Zeros
#-------------------THE ACTUAL PROBLEMATIC PART; ABOVE IS FINE(It Works)------------

list_AlignMatrix[2][0] = 5 
# This is what logically makes sense but here is the output 
# which happens but I don't want (there should be all 0s and 
# only one 5 on the cell [2][0]):

[5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
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1 Answer

  1. Editorial Team

    Each row points to the same sublist. This is the result of appending the same sublist repeatedly. So when you modify one row, you end up modifying the others.

    I would do this:

    ncols = 12
nrows = 8
matrix = [[0] * ncols for i in range(nrows)]
matrix[2][0] = 5

    matrix contains:

    [[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
 [5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]]

    An aside about coding style: It is poor form in Python to include the type of the object in its name. I have chosen to rename int_NumbOfColumns as ncols. If you need something more descriptive use something like column_count. Generally, mixedCase names are to be avoided, while CamelCase is generally used for class names. See PEP 8 — Style Guide for Python Code for more.

    Edit: Since you mentioned that you are new to Python, here’s a little more explanation.

    This is a list comprehension:

    matrix = [[0] * ncols for i in range(nrows)]

    It can also be written as a regular for-loop:

    matrix = []
for i in range(nrows):
    matrix.append([0] * ncols)
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