Sorting matrix columns

I have a matrix 4*5 and I need to sort it by several columns. Please help me figure out why the answer does not match.

I spent a lot of hours and maybe a stupid mistake. I would be grateful if you could point it out to me.

sort_columns = [3, 1, 2, 4, 5, 2]
matrix = [[3, 1, 8, 1, 9],
          [3, 7, 8, 2, 9],
          [2, 7, 7, 1, 2],
          [2, 1, 7, 1, 9]]
sort_matrix_columns(matrix, len(matrix), sort_columns)

def sort_matrix_columns(matrix, n, sort_columns):
    for col in sort_columns:
        column = col - 1
        for i in range(n):
            for j in range(i + 1, n):
                if matrix[i][column] > matrix[j][column]:
                    temp = matrix[i]
                    matrix[i] = matrix[j]
                    matrix[j] = temp

I expect to get the result:

2 1 7 1 9
3 1 8 1 9
2 7 7 1 2
3 7 8 2 9

But I get the following:

3 1 8 1 9 
2 1 7 1 9 
2 7 7 1 2 
3 7 8 2 9 

>Solution :

You can just use the default sorted() function, and give it a key to retrieve the column values, at which point the rows are sorted by comparing the key values directly.

The key is callable (like a function) that is given each item that’s being sorted, and you need to return a value that’s being compared.

In your case, just map the column numbers, in reverse:

def sort_matrix_columns(matrix, sort_columns):
    return sorted(
        matrix,
        key=lambda row: [row[col - 1] for col in sort_columns[::-1]]
    )

Note that the n argument to the function can be dropped, it is not used.

Demo:

>>> sort_columns = [3, 1, 2, 4, 5, 2]
>>> matrix = [[3, 1, 8, 1, 9],
...           [3, 7, 8, 2, 9],
...           [2, 7, 7, 1, 2],
...           [2, 1, 7, 1, 9]]
>>> sorted(matrix, key=lambda row: [row[col - 1] for col in sort_columns[::-1]])
[[2, 1, 7, 1, 9], [3, 1, 8, 1, 9], [2, 7, 7, 1, 2], [3, 7, 8, 2, 9]]

The code returns a new, sorted list. If you must sort your matrix in-place, use list.sort():

def sort_matrix_columns(matrix, sort_columns):
    matrix.sort(
        key=lambda row: [row[col - 1] for col in sort_columns[::-1]]
    )