# sortIdx matrix in both directions gone wrong

What I wish to do, is first sort the rows of the matrix and then sort the columns, so I get this result:

Input: randommatrix
[  6, 197,  39,  29;
97, 110,  86, 193;
76, 129, 151, 138]

Output:
correctly sorted matrix
[  6, 29,  39,  151;
76, 97,  110, 193;
86, 129, 138, 197]


Now I must use sortIdx instead of sort because I need to know the original column and row of each number. The sort method works fine. So what I did was:

sortIdx(matrix, matrixRowIndices, SORT_EVERY_ROW + SORT_ASCENDING);


I reconstruct the sorted matrix in two for loops based on those indices, giving Mat sortedRowmatrix. This prints correctly:

sorted row matrix =
[  6,  29,  39, 197;
86,  97, 110, 193;
76, 129, 138, 151]


Now, I want to sort the columns. I use:

sortIdx(sortedRowmatrix, matrixColIndices, SORT_EVERY_COLUMN + SORT_ASCENDING);


I get the correct matrixColIndices (explanation below):

sorted col indices=
[0, 0, 0, 2;
2, 1, 1, 1;
1, 2, 2, 0]


since: Per column analysis of "sorted row matrix":

• 0th column: nr 0 (6) is the smallest, after that nr 2 (76), after that nr 1 (86)
• 1st column: nr 0 (29) is the smallest, nr 1 (97) after that, nr 2 (129) after that.
• 2nd column: nr 0 (39) is the smallest, nr 1 (110) after that, nr 2 (138) after that.
• 3rd column: nr 2 (151) is the smallest, nr 1 (193) after that en nr 0 (197) after that

Then, I write out my final matrix based on original indices and I expect to get the sorted result. Instead I get:

wrongly sorted matrix =
[  6,  29,  39, 129;
151,  97, 110, 193;
97, 129, 138,  39]


Notice it also duplicates some numbers (97).

I cannot get my head around why my reconstruction apparently does not work. I sort horizontally: this gives me the correct column indices. I sort vertically: the horizontal ordning should stay the same, and this should give me correct row indices.

The normal sort method works (but of course does not give the original positions I need). I also tried sorting columns first, then rows. The first step sorting columns works. The second step again doesn't. What goes wrong?

My code is at: https://pastebin.com/0Li2j3xw

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