Question

I am trying to transpose a matrix, like below. I am passing the array by reference, and somehow it is not working (AFTER is same as BEFORE). I am not sure why.

But, What I am really surprised about is that the array is not transposed even inside the transpose() function (how can INSIDE be the same as BEFORE??). What am I missing?

#include <iostream>

template <int M, int N>
void print1(int (&src)[M][N]) {
    for(int i = 0; i < M; i++) {
        for(int j = 0; j < N; j++)
            printf("%2d\t", src[i][j]);
        std::cout << std::endl;
    }
}

template <int M, int N>
void transpose(int (&src)[M][N]) {
    for(int i = 0; i < M; i++) {
        for(int j = 0; j < N; j++) {
            if (i == j) continue;
            int temp = src[i][j];
            src[i][j] = src[j][i];
            src[j][i] = temp;
        }
    }
    std::cout << "\nINSIDE:\n";
    print1(src);
}

int main() {

    int src[][4] = {
        { 1,  2,  3,  4},
        { 5,  6,  7,  8},
        { 9, 10, 11, 12},
        {13, 14, 15, 16},
    };

    std::cout << "BEFORE:\n";
    print1(src);
    transpose(src);

    std::cout << "\nAFTER:\n";
    print1(src);
}

Output:

$ ./a.out
BEFORE:
 1       2       3       4
 5       6       7       8
 9      10      11      12
13      14      15      16

INSIDE:
 1       2       3       4
 5       6       7       8
 9      10      11      12
13      14      15      16

AFTER:
 1       2       3       4
 5       6       7       8
 9      10      11      12
13      14      15      16
$

Answer 1

Are you maybe transposing it twice:

for(int i = 0; i < M; i++) {
    for(int j = 0; j < N; j++) {
        if (i == j) continue;
        int temp = src[i][j];
        src[i][j] = src[j][i];
        src[j][i] = temp;
    }
}

i.e. substitute src(i,j) for src(j,i)… and then substitute src(j,i) for src(i,j)?