In-place transpose of a 2-d vector passed by reference - c++

Question

I am trying to compute the transpose of a matrix. Now, I have passed the matrix as a 2-d vector, and since I want the function to calculate the transpose in-place, I passed the 2-d vector by reference. But it seems to take no effect.
I tried to use std::swap(mat[i][j], mat[j][i]), it had no effect. Again, I tried to manually swap the integers (using a third variable), still no effect.
#include<iostream>
#include<vector>
#include<algorithm>
#define N 5
void inPlaceTranspose(std::vector<std::vector<int> > &mat) {
for(int i = 0; i < N; i++) {
for(int j = 0; j < N; j++) {
int temp = mat[i][j];
mat[i][j] = mat[j][i];
mat[j][i] = temp;
//std::swap(mat[i][j], mat[j][i]);
}
}
std::cout << "Transpose:\n";
for(int i = 0; i < N; i++) {
for(int j = 0; j < N; j++) {
std::cout << mat[i][j] << " ";
}
std::cout << "\n";
}
}
int main() {
std::vector<std::vector<int> > mat(N, std::vector<int>(N));
mat = {
{1, 2, 3, 4, 5},
{7, 8, 9, 10, 11},
{13, 14, 15, 16, 17},
{19, 20, 21, 22, 23},
{25, 26, 27, 28, 29},
};
std::cout << "Original Matrix:\n";
for(int i = 0; i < N; i++) {
for(int j = 0; j < N; j++) {
std::cout << mat[i][j] << " ";
}
std::cout << "\n";
}
inPlaceTranspose(mat);
return 0;
}
The Original Matrix and Transpose are coming out to be same.

Answer 1

The problem is not in the way you pass the matrix, or in the way you do the swap. The problem is in your logic here:
for(int j = 0; j < N; j++) {
which results in swapping the elements twice, thus cancelling out your attempt to transpose the matrix.
Try looping until i, instead of N, like this:
for(int j = 0; j < i; j++) {
instead.