Transpose matrix: swapping elements doesn't alter values - c++

Question

I am trying to transpose a matrix built with vectors.
Here is the transpose function I wrote:
void transpose(std::vector<std::vector<int>>& fill_mat) {
for (int i = 0; i < fill_mat.size(); ++i) {
for (int j = 0; j < fill_mat.size(); ++j) {
std::swap(fill_mat[i][j], fill_mat[j][i]);
}
}
}
It doesn't seem to be doing anything: my final results are the same as the starting ones.
Here is my full program:
#include<iostream>
#include<vector>
#include<utility>
void print_matrix(std::vector<std::vector<int>>& to_print) {
for (int i = 0; i < to_print.size(); ++i) {
for (int j = 0; j < to_print.size(); ++j) {
std::cout << " " << to_print[i][j];
}
std::cout << std::endl;
}
}
void make_matrix(std::vector<std::vector<int>>& fill_mat) {
for (int i = 0; i < fill_mat.size(); ++i) {
for (int j = 0; j < fill_mat.size(); ++j) {
fill_mat[i][j] = rand() % 15;
}
}
}
void transpose(std::vector<std::vector<int>>& fill_mat) {
for (int i = 0; i < fill_mat.size(); ++i) {
for (int j = 0; j < fill_mat.size(); ++j) {
std::swap(fill_mat[i][j], fill_mat[j][i]);
}
}
}
int main() {
int size = 3;
std::vector<std::vector<int>> matrix_sample(size, std::vector<int>(size));
make_matrix(matrix_sample);
print_matrix(matrix_sample);
transpose(matrix_sample);
std::cout << "----## transpose ##-----" << std::endl;
print_matrix(matrix_sample);
}
I expected to print out the transposed matrix but the input ends up being the same as the output. What am I doing wrong?

Answer 1

You swap (i,j) with (j,i) twice! That's why it has no effect.
You should only work on one half of your matrix. Plus minors improvements, we get:
void transpose(std::vector<std::vector<int>>& fill_mat)
{
using size_type = decltype(fill_mat)::size_type; // better use your matrix' size type
for (size_type i = 0; i < fill_mat.size(); ++i) {
for (size_type j = 0; j < i; ++j) {
using std::swap; // see swap idiom
swap(fill_mat[i][j], fill_mat[j][i]);
}
}
}