Averaging elements of a 2D vector in C++

[DanielDeChamps]

Hello, my code is returning only the last element calculated inside the loops.
I am "binning" (resize averaging the neighbors elements) a matrix 4x4 into a 2x2. I am taking the average of values in a square, for instance 1.0 +2.0 +5.0+ 6.0 divided by four, this is the [0][0] elements of the new matrix. Below the matrix called "a" from .txt file.

1.0 2.0 3.0 4.0
5.0 6.0 7.0 8.0
9.0 10.0 11.0 12.0
13.0 14.0 15.0 16.0 

The expected result 2x2 "matrix_binned":

3.5 5.5
11.5 13.5

My code:

    std::vector<std::vector<float>> matrix_binned(2, std::vector<float>(2, 0));
    int n, m, i, j;
    for (n = 0; n < 4; n+=2) {
        for (m = 0; m < 4; m+=2) {

            for (i = 0; i < 2; ++i) {
                for (j = 0; j < 2; ++j) {

                    matrix_binned[i][j] = (a[n][m] + a[n][m + 1] + a[n + 1][m] + a[n + 1][m + 1]) / 4;
                    
                 }
            }  
        }
        
    }  

When printed, all the elements shown are 13.5.

[dutch]

You want to map the elements of the 4x4 array to the 2x2 array like this:

  0       1
0 0,0 1,0 2,0 3,0
  0,1 1,1 2,1 3,1
1 0,2 1,2 2,2 3,2
  0,3 1,3 2,3 3,3

You just need a double loop (not a quadruple loop!).

Note that I've put the y (row) coordinate first so that it matches how the initializer looks. It may seem a little weird but in C++ matrices are (IMO at least) indexed [row][col] or [y][x]. (IIRC you were doing it the other way around in the last program.)

#include <iostream>
#include <iomanip>
#include <vector>

using InnerVec = std::vector<float>;
using Matrix = std::vector<InnerVec>;

int main()
{
    Matrix a
    {
        {  1.0,  2.0,  3.0,  4.0 },
        {  5.0,  6.0,  7.0,  8.0 },
        {  9.0, 10.0, 11.0, 12.0 },
        { 13.0, 14.0, 15.0, 16.0 }
    };

    Matrix binned(2, InnerVec(2));
    for (int y = 0; y < 2; ++y)
        for (int x = 0; x < 2; ++x)
            binned[y][x] = (a[y*2  ][x*2  ]
                          + a[y*2  ][x*2+1]
                          + a[y*2+1][x*2  ]
                          + a[y*2+1][x*2+1]) / 4.0;

    std::cout << std::fixed << std::setprecision(2);
    for (int y = 0; y < 2; ++y)
    {
        for (int x = 0; x < 2; ++x)
            std::cout << std::setw(6) << binned[y][x] << ' ';
        std::cout << '\n';
    }
}

[DanielDeChamps]

Hello dutch! it worked perfectly.

I also test it with a 1024x1024 .txt file, doing some adjustments, and was perfect. I am now analyzing my code and yours comparatively, but anyway I think your code is less expensive certainty.

Yes, I was doing [y][x] in the last program, I thought that behavior was only to write & read .txt and .csv, but not when writing or reading a matrix. Very important to know.

Thank you so much dutch.