Transform NxN upper triangular matrix to a 1D row-major vector

upper_triangular_matrix.cpp

#include <stdio.h>

// fhm - felso haromszog matrix

// nxn-es fhm nem-nulla elemeinek a szamossaga
int fhm_elemei(int n)
{
    return (n*n + n)/2;
}

// lekepezes ami megadja hogy M nxn-es fhm i-edik soranak
// j-edik eleme hanyadik indexu eleme az fhm-et reprezentalo
// sorfolytonos 1-dim vektorban
int f(int i, int j, int n)
{
    return j >= i ? (n * i + j - (i*i + i)/2) : -1;
}

void print_vec(int n, int* vec)
{
    printf("%s", "[");
    for (int j = 0; j < n; ++j)
    {
        printf("%d,", vec[j]);
    }
    printf("%s", "]");
}

int main()
{
    int n = 4;
    int k = 1;
    int m[n][n];
    int len = fhm_elemei(n);
    int vec[len];
    
    //feltoltes
    for (int i = 0; i < n; ++i)
    {
        for (int j = 0; j < n; ++j)
        {
            if (j >= i)
            {
                m[i][j] = k++;
            }
            else
            {
                m[i][j] = 0;
            }
        }
    }
    
    /*
    m :
      [1,2,3,4]
      [0,5,6,7]
      [0,0,8,9]
      [0,0,0,10]
    */
    //atalakitas sorfolytonossa
    for (int i = 0; i < n; ++i)
    {
        for (int j = 0; j < n; ++j)
        {
            if (j >= i)
            {
                int idx = f(i,j,n);
                if (idx != -1)
                {
                    vec[idx] = m[i][j];
                }
            }
        }
    }
 
    print_vec(len, vec); //prints [1,2,3,4,5,6,7,8,9]
       
    return 0;
}