How do I multiply two square matrices by 1 nested loop?

I managed to print the matrix in one loop, but how can I multiply two matrices in 2 loops (1 nested loop), please help!!

Code:

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#include<iostream>
using namespace std;
#define size 4
int main()
{
  int M[size][size] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 };
  int *ptrMatris = &M[0][0];
  for (int i = 1; i <= 16; i++) {
    cout << *ptrMatris++ << " ";
    if (i % size == 0)
      cout << endl;
  }
  return 0;
}

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This way?

Code:

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void m4x4_mult( double *m, double *n, double *r )
{
  int i, j;

  for ( i = 0; i < 4; i++ )
    for ( j = 0; j < 4; j++ )
    {
      r[i+4*j] = m[4*j]  * n[i]  +
                m[4*j+1] * n[i+4] +
                m[4*j+2] * n[i+8] +
                m[4*j+3] * n[i+12];
    }
}

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yes, this way works but how can i write for any value of 0-10 not only 4. thanks... :))

You'll need a third loop for a variable square matrix dimensions...