int index1, index2, index3, indexLimit;
double firstFactor[indexLimit][indexLimit],
  secondFactor[indexLimit][indexLimit],
  productMatrix[indexLimit][indexLimit];
for ( index1 = 0; index1 < indexLimit; index1++ )
  for ( index2 = 0; index2 < indexLimit; index2++ )
  {
    productMatrix[index1][index2] = 0;
    for ( index3 = 0; index3 < indexLimit; index3++ )
      productMatrix[index1][index2] =
        productMatrix[index1][index2] + 
          firstFactor[index1][index3] *
            secondFactor[index3][index2];
  }