For loop condition not met, not sure why

Question

With the following code:

void pivot(int n, int m, int evi, int lvi, float a[m][n]) {
  int i,j;
  float s = a[lvi][evi];
  for (i = 0; i < n; i++) a[lvi][i] /= s;
  for (j = 0; (j < m) && (j != lvi); j++) {
    s = a[j][evi];
    for (i = 0; i < n; i++) {
      a[j][i] -= s * a[lvi][i];
      printf("tab[%d][%d] = %f\n", j, i, a[j][i]);
    }
  }
}
float tab[3][6] = {{0.0}};
tab[0][0] = 200;
tab[0][1] =-200;
tab[0][2] = 60;
tab[0][3] =-60;
tab[0][4] = 1;
tab[1][0] =-100;
tab[1][1] = 100;
tab[1][2] =-150;
tab[1][3] = 150;
tab[1][5] = 1;

When I call

pivot(6,3,0,0,tab)

the second for loop in the pivot function (with && operator) never gets entered, why?

Answer 1

A for loop stops as soon as the condition becomes false. If it's false when the loop starts, the loop is never entered at all.

If you want to skip certain rows, you should put the check in the loop body.

  for (j = 0; j < m; j++) {
    if (j == lvi) {
      continue;
    }
    s = a[j][evi];
    for (i = 0; i < n; i++) {
      a[j][i] -= s * a[lvi][i];
      printf("tab[%d][%d] = %f\n", j, i, a[j][i]);
    }
  }