Array increment doesnt give the correct value

Question

This is a very strange problem. I cannot see any differences between code1 and code2 . However, there should be a difference because they produce different results : (notice f0 and f0A (acts as a buffer))

code1 :

    for (k = 0; k < 6; k++) {
      r1 = i0 + 6 * k;
      f0 = 0.0F;
      for (r2 = 0; r2 < 6; r2++) {
        f0 += (float)b_a[i0 + 6 * r2] * p_est[r2 + 6 * k];
      }

      a[r1] = f0;
    }

code2:

float f0A[6] = {0};
for (k = 0; k < 6; k++) {
  r1 = i0 + 6 * k;
  for (r2 = 0; r2 < 6; r2++) {
    f0A[r2] += (float)b_a[i0 + 6 * r2] * p_est[r2 + 6 * k];
  }
}
for (r2 = 0; r2 < 6; r2++) {
  r1 = i0 + 6 * r2;
  a[r1] = f0A[r2];
}

Please post a [mcve] containing all relevant variable declarations. We can't guess the variable types. — Lundin, Nov 11 '19 at 14:29
Your code is incomplete. Please wrap your code in a `main` function to allow compiling and running it and show the expected and actual output. — Bodo, Nov 11 '19 at 14:30
_"doesnt give the correct value"_ is not a problem description. What value do you expect? Why? What do you get instead? — underscore_d, Nov 11 '19 at 14:37

score 1 · Accepted Answer · answered Nov 11 '19 at 14:30

In the first loop, you are setting a[r1] to a summation stored in f0. It is being added to each loop.

In the second loop, you aren't doing a summation, your loop is using += but it is storing each one in a different f0A index. Thus a[r1] is not given the correct value

There is the difference

Array increment doesnt give the correct value

1 Answers1