Right circular loop

Question

I have to cycle from 0 to 5 with step 0.5. But the cycle should be reversed when the value is 5.

bool forward = true;
int counter = 0;
float val = 5.0f;

// And somewhere in global app cycle:
if (val <= 0.0 && forward) forward = false;
if (forward) val -= .5f;

if (val >= 5.0 && !forward) forward = true;
if (!forward) val += .5f;

But the result is sometimes negative numbers and, I think, that's a bit ugly code.

0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
2.98023e-08
-0.02
2.98023e-08
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0.2
0.18

Answer 1

The problem you are experiencing is that 0.2 cannot be exactly represented in floating-point. So when you write val -= 0.2, what you're really doing is val -= 0.20000000298023224;. Therefore, you accumulate errors.

The solution is to not use floating-point to control your loop. Use an int instead, and multiply by a constant to get val. e.g.:

int inc = -1;
int i = 10;

while (whatever)
{
    if (i == 0) inc = +1;
    if (i == 10) inc = -1;

    i += inc;

    float val = i * 0.02;
}