floating point while loop

Question

int main()
{
    float x = 50;
    float y = 1/x;
    float result = y * x;

    float test = 41;
    float z = 1/test;
    float testRes = z * test;


    while (result == 1)
    {    
        x--;
    }

    printf("%f\n", x);
    printf("%.6f\n", testRes);
}

My while loop is infinite, it never ends, but it should end at 41 because when I test 41, it equals 0.99999404

score 3 · Answer 1 · answered Sep 29 '15 at 21:32

3

result never changes within the loop, therefore there is no reason to believe that, once it starts, the loop will ever end.

answered Sep 29 '15 at 21:32

Ignacio Vazquez-Abrams

776,304
153
1,341
1,358

but won't the result be 0.9999404 when x is 41? – whenyounodyourheadyes Sep 29 '15 at 21:39
If `result` is not 1 then the loop will never be entered. If `result` is 1 then the loop will never be exited. – Ignacio Vazquez-Abrams Sep 29 '15 at 21:41
but when x--; doesn't that mean i go from 50, 49, 48, ..., 41, which doesn't enter the loop, and then since 41 gives 0.999404 41 should be printed? – whenyounodyourheadyes Sep 29 '15 at 21:45
Where are you changing `result`? – Ignacio Vazquez-Abrams Sep 29 '15 at 21:46
from float result = y * x, since x is changing shouldn't result change as well? – whenyounodyourheadyes Sep 29 '15 at 21:49
1

No. Unlike in make or Verilog, execution in C is immediate. The operation is performed before the loop starts, and then never again. – Ignacio Vazquez-Abrams Sep 29 '15 at 21:51
sorry so how would I get the loop to go to 41 and stop? – whenyounodyourheadyes Sep 29 '15 at 22:07
1

Change `result` inside the loop. – Ignacio Vazquez-Abrams Sep 29 '15 at 22:08
Also, there's no reason to believe that `x * (1/x)` will equal 1, when using floats. Never base program flow on a floating point test for equality. – Lee Daniel Crocker Sep 29 '15 at 23:56

floating point while loop

1 Answers1