浮点运算后四舍五入不一致

Question

Take the following example (with contrived numbers):

double a = 9.0, b = 2.0;
double c = a / b;
int d = RoundAndCastToInt(c);

Across multiple runs, the value of d isn't consistent. Here are two contrived ways the math gets performed:

Execution 1:

9.0 / 2.0 == 4.49999...

RoundAndCastToInt(4.49999...) == 4

Execution 2:

9.0 / 2.0 == 4.5

RoundAndCastToInt(4.5) == 5

I want to get the same value consistently across different executions and different machines. In the example above, a result of either 4 or 5 is fine as long as it is always 4 or always 5 .

Answer 1

This might be stupid with a lot of unsolved details, but I guess it can work when you know the precision of the solution you want...

#include <stdio.h>
#include <math.h>

int main(int argc, char * argv[]) {
  float i = 5.3 / 4.7;
  float j = 5.3 / 4.7;

  printf("i %f \n", round(i * 10000.0) / 10000.0);
  printf("j %f \n", round(j * 10000.0) / 10000.0);


  return 0;
}