Question

I can’t reproduce this with a simple program, but somewhere in my program I have something like:

float e = f(...);
if (e > 0.0f) {
    ...

printf("%f", e) shows that e is 0.000000, yet e > 0.0f is true… So is e > 0 and e > 0.0. What am I missing?

Answer 1

The problem is that the floating point value is greater than 0, but less than the precision that printf uses to print floating point numbers with %f. You can use %e or %g for better results as illustrated with the following program.

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

void main(void)
{
  int i;
  float e;

  for (i = 1; i < 64; i++) {
    printf("Decimal places: %d\n", i);

    e = 1.0 / pow(10, i);

    if (e > 0.0f) {
      printf("Value displayed with %%e: %e > 0.0f\n", e);
      printf("Value displayed with %%f: %f > 0.0f\n", e);
      printf("Value displayed with %%g: %g > 0.0f\n\n", e);

    }
  }
}

You will need to compile this with the maths library. For gcc use: -lm