Machine precision test

add_test.c

/* add_test.c -- test machine precision for float and double datatypes
 Compile: gcc -lm add_test.c -o add_test
 
 Adapted from: http://virtuallyshocking.com/2008/01/14/float-vs-double/
 
 This code iteratively increments a Real variable with a small amount, 
 leading to round-off errors. When the round-off error is bigger than
 a preset value 'diff', it prints the number of iterations and the round-off
 error at that point. Additionally it prints the error at each 100 iterations.
 
 
 This tests three combinations of machine precision:

 * Float counter with float increment
 * Double counter with double increment
 * Double counter with float increment
*/

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

int main (void) {
  float float_v=0, fdt;
  double double_v=0, cross_v=0, ddt, diff;
  int i;

  ddt = 1e-5;
  fdt = 1e-5;
  diff = 1e-8;
  
  for (i=1; i< 1e6; i++) {
    double_v += ddt;
    float_v += fdt;
    cross_v += fdt;
    
    if (i % 100 == 0) {
      printf("%d: float: %.3g, dbl: %.3g, cross: %.3g, diff: %g\n", 
      	i,
      	fabs(float_v - i*ddt),
      	fabs(double_v - i*ddt),
      	fabs(cross_v - i*ddt),
      	double_v - float_v);
    }

	if (fabs(double_v - float_v) > diff) { 
		printf("Found discrepancy of %g at step %d!\n", double_v - float_v, i);
		break;
	}
  }

  return 0;
}