Relative error reduction calculation

rer.c

// Computes relative error reduction given two percentages.
//
// This computes relative error reduction (RER) given two percentages, the
// "before" and "after" accuracy.
//
// This is given by:
//
// RER = 1 - (1 - new_accuracy) / (1 - old_accuracy)
//
// To compile: gcc -O3 -std=c99 -o rer rer.c

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv) {
  if (argc != 3) {
    fprintf(stderr, "USAGE: %s old_accuracy new_accuracy\n", argv[0]);
    return 1;
  }

  const double old_error = 1. - atof(argv[1]);
  const double new_error = 1. - atof(argv[2]);
  if (old_error < new_error) {
    fprintf(stderr, "FATAL: Old accuracy > new_accuracy\n");
    return 1;
  }
  const double rel_error = 1. - new_error / old_error;

  printf("%.4f\n", rel_error);

  return 0;
}