#include <stdio.h>

void swap_doubles_ptr(double *d1, double *d2){
  double temp;
  temp = *d1;
  *d1 = *d2;
  *d2 = temp;
}

int main(void) {
  double e, f;

  e = 5.0, f = 7.0;
  printf("%f %f\n", e, f);    // 5.0 7.0
  swap_doubles_ptr(&e, &f);  
  printf("%f %f\n", e, f);    // 7.0 5.0
  
  return 0;
}