Untitled

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

#define pi 3.14159265358979323846

/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::  Function prototypes                                           :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double deg2rad(double);
double rad2deg(double);

double distance(double lat1, double lon1, double lat2, double lon2, char unit) {
  double theta, dist;
  if ((lat1 == lat2) && (lon1 == lon2)) {
    return 0;
  }
  else {
    theta = lon1 - lon2;
    dist = sin(deg2rad(lat1)) * sin(deg2rad(lat2)) + cos(deg2rad(lat1)) * cos(deg2rad(lat2)) * cos(deg2rad(theta));
    dist = acos(dist);
    dist = rad2deg(dist);
    dist = dist * 60 * 1.1515;
    switch(unit) {
      case 'M':
        break;
      case 'K':
        dist = dist * 1.609344;
        break;
      case 'N':
        dist = dist * 0.8684;
        break;
    }
    return (dist);
  }
}

/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::  This function converts decimal degrees to radians             :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double deg2rad(double deg) {
  return (deg * pi / 180);
}

/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::  This function converts radians to decimal degrees             :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double rad2deg(double rad) {
  return (rad * 180 / pi);
}

int main()
{
    double a = distance(5.0,10.0,2.0,4.0,'M');
    printf("%f",a);
}