Use string as index in the array in C

Question

I know a char can be used as the index in the array. But using string as the index becomes difficult.

For example, I want to record the distance between two cities.

MAP["NewYork"]["Washington"] = 2000;

The only way I know is to

strcpy(city[0], "NewYork"); strcpy(city[1], "Washington");

and use 0 and 1 as the index in MAP

MAP[0][1] = 2000

But things get complicated if I need to calculate the total amount from Places A to B then to C.

I am looking for suggestions as to how my approach to implementing these things can be improved.

Answer 1

There are no dictionaries in the C language. You need to write it yourself. You may have an array of structures where one member is a string another hold something else. Then you need to implement the function to find the proper structure using the string

Answer 2

You could also hash your strings.

A hash function can turn a string into a number, then you could modulo(%) it to the maximum map size.

Downside is that there can be collisions(two strings pointing to the same number) which you may have to treat by comparing the original strings themselves, but then you would need to store which string is associated with each number to compare them later.

There are some string hashes discussed in this question.

If you want to read more about hash tables: The wikipedia page might be a good start

Answer 3

..."But using string as the index becomes difficult."
..."I want to record the distance between two cities"...

You cannot use a string as an array index, but you can associate a specific array index with a particular string using a combination of an enum (array index) and const char array. The following address the two stated issues above by illustrating how a struct, enum and const char arrays may be used in calulating distances between city locations.

//from Google:
//Longest city name: Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch

#define MAX_NAME 60 //59+1
#define earthRadiusKm 6371.0
#define PI 3.14159265359

enum {
    LAT,
    LON
};

enum {
    NEW_YORK,
    WASHINGTON,
    MIAMI,
    MAX_CITIES
};

typedef struct {
    char sCity[MAX_NAME];
    double lat;
    double lon;
}CITY_LOC;

// Note:
// Normally the following two items would be accessed from a lookup
// table, or database, but for purposes of illustration, will
// include a very abbreviated collection of values for city strings
// and latitude/longitude values here:    
const char cities[MAX_CITIES][MAX_NAME] = {"New York", "Washington", "Miami"};
double lat_lon[MAX_CITIES][2] = {{40.730610, -73.935242},
                                 {38.889931, -77.009003},
                                 {25.761681,  -80.191788}};

double distanceEarth(double lat1d, double lon1d, double lat2d, double lon2d);
double deg2rad(double deg);
double rad2deg(double rad);

int GetCityIndexFromString(char *city);

/// enter two city names
/// use double quotes if city name include spaces. eg. "New York"
int main(int argc, char *argv[])
{
    int i, j;
    double MAP[MAX_CITIES][MAX_CITIES] = {0};
    CITY_LOC cLoc[MAX_CITIES] = {0};

    if(argc != 3)
    {
        printf("Usage: %s <city1> <city2>\nExiting", argv[0]);
        getchar();
        return 0;
    }

    //populate struct
    for(i=0;i<MAX_CITIES;i++)
    {
        strcpy(cLoc[i].sCity, cities[i]);
        cLoc[i].lat = lat_lon[i][LAT];
        cLoc[i].lon = lat_lon[i][LON];
    }

    //calculate distances between all cities
    for(i=NEW_YORK;i<MAX_CITIES;i++)
    {
        for(j=NEW_YORK;j<MAX_CITIES;j++)
        {
            MAP[i][j] = distanceEarth(cLoc[i].lat, cLoc[i].lon, cLoc[j].lat, cLoc[j].lon);
        }
    }

    //distance in kilometers between requested cities
    int city1 = GetCityIndexFromString(argv[1]);
    int city2 = GetCityIndexFromString(argv[2]);        
    double distance = MAP[city1][city2];

    return 0;
}

int GetCityIndexFromString(char *city)
{
    int i;
    for(i=0;i<MAX_CITIES;i++)
    {
        if(stricmp(city,cities[i]) == 0)
        {
            return i;
        }
    }
    return -1;
}


double distanceEarth(double lat1d, double lon1d, double lat2d, double lon2d) {
  double lat1r, lon1r, lat2r, lon2r, u, v;
  lat1r = deg2rad(lat1d);
  lon1r = deg2rad(lon1d);
  lat2r = deg2rad(lat2d);
  lon2r = deg2rad(lon2d);
  u = sin((lat2r - lat1r)/2);
  v = sin((lon2r - lon1r)/2);
  return 2.0 * earthRadiusKm * asin(sqrt(u * u + cos(lat1r) * cos(lat2r) * v * v));
}

// 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);
}

Where credit for distanceEarth() goes to:
double distanceEarth(double lat1d, double lon1d, double lat2d, double lon2d)