Thread: How to use local variables in other functions

So I am new to C programming but was tasked to create a program that reads data from a file and then prints it out.

One of the sub-tasks is to calculate distance between 2 points through the haversine formula.

Now here is where my question comes in. How can I use some local variables defined from one of my functions in another function? If I can work this out, I will be able to do most of the rest of my task so I am really hoping there is a way to do this. I have tried working this out for the last day and have gotten no where.

I have included the code here. I will highlight the variables I want to use between functions. If it isn't immediately obvious, I am hoping to keep token[1] and token[2].

Code:

int main() {    int i = 0;
    double readfile, stage2;
    readfile = read_file(i);
    stage2 = stage_2_distance(i);
    printf("%f %f", readfile, stage2);
    
    return 0;
}


double read_file(double i) {
    /* This function takes the data from the input file,reading and printing the 
    User ID, Location (longitude and latitude), Date, Time, and Distance*/
    char length[256];
    char *token[6];
    double r_lat, r_long;


    if (fgets(length, 256, stdin) != NULL) {
        token[0] = strtok(length, " ");
        int i = 0;
        double dist = 0;
        for (i = 1; i < 6; i++)
            token[i] = strtok(NULL, " "); /*C programming is fun*/
            printf("Stage 1\n==========\n");
            printf("User: #%s\n", token[0]); 
            printf("Location: <%s, %s>\n", token[1], token[2]); /* token [1] and token [2] are the variables I want to use elsewhere. */
            printf("Date: %s\n", token[3]);
            printf("Time: %s\n", token[4]);
            printf("Distance to reference: %.2f\n", distance(dist));
            r_lat = *token[1]; /* These next two lines are what I have been playing around with*/
            r_long = *token[2];
    }
    else {
        printf("Error opening file. Check file and try again.");
        exit(EXIT_FAILURE);
    }
    exit(EXIT_SUCCESS);
}


double distance(double d) {
    /* This function is designed to calculate the distance between the check-in 
    POI and the reference point provided*/
    double angle_distance, chord_length;
    double lat_2, long_2;
    int i = 0;
    
    char length[256];
    char *token[6];
    
    token[0] = strtok(length, " ");
    token[i] = strtok(NULL, " ");
    double dist;
    
    lat_2 = atof(token[3]);
    long_2 = atof(token[4]);
    
    double var_lat = toRadian(lat_2 - LAT_1);
    double var_long = toRadian(long_2 - LONG_1);
 
    double nLAT_1 = toRadian(LAT_1);
    double nLat_2 = toRadian(lat_2);
        
    chord_length = pow(sin(var_lat/2),2) + (cos(nLAT_1) * cos(nLat_2) * 
    pow(sin(var_long/2),2));
    
    angle_distance = 2 * atan2(sqrt(chord_length), sqrt(1 - chord_length)); 
    
    dist = 6371 * angle_distance;
    
    return dist;
}


double toRadian(double x) {
    x = PI/DEGREES;
    return x;
}

In your case, I'd suggest reading from the file into an array of location struct objects, each of which consists of two doubles (i.e., the longitude and latitude). You can then pass this array to the read function along with the array size so that the read function reads into this array rather than an array local to the function.

A struct is not a function; it's a way of grouping data such that you can refer to disparate pieces of data (the members) as a single object. This makes it both easier to work with the data (since it is grouped together) and to reason about it (since you name the struct type and the object, thereby conferring meaning to the data).

What you're doing right now with read_file is to mostly read, print, then discard the data. What I'm suggesting is to come up with a struct (or two) to represent the data read, and from there you can figure out how to get the data out of the function instead of discarding it.