How to sort an array of structs nested in a array of structs C

Question

I know there are similiar posts to this one but I can't get my head around them so I'm making one too so please don't mark this as a duplicate :)

I've got 2 nested arrays of structs where the outer structs are races and the inner structs are the number of boats from those races. I want to sort the inner struct so that the boats are in ascending order of time_to_complete_race and once I've done this I then assign the first boat in the array points = 4, the second points = 2 and the thirdpoints = 1. I haven't however implemented the points assinging yet so please ignore this part.

struct boat_data {
    int ID;
    int time_to_complete_race;
    int points;
} boat_node;
typedef struct race_result {
    char race_date[80];
    int start_time;
    int num_boats_competing;
    struct boat_data boat_data[MAX_BOAT_NUMBER];
} race_node;

Here is the code that I am using to sort the inner stucts:

void print_races(int races, race_ptr results[], member_node_ptr member) {

    for(int race = 0; race < races; race++) {
        printf("Race Date: %s\n", results[race].race_date);
        printf("Race Start Time: %d\n", results[race].start_time);
        printf("Number of Skippers: %d\n", results[race].num_boats_competing);
        for(int boats = 0; boats < results[race].num_boats_competing; boats++) {
            find_node_by_id(member, results[race].boat_data[boats].ID);
            printf("\tTime to Complete Race in Seconds: %d\n",
                    results[race].boat_data[boats].time_to_complete_race);
        }
        printf("\n");
    }
}

void print_sorted_races(int races, race_ptr results[], member_node_ptr member) {

    race_ptr sorted_results[races];
    struct boat_data temp;

    int race, swap, boat;

    for (race = 0; race < races; race++) {
        sorted_results[race] = results[race];
    }

    for (race = 0; race < races; race++) {

        for (boat = 0; boat< (sorted_results[race].num_boats_competing -1); boat++) {
            for (swap = race + 1; swap < sorted_results[race].num_boats_competing; swap++) {
                if (sorted_results[race].boat_data[swap].time_to_complete_race >
                    sorted_results[race].boat_data[boat].time_to_complete_race) {
                    temp = sorted_results[race].boat_data[boat];
                    sorted_results[race].boat_data[boat] = sorted_results[race].boat_data[swap];
                    sorted_results[race].boat_data[swap] = temp;
                }
            }
        }
    }
    print_races(races, results, member);
}

Answer 1

You are making things much harder and much more error prone by trying to roll-your-own sort routine rather than using qsort provided by the C library. With qsort all you need to do is write a compare function that compares elements of the boat_data[] array member of race_node. The compare function prototype is:

 int compare (const void *a, const void *b)

Where all a and b are, are pointers to elements of the array boat_data[]. So within compare you simply need to cast a and b to the correct type (e.g. struct boat_node const *pa = a, *pb = b; or if you complete your typedef on your first struct, simply boat_node const *pa = a, *pb = b;).

Then compare pa->time_to_complete_race and pb->time_to_complete_race returning -1 if pa->time_to_complete_race sorts before pb->time_to_complete_race or 1 if pb->time_to_complete_racesorts before pa->time_to_complete_race, or 0 if they are equal (note: exactly the way strcmp() does)

Your compare function is then:

int compare (const void *a, const void *b)
{
    boat_node const *pa = a, *pb = b;

    return (pa->time_to_complete_race > pb->time_to_complete_race) -
            (pa->time_to_complete_race < pb->time_to_complete_race);
}

note: after completing your typedef, e.g.

typedef struct boat_data {
    int ID;
    int time_to_complete_race;
    int points;
} boat_node;

Then to sort your boat_data[] array which is a member of race_node race , all you do is call:

    qsort (race.boat_data, race.num_boats_competing, 
            sizeof *race.boat_data, compare);

(done!)

New C programmers are often hesitant to use qsort because they don't know how to write the compare function. After you make friends with the fact that a and b are just pointers to elements of whatever you are sorting, you can easily provide a cast to the proper type and then a comparison that tells qsort how you want it sorted.

In this case you simply want the array of boat_data[] sorted by time_to_complete_race. The return (a > b) - (a < b) form is simply a convenient way to avoid potential overflow were you tempted to return a - b; where, e.g., a is a large negative integer and b a large positive integer.

Full Example

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

#define MAX_BOAT_NUMBER 10

typedef struct boat_data {
    int ID;
    int time_to_complete_race;
    int points;
} boat_node;

typedef struct race_result {
    char race_date[80];
    int start_time;
    int num_boats_competing;
    boat_node boat_data[MAX_BOAT_NUMBER];
} race_node;

int compare (const void *a, const void *b)
{
    boat_node const *pa = a, *pb = b;

    return (pa->time_to_complete_race > pb->time_to_complete_race) -
            (pa->time_to_complete_race < pb->time_to_complete_race);
}

int main (void) {

    race_node race = { .race_date = "11/26/19",
                       .start_time = 1400,
                       .num_boats_competing = 3,
                       .boat_data = {{ 1, 23, 0 },
                                     { 2, 21, 0 },
                                     { 3, 22, 0 }} };

    qsort (race.boat_data, race.num_boats_competing, 
            sizeof *race.boat_data, compare);

    for (int i = 0; i < race.num_boats_competing; i++)
        printf ("%2d  %4d  %d\n", race.boat_data[i].ID,
                race.boat_data[i].time_to_complete_race,
                race.boat_data[i].points);
}

Example Use/Output

The points members were left all 0:

$ ./bin/boat_race
 2    21  0
 3    22  0
 1    23  0

Much easier than trying to write your own sort. Look things over and let me know if you have further questions.