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

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.