Thread: Help with C program sorting structs

I have everything except for the last two prototype functions, I think. Please help.

Code:

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


#define MAX_STUDENTS 20
#define MAX_LINE 100


// field widths
#define FIRST 7
#define MIDDLE 1
#define LAST 9
#define STREET 16
#define CITY 11
#define STATE 2
#define ZIP 5
#define AGE 2
#define GPA 4


typedef struct {
    char street[STREET + 1];
    char city[CITY + 1];
    char state[STATE + 1];
    char zip[ZIP + 1];
} Address;


typedef struct {
    char first[FIRST + 1];
    char initial[MIDDLE + 1];
    char last[LAST + 1];
    Address address;
    int age;
    double gpa;
} Student;


// prototypes
void strsub(char *buffer, char *s, int start, int end);
int readData(Student students[]);
void outputData(Student students[], int num);
void bestGPA(Student students[], int num);
double averageGPA(Student students[], int num);
void aboveAverage(Student students[], int num, double avgGPA);
void youngestBelowAverage(Student students[], int num, double avgGPA);
void sort(Student students[], int num);


// Lab_6.c: Students GPA
void main(void)
{
    Student students[MAX_STUDENTS];
    int num;
    double avgGPA;
    
    num = readData(students);
    outputData(students, num);
    bestGPA(students, num);
    avgGPA = averageGPA(students, num);
    aboveAverage(students, num, avgGPA);
    youngestBelowAverage(students, num, avgGPA);
    sort(students, num);
    outputData(students, num);
}


// strsub: copy buffer to s from start for size
void strsub(char *buffer, char *s, int start, int size)
{
    int i, j, finish;
    
    i = start;
    j = 0;
    finish = start + size;
    
    while (i < finish)
        s[j++] = buffer[i++];
        
    s[j] = '\0';
}


// readData: read file, populate array of structs, return # of Students
int readData(Student students[])
{
    int i = 0;
    char buf[MAX_LINE], temp[20];
    FILE *fp;
    
    if (!(fp = fopen("students.dat", "r"))) {
        puts("File not found.");
        exit(1);
    }
    
    while (!feof(fp)) {
        fgets(buf, MAX_LINE, fp);
        strsub(buf, students[i].first, 0, FIRST);
        strsub(buf, students[i].initial, 8, MIDDLE);
        strsub(buf, students[i].last, 10, LAST);
        strsub(buf, students[i].address.street, 20, STREET);
        strsub(buf, students[i].address.city, 37, CITY);
        strsub(buf, students[i].address.state, 49, STATE);
        strsub(buf, students[i].address.zip, 52, ZIP);
        strsub(buf, temp, 58, AGE);
        students[i].age = atoi(temp);
        strsub(buf, temp, 61, GPA);
        students[i].gpa = atof(temp);
        ++i;
    }
    return i;
}


// outputData: output array of structs in easily read format
void outputData(Student students[], int num)
{
    int i;
    
    printf("\nOUTPUT DATA\n");
    printf("-----------\n");
    for (i = 0; i < num; i++) {
        printf("%s ", students[i].first);
        printf("%s ", students[i].initial);
        printf("%s ", students[i].last);
        printf("%s ", students[i].address.street);
        printf("%s ", students[i].address.city);
        printf("%s ", students[i].address.state);
        printf("%s ", students[i].address.zip);
        printf("%d ", students[i].age);
        printf("%.2lf\n", students[i].gpa);
    }
}


// bestGPA: print the full name of the student with the best GPA
void bestGPA(Student students[], int num)
{
    int i, best;
    
    best = 0;
    
    for (i = 0; i < num; i++)
        if (students[i].gpa > students[best].gpa)
            best = i;
            
    printf("\nThe student with the best GPA is: %s %s %s\n", students[best].first, students[best].initial, students[best].last);
}


// averageGPA: calculate and print the average GPA
double averageGPA(Student students[], int num)
{
    int i;
    double avg;
    
    avg = 0;
    
    for (i = 0; i < num; i++)
        avg += students[i].gpa;
        
    printf("\nThe average GPA of all students is: %.2f\n", avg/num);
    
    return avg/num;     // required to compile
}


// aboveAverage: print the names of all students with GPAs above the average
void aboveAverage(Student students[], int num, double avgGPA)
{
    int i;
    
    printf("\nStudents with above average GPA:\n");
    
    for (i = 0; i < num; i++)
        if (students[i].gpa > avgGPA)
            printf("        %s %s %s\n", students[i].first, students[i].initial, students[i].last);
}


// youngestBelowAverage: print the name of the youngest student who has a GPA below average
void youngestBelowAverage(Student students[], int num, double avgGPA)
{
    
}


//sort: sort the structures in the array into order from lowest to highest GPA
void sort(Student students[], int num)
{
    
}

You should also check that you don't overflow your array in readData:

Code:

while (fgets(...) != NULL) {
    if (i >= MAX_STUDENTS) {
        printf("yikes\n");
        exit(EXIT_FAILURE);
    }
    ...
}