Example: qsort Structs by ID in C

// qsort Structs by ID
#include <stdio.h>
#include <stdlib.h> // For qsort
#include <string.h> // For strcpy

// Define a custom structure
typedef struct {
    int id;
    char name[50];
    float score;
} Student;

// Comparison function to sort Students by ID (ascending)
int compareStudentsByID(const void *a, const void *b) {
    // Cast void* to Student* and dereference to access members
    const Student *student_a = (const Student *)a;
    const Student *student_b = (const Student *)b;
    return student_a->id - student_b->id; // Compare IDs
}

// Comparison function to sort Students by Name (alphabetical)
int compareStudentsByName(const void *a, const void *b) {
    const Student *student_a = (const Student *)a;
    const Student *student_b = (const Student *)b;
    return strcmp(student_a->name, student_b->name); // Compare names using strcmp
}

void printStudents(const char *msg, Student students[], int n) {
    printf("%s:\n", msg);
    for (int i = 0; i < n; i++) {
        printf("  ID: %d, Name: %s, Score: %.1f\n", students[i].id, students[i].name, students[i].score);
    }
    printf("\n");
}

int main() {
    // Step 1: Initialize an array of Student structures
    Student students[] = {
        {103, "Alice", 85.5},
        {101, "Bob", 92.0},
        {105, "Charlie", 78.0},
        {102, "David", 88.5},
        {104, "Eve", 95.0}
    };
    int n = sizeof(students) / sizeof(students[0]);

    printStudents("Original student list", students, n);

    // Step 2: Sort students by ID
    qsort(students, n, sizeof(Student), compareStudentsByID);
    printStudents("Sorted students by ID", students, n);

    // Step 3: Sort students by Name (using the same array)
    qsort(students, n, sizeof(Student), compareStudentsByName);
    printStudents("Sorted students by Name", students, n);

    return 0;
}