Structures & File I/O

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

// struct definition
typedef struct {
    int id;
    char name[50];
    float gpa;
} Student;

// Enum
typedef enum { MON, TUE, WED, THU, FRI, SAT, SUN } Day;

// Union (shared memory)
typedef union {
    int intVal;
    float floatVal;
    char strVal[20];
} DataValue;  // Size = largest member (20 bytes)

// Function taking struct pointer (efficient — no copy)
void printStudent(const Student *s) {
    printf("ID: %d, Name: %s, GPA: %.2f\n", s->id, s->name, s->gpa);
}

int main() {
    // Create struct
    Student alice = {1, "Alice Johnson", 3.85};
    printStudent(&alice);

    // Array of structs
    Student class[3] = {
        {1, "Alice", 3.85},
        {2, "Bob", 3.42},
        {3, "Charlie", 3.91}
    };

    // Dynamic struct allocation
    Student *s = (Student *)malloc(sizeof(Student));
    s->id = 4;
    strcpy(s->name, "Diana");
    s->gpa = 3.67;
    free(s);

    // File I/O: Text files
    FILE *fp = fopen("students.txt", "w");
    if (fp == NULL) { perror("Error"); return 1; }
    
    for (int i = 0; i < 3; i++) {
        fprintf(fp, "%d,%s,%.2f\n", class[i].id, class[i].name, class[i].gpa);
    }
    fclose(fp);

    // Read text file
    fp = fopen("students.txt", "r");
    char line[100];
    while (fgets(line, sizeof(line), fp)) {
        printf("%s", line);
    }
    fclose(fp);

    // Binary file I/O (faster, compact)
    fp = fopen("students.bin", "wb");
    fwrite(class, sizeof(Student), 3, fp);
    fclose(fp);

    Student loaded[3];
    fp = fopen("students.bin", "rb");
    fread(loaded, sizeof(Student), 3, fp);
    fclose(fp);

    return 0;
}