Arrays of Structures and Pointers to Structures

Real programs work with many records: all the students in a class, all the products in a shop. An array of structures stores them, and pointers to structures let functions read and update records efficiently. The "student record" or "employee database" program built here is a standard lab and mini-project exercise.

An array of structures

#include <stdio.h>

typedef struct {
    int roll;
    char name[20];
    int marks[3];
} Student;

int main(void) {
    Student cls[] = {
        {1, "Asha",  {78, 85, 92}},
        {2, "Ravi",  {65, 70, 58}},
        {3, "Meera", {90, 88, 95}},
    };
    int n = sizeof cls / sizeof cls[0];

    int top = 0, best_total = -1;
    for (int i = 0; i < n; i++) {
        int total = cls[i].marks[0] + cls[i].marks[1] + cls[i].marks[2];
        printf("%d %-6s total %3d  average %.1f\n", cls[i].roll, cls[i].name, total, total / 3.0);
        if (total > best_total) { best_total = total; top = i; }
    }
    printf("Topper: %s with %d\n", cls[top].name, best_total);
    return 0;
}

1 Asha   total 255  average 85.0
2 Ravi   total 193  average 64.3
3 Meera  total 273  average 91.0
Topper: Meera with 273

Pointers to structures and the arrow operator

#include <stdio.h>

typedef struct {
    char name[20];
    double balance;
} Account;

int main(void) {
    Account acc = {"Asha", 5000.0};
    Account *p = &acc;

    printf("%s has %.2f\n", (*p).name, (*p).balance);   // dereference, then dot
    p->balance += 1500;                                 // arrow: the same, shorter
    printf("%s has %.2f\n", p->name, p->balance);
    return 0;
}

Asha has 5000.00
Asha has 6500.00

p->member means (*p).member. The brackets in the long form are required, because . binds tighter than *.

Passing structures to functions

#include <stdio.h>
#include <stdbool.h>

typedef struct {
    char name[20];
    double balance;
} Account;

void print_account(const Account *a) {          // read-only: const pointer, no copy
    printf("%-6s %10.2f\n", a->name, a->balance);
}

bool withdraw(Account *a, double amount) {      // modifies the caller's struct
    if (amount <= 0 || amount > a->balance) return false;
    a->balance -= amount;
    return true;
}

Account with_interest(Account a, double rate) { // by value: works on a copy, returns a new struct
    a.balance *= 1 + rate / 100;
    return a;
}

int main(void) {
    Account acc = {"Ravi", 2000.0};
    print_account(&acc);
    printf("withdraw 500: %s\n", withdraw(&acc, 500) ? "ok" : "refused");
    printf("withdraw 9000: %s\n", withdraw(&acc, 9000) ? "ok" : "refused");
    Account next_year = with_interest(acc, 7.5);
    print_account(&acc);
    print_account(&next_year);
    return 0;
}

Ravi      2000.00
withdraw 500: ok
withdraw 9000: refused
Ravi      1500.00
Ravi      1612.50

Sorting and searching records

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

typedef struct {
    int id;
    char name[20];
    double salary;
} Employee;

int by_salary_desc(const void *a, const void *b) {
    const Employee *x = a, *y = b;
    return (y->salary > x->salary) - (y->salary < x->salary);
}

int by_name(const void *a, const void *b) {
    return strcmp(((const Employee *)a)->name, ((const Employee *)b)->name);
}

Employee *find_by_id(Employee *list, int n, int id) {
    for (int i = 0; i < n; i++) if (list[i].id == id) return &list[i];
    return NULL;
}

int main(void) {
    Employee staff[] = {{11, "Kiran", 72000}, {12, "Asha", 95000}, {13, "Ravi", 61000}, {14, "Dev", 88000}};
    int n = sizeof staff / sizeof staff[0];

    qsort(staff, n, sizeof staff[0], by_salary_desc);
    printf("By salary:\n");
    for (int i = 0; i < n; i++) printf("  %-6s %.0f\n", staff[i].name, staff[i].salary);

    qsort(staff, n, sizeof staff[0], by_name);
    printf("By name:");
    for (int i = 0; i < n; i++) printf(" %s", staff[i].name);
    printf("\n");

    Employee *e = find_by_id(staff, n, 13);
    if (e != NULL) {
        e->salary *= 1.10;                       // raise via the pointer
        printf("%s now earns %.0f\n", e->name, e->salary);
    }
    return 0;
}

By salary:
  Asha   95000
  Dev    88000
  Kiran  72000
  Ravi   61000
By name: Asha Dev Kiran Ravi
Ravi now earns 67100

Practice

Summary