Arrays in C

Storing the marks of 60 students in 60 separate variables would be absurd. An array stores many values of the same type under one name, side by side in memory, and lets you reach each by its position. Arrays are the most used data structure in C – and the basis of strings, matrices and most algorithms.

Declaring and using an array

#include <stdio.h>

int main(void) {
    int marks[5] = {78, 91, 64, 85, 70};   // 5 ints, indexes 0 to 4
    marks[2] = 68;                          // change the third element

    printf("first = %d, last = %d\n", marks[0], marks[4]);
    for (int i = 0; i < 5; i++) {
        printf("marks[%d] = %d\n", i, marks[i]);
    }

    int zeros[6] = {0};                     // all six elements set to 0
    int primes[] = {2, 3, 5, 7, 11};        // size worked out from the list: 5
    int n = sizeof primes / sizeof primes[0];
    printf("zeros[5] = %d, primes has %d elements\n", zeros[5], n);
    return 0;
}

first = 78, last = 70
marks[0] = 78
marks[1] = 91
marks[2] = 68
marks[3] = 85
marks[4] = 70
zeros[5] = 0, primes has 5 elements

Indexes start at 0, so an array of size n has indexes 0 to n − 1. Because elements are consecutive, the address of marks[i] is simply start + i × sizeof(int) – that's why indexing is instant, whatever the size.

Common array operations

#include <stdio.h>

int main(void) {
    int a[] = {34, 72, 15, 90, 48, 63};
    int n = sizeof a / sizeof a[0];

    int sum = 0, max = a[0], min = a[0], max_pos = 0;
    for (int i = 0; i < n; i++) {
        sum += a[i];
        if (a[i] > max) { max = a[i]; max_pos = i; }
        if (a[i] < min) min = a[i];
    }
    printf("sum=%d avg=%.2f max=%d (at index %d) min=%d\n", sum, (double)sum / n, max, max_pos, min);

    for (int i = 0, j = n - 1; i < j; i++, j--) {      // reverse in place
        int t = a[i]; a[i] = a[j]; a[j] = t;
    }
    printf("reversed:");
    for (int i = 0; i < n; i++) printf(" %d", a[i]);
    printf("\n");
    return 0;
}

sum=322 avg=53.67 max=90 (at index 3) min=15
reversed: 63 48 90 15 72 34

Passing arrays to functions

An array passed to a function isn't copied: the function receives the address of the first element. So the function can modify the caller's array – and it doesn't know the length, so you must pass it too:

#include <stdio.h>

void print_array(const int a[], int n) {        // const: promises not to modify
    for (int i = 0; i < n; i++) printf("%d ", a[i]);
    printf("\n");
}

void add_bonus(int a[], int n, int bonus) {     // modifies the caller's array
    for (int i = 0; i < n; i++) a[i] += bonus;
}

int count_above(const int a[], int n, int limit) {
    int count = 0;
    for (int i = 0; i < n; i++) if (a[i] > limit) count++;
    return count;
}

int main(void) {
    int marks[] = {38, 72, 55, 91, 40};
    int n = sizeof marks / sizeof marks[0];
    print_array(marks, n);
    add_bonus(marks, n, 5);
    print_array(marks, n);
    printf("%d students above 50\n", count_above(marks, n, 50));
    return 0;
}

38 72 55 91 40 
43 77 60 96 45 
3 students above 50

Out-of-bounds access

C doesn't check indexes. Reading or writing marks[5] in a 5-element array touches whatever memory comes next – a silent bug that can corrupt other variables or crash the program. It's also behind many real security vulnerabilities (buffer overflows). The sanitizer flags it immediately:

gcc -fsanitize=address -g program.c -o program   # AddressSanitizer reports out-of-bounds reads and writes

Always loop with i < n, never i <= n.

Practice

Summary