An array is a fixed-size block of contiguous elements of one type, indexed from 0. C never checks the index — staying in bounds is your job.

Declaring and initialising

C

#include <stdio.h>

int main(void) {
    int a[5] = {10, 20, 30, 40, 50};
    int b[5] = {1, 2};              /* rest are zero: 1 2 0 0 0 */
    int c[] = {7, 8, 9};            /* size deduced: 3 */
    int d[5] = {0};                 /* all zeros */
    int e[6] = {[2] = 5, [4] = 9};  /* designated initializers (C99) */

    size_t n = sizeof a / sizeof a[0];     /* element count */
    printf("n=%zu sizeof(c)=%zu\n", n, sizeof c);
    for (int i = 0; i < 5; i++) printf("%d ", b[i]);
    printf("| %d | ", d[3]);
    for (int i = 0; i < 6; i++) printf("%d ", e[i]);
    printf("\n");
    return 0;
}

Output

n=5 sizeof(c)=12
1 2 0 0 0 | 0 | 0 0 5 0 9 0

Memory layout

Index	0	1	2	3	4
Value	10	20	30	40	50
Offset (bytes)	0	4	8	12	16

a[i] is defined as *(a + i): start address plus i × sizeof(int). That is why indexing is O(1) — and why an index past the end silently reads whatever lies next in memory.

Passing arrays to functions

C

#include <stdio.h>

/* the array decays to a pointer: the size must travel separately */
int max_of(const int arr[], size_t n) {
    int best = arr[0];
    for (size_t i = 1; i < n; i++)
        if (arr[i] > best) best = arr[i];
    return best;
}

void double_all(int *arr, size_t n) {     /* same as int arr[] */
    for (size_t i = 0; i < n; i++) arr[i] *= 2;
}

int main(void) {
    int v[] = {3, 9, 2, 7};
    size_t n = sizeof v / sizeof v[0];
    double_all(v, n);                      /* modifies the caller's array */
    printf("max = %d, v[0] = %d\n", max_of(v, n), v[0]);
    return 0;
}

Output

max = 18, v[0] = 6

Common array algorithms

C

#include <stdio.h>

int main(void) {
    int a[] = {4, -1, 7, 3, 7, 0};
    int n = sizeof a / sizeof a[0];

    int sum = 0, maxi = 0;
    for (int i = 0; i < n; i++) {
        sum += a[i];
        if (a[i] > a[maxi]) maxi = i;
    }
    printf("sum=%d max=%d at %d\n", sum, a[maxi], maxi);

    /* linear search */
    int key = 3, pos = -1;
    for (int i = 0; i < n; i++) if (a[i] == key) { pos = i; break; }
    printf("3 found at %d\n", pos);

    /* rotate left by one */
    int first = a[0];
    for (int i = 0; i < n - 1; i++) a[i] = a[i + 1];
    a[n - 1] = first;
    for (int i = 0; i < n; i++) printf("%d ", a[i]);
    printf("\n");
    return 0;
}

Output

sum=20 max=7 at 2
3 found at 3
-1 7 3 7 0 4

Variable-length arrays (C99)

C

#include <stdio.h>

int main(void) {
    int n = 4;
    int squares[n];                   /* size known only at run time: VLA on the stack */
    for (int i = 0; i < n; i++) squares[i] = i * i;
    printf("%d %zu\n", squares[3], sizeof squares);
    return 0;
}

Output

9 16

Out of bounds is undefined behaviour

C — caught by the sanitizer

#include <stdio.h>

int main(void) {
    int a[5] = {1, 2, 3, 4, 5};
    int sum = 0;
    for (int i = 0; i <= 5; i++)    /* off by one: touches a[5] */
        sum += a[i];
    printf("%d\n", sum);
    return 0;
}

UBSan

main.c:7:16: runtime error: index 5 out of bounds for type 'int[5]'

Why can't you assign one array to another with =?

What is array decay?