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Arrays

Store many values of the same type in one variable.

An array is a fixed-size row of values of the same type, stored next to each other in memory. You choose its size when you declare it, and you reach each element by its index, starting at 0.

C
Open in playground
#include <stdio.h>

int main(void) {
    int temps[5] = {31, 29, 33, 35, 30};

    printf("First: %d\n", temps[0]);
    printf("Last: %d\n", temps[4]);

    temps[1] = 28;   /* change an element */
    for (int i = 0; i < 5; i++) {
        printf("%d ", temps[i]);
    }
    printf("\n");
    return 0;
}
Output
First: 31
Last: 30
31 28 33 35 30

Finding the size

sizeof on an array gives its total size in bytes. Dividing by the size of one element gives the number of elements.

C
Open in playground
#include <stdio.h>

int main(void) {
    double prices[] = {9.5, 12.0, 4.25, 30.0};
    int n = sizeof(prices) / sizeof(prices[0]);

    double max = prices[0];
    for (int i = 1; i < n; i++) {
        if (prices[i] > max) {
            max = prices[i];
        }
    }
    printf("%d prices, highest is %.2f\n", n, max);
    return 0;
}
Output
4 prices, highest is 30.00

Passing arrays to functions

When you pass an array to a function, pass its length too; the function cannot work it out on its own.

C
Open in playground
#include <stdio.h>

void reverse(int arr[], int n) {
    for (int i = 0; i < n / 2; i++) {
        int tmp = arr[i];
        arr[i] = arr[n - 1 - i];
        arr[n - 1 - i] = tmp;
    }
}

int main(void) {
    int nums[] = {1, 2, 3, 4, 5, 6};
    reverse(nums, 6);
    for (int i = 0; i < 6; i++) {
        printf("%d ", nums[i]);
    }
    printf("\n");
    return 0;
}
Output
6 5 4 3 2 1

Unlike ordinary variables, the function changes the caller's array. The reason is explained in the lesson on pointers and arrays.

Two-dimensional arrays

C
Open in playground
#include <stdio.h>

int main(void) {
    int grid[2][3] = {
        {1, 2, 3},
        {4, 5, 6}
    };
    for (int r = 0; r < 2; r++) {
        for (int c = 0; c < 3; c++) {
            printf("%3d", grid[r][c] * 10);
        }
        printf("\n");
    }
    return 0;
}
Output
 10 20 30
 40 50 60

GATE-style practice questions

GATE-styleMCQ · one correct answer

What does the following program print?

#include <stdio.h>
int main(void) {
    int a[] = {10, 20, 30, 40, 50};
    int n = sizeof(a) / sizeof(a[0]);
    int s = 0;
    for (int i = 0; i < n; i += 2) s += a[i];
    printf("%d %d\n", n, s);
    return 0;
}
Show answer and explanationAnswer: B

sizeof(a) / sizeof(a[0]) counts the elements: 5. The loop adds every second element, a[0] + a[2] + a[4] = 10 + 30 + 50 = 90.

GATE-styleNAT · numerical answer

What does the following program print?

#include <stdio.h>
void f(int a[]) {
    a[0] = 99;
}
int main(void) {
    int x[3] = {1, 2, 3};
    f(x);
    printf("%d\n", x[0] + x[2]);
    return 0;
}
Show answer and explanationAnswer: 102

An array argument is passed as a pointer to its first element, so f changes the caller's array: x[0] becomes 99. Then x[0] + x[2] = 99 + 3 = 102.

GATE-styleNAT · numerical answer

What does the following program print?

#include <stdio.h>
int main(void) {
    int a[5] = {1, 2};
    printf("%d\n", a[0] + a[1] + a[4]);
    return 0;
}
Show answer and explanationAnswer: 3

When an array is partly initialised, the remaining elements are set to 0. So a = {1, 2, 0, 0, 0} and a[0] + a[1] + a[4] = 1 + 2 + 0 = 3.