Bytebook Compiler

Arrays & Strings

Arrays

Store many values of one type under one name: declaring, indexing, looping, and the sum, max, search and reverse programs.

5 min read5 runnable examples4-question quiz

An array stores many values of the same type in one contiguous block of memory, under one name. Instead of mark1, mark2, mark3… you write marks[0], marks[1], marks[2].

C
#include <stdio.h>

int main(void) {
    int marks[5] = {78, 92, 65, 88, 71};
    printf("First mark: %d\n", marks[0]);
    printf("Last mark:  %d\n", marks[4]);
    marks[2] = 70;                    // change one element
    for (int i = 0; i < 5; i++)
        printf("marks[%d] = %d\n", i, marks[i]);
    return 0;
}
Output
First mark: 78
Last mark:  71
marks[0] = 78
marks[1] = 92
marks[2] = 70
marks[3] = 88
marks[4] = 71
Try it Yourself
marks780921702883714
int marks[5] — five ints side by side in memory. Indexes run from 0 to size − 1.

Declaring and initialising

C
int a[5];                       // 5 ints, values are garbage (if local)
int b[5] = {1, 2, 3, 4, 5};     // fully initialised
int c[5] = {1, 2};              // rest become 0  -> {1, 2, 0, 0, 0}
int d[5] = {0};                 // all zeros
int e[] = {4, 8, 15, 16, 23};   // size worked out from the list: 5

The number of elements is sizeof(arr) / sizeof(arr[0]) — handy when you initialise without writing the size.

Reading an array and finding sum, average, max and min

C
#include <stdio.h>

int main(void) {
    int n, a[100];
    printf("How many numbers? ");
    scanf("%d", &n);
    printf("Enter %d numbers: ", n);
    for (int i = 0; i < n; i++) scanf("%d", &a[i]);

    int sum = 0, max = a[0], min = a[0];
    for (int i = 0; i < n; i++) {
        sum += a[i];
        if (a[i] > max) max = a[i];
        if (a[i] < min) min = a[i];
    }
    printf("Sum = %d, Average = %.2f\n", sum, (float)sum / n);
    printf("Max = %d, Min = %d\n", max, min);
    return 0;
}
Output
How many numbers? 6
Enter 6 numbers: 45 12 89 33 89 7
Sum = 275, Average = 45.83
Max = 89, Min = 7
Sample input is includedTry it Yourself

Check each element until you find the target. In the worst case you look at all n elements: O(n).

C
#include <stdio.h>

int linearSearch(int a[], int n, int key) {
    for (int i = 0; i < n; i++)
        if (a[i] == key) return i;     // found: return its index
    return -1;                         // not found
}

int main(void) {
    int a[] = {14, 3, 27, 9, 41, 6};
    int n = sizeof(a) / sizeof(a[0]);
    printf("27 found at index %d\n", linearSearch(a, n, 27));
    printf("50 found at index %d\n", linearSearch(a, n, 50));
    return 0;
}
Output
27 found at index 2
50 found at index -1
Try it Yourself

Notice how arrays are passed to functions: int a[] plus the size n. Inside the function, sizeof(a) would not give the array size — the function only receives the address of the first element. That is why the size is passed separately.

Reverse an array in place

Swap the first and last, then the second and second-last, meeting in the middle:

C
#include <stdio.h>

int main(void) {
    int a[] = {1, 2, 3, 4, 5, 6, 7};
    int n = 7;
    for (int i = 0, j = n - 1; i < j; i++, j--) {
        int t = a[i];
        a[i] = a[j];
        a[j] = t;
    }
    for (int i = 0; i < n; i++) printf("%d ", a[i]);
    printf("\n");
    return 0;
}
Output
7 6 5 4 3 2 1
Try it Yourself

Counting frequencies

Use a second array as a tally. Here count[d] counts how often digit d appears:

C
#include <stdio.h>

int main(void) {
    int votes[] = {2, 3, 2, 1, 2, 3, 3, 3, 0, 2};
    int count[4] = {0};
    for (int i = 0; i < 10; i++) count[votes[i]]++;
    for (int c = 0; c < 4; c++) printf("Candidate %d: %d votes\n", c, count[c]);
    return 0;
}
Output
Candidate 0: 1 votes
Candidate 1: 1 votes
Candidate 2: 4 votes
Candidate 3: 4 votes
Try it Yourself

Test yourself

4 questions · pick an answer to check it instantly

  1. 1

    What is the index of the last element in int a[10];?

  2. 2

    What does int a[5] = {3, 4}; contain?

  3. 3

    Why do functions that take arrays usually also take the size?

  4. 4

    What is the worst-case time of linear search on n elements?

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