Arrays and strings

A class has 25 students, and you need to store a mark for each of them. Creating 25 separate variables — mark1, mark2, …, mark25 — is tiring and awkward: you cannot walk through them with a loop. That is what an array is for: a row of values of the same type placed side by side in memory. In C, text is an array too — simply an array of characters.

Arrays

To declare an array, you write the type, the name and the number of elements in square brackets: int marks[5];. Elements are accessed by index, and indexes start at 0: the first element is marks[0] and the last is marks[4]. Starting values can be given in curly braces; if you give fewer values, the remaining elements become 0. For example, int counts[10] = {0}; sets every element to zero.

#include <stdio.h>

int main(void) {
    int marks[5] = {5, 4, 3, 5, 4};
    int n = sizeof(marks) / sizeof(marks[0]);
    int sum = 0;
    int best = marks[0];
    for (int i = 0; i < n; i++) {
        sum += marks[i];
        if (marks[i] > best) {
            best = marks[i];
        }
    }
    printf("Count: %d\n", n);
    printf("First: %d, last: %d\n", marks[0], marks[n - 1]);
    printf("Average: %.2f\n", (double) sum / n);
    printf("Best: %d\n", best);
    return 0;
}

Count: 5
First: 5, last: 4
Average: 4.20
Best: 5

When you pass an array to a function, the function does not know its length, so the length is passed as a separate parameter. There is another difference: unlike an ordinary variable, the array is not copied — the function works with the original elements and can change them. You will see why in the next lesson, together with pointers. Writing const on the parameter of a function that does not change the array is a good habit.

#include <stdio.h>

int find(const int arr[], int n, int target) {
    for (int i = 0; i < n; i++) {
        if (arr[i] == target) {
            return i;
        }
    }
    return -1;
}

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

int main(void) {
    int prices[4] = {3, 7, 1, 10};
    printf("7 is at index %d\n", find(prices, 4, 7));
    printf("5 is at index %d\n", find(prices, 4, 5));
    double_all(prices, 4);
    printf("After: %d %d %d %d\n", prices[0], prices[1], prices[2], prices[3]);
    return 0;
}

7 is at index 1
5 is at index -1
After: 6 14 2 20

Strings: character arrays ending in '\0'

C has no separate string type. A string is a char array with a special null character '\0' at the end. Functions such as printf and strlen find the end of a string by looking for this character. When you write char word[] = "milk";, the compiler creates a 5-byte array: 4 letters plus '\0'.

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

int main(void) {
    char word[] = "milk";
    printf("Word: %s\n", word);
    printf("strlen: %zu\n", strlen(word));
    printf("sizeof: %zu\n", sizeof(word));
    printf("First letter: %c\n", word[0]);
    word[0] = 's';
    printf("Changed: %s\n", word);
    return 0;
}

Word: milk
strlen: 4
sizeof: 5
First letter: m
Changed: silk

The string.h functions

You cannot assign one array to another with = or compare strings with ==. For this you use the functions from the string.h header: strcpy(dest, src) copies a string, strcat(dest, src) appends one to the end, and strcmp(a, b) compares. strcmp returns 0 if the strings are the same, a negative number if a comes first in alphabetical order, and a positive number if it comes later.

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

int main(void) {
    char first[20] = "Baku";
    char second[20];
    strcpy(second, first);
    printf("Copy: %s\n", second);
    strcat(second, " city");
    printf("Joined: %s (%zu chars)\n", second, strlen(second));
    if (strcmp(first, "Baku") == 0) {
        printf("Equal\n");
    }
    if (strcmp("apple", "banana") < 0) {
        printf("apple comes first\n");
    }
    return 0;
}

Copy: Baku
Joined: Baku city (9 chars)
Equal
apple comes first