Bytebook Compiler

Arrays & Strings

Strings

Character arrays ending in '\0': reading strings, the string.h functions, and writing length, reverse, palindrome and vowel-count programs yourself.

4 min read5 runnable examples4-question quiz

C has no special string type. A string is a char array that ends with the null character '\0'. Every string function relies on that terminator to know where the text stops.

C
#include <stdio.h>

int main(void) {
    char name[] = "ASHA";            // the compiler adds '\0' for you
    printf("%s has %zu bytes of storage\n", name, sizeof(name));
    for (int i = 0; name[i] != '\0'; i++)
        printf("name[%d] = '%c' (%d)\n", i, name[i], name[i]);
    return 0;
}
Output
ASHA has 5 bytes of storage
name[0] = 'A' (65)
name[1] = 'S' (83)
name[2] = 'H' (72)
name[3] = 'A' (65)
Try it Yourself
nameA0S1H2A3\04
char name[] = "ASHA" takes 5 bytes: four letters plus the terminating '\0'.

Declaring strings

C
char a[20] = "Hello";        // 20 bytes, "Hello" + '\0' + unused space
char b[] = "Hello";          // exactly 6 bytes
char c[] = {'H', 'i', '\0'}; // character by character — don't forget '\0'

string.h functions

Function What it does
strlen(s) length, not counting '\0'
strcpy(dst, src) copy src into dst
strncpy(dst, src, n) copy at most n characters
strcat(dst, src) append src to the end of dst
strcmp(a, b) 0 if equal, negative if a < b, positive if a > b (dictionary order)
strchr(s, c) pointer to first c in s, or NULL
strstr(s, t) pointer to first occurrence of t in s, or NULL
C
#include <stdio.h>
#include <string.h>

int main(void) {
    char first[30] = "Data";
    char second[] = "Structures";
    printf("Length of \"%s\": %zu\n", first, strlen(first));
    strcat(first, " ");
    strcat(first, second);
    printf("Joined: %s\n", first);

    char copy[30];
    strcpy(copy, first);
    printf("Copy: %s\n", copy);

    printf("strcmp(\"apple\", \"banana\") = %d\n", strcmp("apple", "banana") < 0 ? -1 : 1);
    printf("strcmp(\"cat\", \"cat\") = %d\n", strcmp("cat", "cat"));
    printf("Found \"Struct\" at position %td\n", strstr(first, "Struct") - first);
    return 0;
}
Output
Length of "Data": 4
Joined: Data Structures
Copy: Data Structures
strcmp("apple", "banana") = -1
strcmp("cat", "cat") = 0
Found "Struct" at position 5
Try it Yourself

Write them yourself

Exams frequently say "without using library functions". These four are the most common:

C
#include <stdio.h>

int myStrlen(const char s[]) {
    int n = 0;
    while (s[n] != '\0') n++;
    return n;
}

void myReverse(char s[]) {
    for (int i = 0, j = myStrlen(s) - 1; i < j; i++, j--) {
        char t = s[i]; s[i] = s[j]; s[j] = t;
    }
}

int isPalindrome(const char s[]) {
    for (int i = 0, j = myStrlen(s) - 1; i < j; i++, j--)
        if (s[i] != s[j]) return 0;
    return 1;
}

int countVowels(const char s[]) {
    int v = 0;
    for (int i = 0; s[i]; i++) {
        char c = s[i] | 32;           // quick lowercase for letters
        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') v++;
    }
    return v;
}

int main(void) {
    char word[] = "Programming";
    printf("Length: %d\n", myStrlen(word));
    printf("Vowels: %d\n", countVowels(word));
    myReverse(word);
    printf("Reversed: %s\n", word);
    printf("\"madam\" palindrome? %s\n", isPalindrome("madam") ? "yes" : "no");
    printf("\"kalvium\" palindrome? %s\n", isPalindrome("kalvium") ? "yes" : "no");
    return 0;
}
Output
Length: 11
Vowels: 3
Reversed: gnimmargorP
"madam" palindrome? yes
"kalvium" palindrome? no
Try it Yourself

Changing case with ctype.h

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

int main(void) {
    char line[100];
    int letters = 0, digits = 0, spaces = 0;
    printf("Type a line: ");
    fgets(line, sizeof line, stdin);
    line[strcspn(line, "\n")] = '\0';
    for (int i = 0; line[i]; i++) {
        if (isalpha((unsigned char)line[i])) letters++;
        else if (isdigit((unsigned char)line[i])) digits++;
        else if (line[i] == ' ') spaces++;
        line[i] = toupper((unsigned char)line[i]);
    }
    printf("Upper case: %s\n", line);
    printf("Letters %d, digits %d, spaces %d\n", letters, digits, spaces);
    return 0;
}
Output
Type a line: Hello, CSE 101!
Upper case: HELLO, CSE 101!
Letters 8, digits 3, spaces 2
Sample input is includedTry it Yourself

Arrays of strings

A list of words is a 2D char array — one row per word:

C
#include <stdio.h>

int main(void) {
    char days[7][10] = {"Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"};
    for (int i = 0; i < 7; i++)
        printf("%d: %s\n", i + 1, days[i]);
    return 0;
}
Output
1: Mon
2: Tue
3: Wed
4: Thu
5: Fri
6: Sat
7: Sun
Try it Yourself

Test yourself

4 questions · pick an answer to check it instantly

  1. 1

    How many bytes does char s[] = "C101"; occupy?

  2. 2

    What does strlen("Kalvium") return?

  3. 3

    How should you check if two strings a and b have the same text?

  4. 4

    What does '\0' mark?

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