📘 Section 12 — Strings in C

A string in C is just an array of characters that ends with a special character '\0' (the null terminator). That '\0' is what tells C “the string ends here.”

🎯 What You’ll Practice

• Understanding how strings are stored in memory as character arrays
• Declaring and initializing strings in different ways
• Reading strings with scanf and fgets
• Using common string functions: strlen, strcpy, strcat, strcmp, strncmp
• Writing your own versions of strlen and strcat from scratch
• Reversing a string, counting characters, and applying simple string-based rules

1. What Is a String in C?

Unlike many other languages, C does not have a built-in string type. A string in C is simply a character array that ends with the null terminator '\0'.

#include <stdio.h>
int main() {
    char text[20];                  // Declaration (19 chars plus '\0' terminator)
    char text2[] = "Hello Ahmed!";  // Initialized — size auto-calculated

    return 0;
}

💡 When you write a string literal like "Hello Ahmed!" between double quotes, C automatically appends '\0' at the end for you. So "Hello Ahmed!" actually takes 13 characters in memory, not 12.

How a String Looks in Memory

Take the string "HELLO!". Here is exactly how it sits in a char array:

 Index:    0     1     2     3     4     5     6
        +-----+-----+-----+-----+-----+-----+------+
        | 'H' | 'E' | 'L' | 'L' | 'O' | '!' | '\0' |
        +-----+-----+-----+-----+-----+-----+------+

⚠️ Without the '\0', C functions like strlen or printf("%s", ...) keep reading past the end of your array into garbage memory — until they accidentally find a zero byte somewhere. Always make sure your strings are properly terminated.

2. Reading and Manipulating Strings

C gives you a small set of helper functions in <string.h> for working with strings:

Reading a String from the User

There are several ways to read a string in C, but they differ in whether they handle spaces and whether they’re safe from buffer overflows.

#include <stdio.h>

int main() {
    char input[20];

    // scanf("%s", input);        // stops at the first space
    // scanf("%[^\n]", input);    // reads until '\n', but no size limit
    // gets(input);               // removed from the C standard — not recommended use
    fgets(input, 20, stdin);      // ✅ reads a full line, with a size limit

    printf("%s\n", input);
    return 0;
}

💡 With scanf("%s", ...), you don’t need & — the array name already decays to its address.

⚠️ fgets keeps the trailing newline. If the user types ahmed mohamed and presses Enter, the buffer holds "ahmed mohamed\n\0". Strip the '\n' yourself if you don’t want it.

Quick Example — strlen in Action

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

int main() {
    char text[] = "Hello Ahmed!";
    int len = strlen(text);
    printf("Length: %d\n", len);    // Length: 12
    return 0;
}

Quick Example — strcat in Action

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

int main() {
    char dest[50] = "Hello, ";
    char src[]    = "World!";
    strcat(dest, src);              // dest is now "Hello, World!"
    printf("%s\n", dest);
    return 0;
}

⚠️ The destination array (dest) must be big enough to hold both strings combined plus the '\0'. Otherwise you overflow and corrupt memory.

🧪 Exercises

Exercise 1: Length of a String Without strlen()

Task: Write a C program that calculates the length of a string without using the built-in strlen() function.

#include <stdio.h>

int main()
{
    char s[] = "okayy";
    int i = 0;
    while (s[i] != '\0')
    {
        i++;
    }
    printf("count: %d\n", i);
    return 0;
}

count: 5

Exercise 2: Concatenate Two Strings Without strcat()

Task: Write a C program that concatenates two strings without using the built-in strcat() function.

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

int main(){
    char str2[20] = "okay1|";
    char str1[]   = "okay2";
    int len1 = strlen(str1);
    int j    = strlen(str2);

    for (int i = 0; i <= len1; i++)   // <= to also copy the '\0' at the end
    {
        str2[j] = str1[i];
        j++;
    }
    printf("%s\n", str2);
    return 0;
}

okay1|okay2

Exercise 3: Reverse a String

Task: Write a C program to reverse a given string.

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

// Solution 1 — actually reverses the array
void reverseString(char str[]) {
    int length = strlen(str);
    for (int i = 0; i < length / 2; i++) {
        // swap str[i] with str[length - i - 1]
        char temp = str[i];
        str[i] = str[length - i - 1];
        str[length - i - 1] = temp;
    }
}

// Solution 2 — only prints in reverse (does NOT modify the original)
void reverseString2(char str[]) {
    int length = strlen(str);
    for (int i = length - 1; i >= 0; i--) {
        printf("%c", str[i]);
    }
}

int main(){
    char str[] = "teacher";
    reverseString2(str);          // prints: rehcaet
    reverseString(str);           // now str itself becomes "rehcaet"
    printf("\n%s\n", str);
    return 0;
}

i = 0:  swap 't' and 'r'  →  "reachet"   (wait, let's do it properly)

Original:  t  e  a  c  h  e  r
Indices:   0  1  2  3  4  5  6

i = 0: swap [0] and [6]  → r e a c h e t
i = 1: swap [1] and [5]  → r e a c h e t  → r e a c h e t (e and e, no change)
i = 2: swap [2] and [4]  → r e h c a e t

Final:     r  e  h  c  a  e  t

Exercise 4: Count Occurrences of a Character

Task: Write a C program to count how many times a specific character appears in a string.

#include <stdio.h>

int main()
{
    char str[]   = "ahmad";
    char target  = 'a';
    int count    = 0;

    for (int i = 0; str[i] != '\0'; i++)
    {
        if (str[i] == target)
        {
            count++;
        }
    }
    printf("Count: %d", count);
    return 0;
}

Count: 2

Exercise 5: Verb → Past Tense

Task: Write a program that takes verbs and forms their past tense based on these rules:

1. If a verb ends in "e" → add "d".
2. If a verb ends in "ss" or "gh" → add "ed".
3. In all other cases → tell the user the verb may have an irregular past tense.

Print each verb and its past tense. Test with this data:

smile  discuss  confess  declare  laugh  run  cough  teach  buy

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

void past(char verb[]) {
    int length = strlen(verb);

    if (verb[length - 1] == 'e') {
        strcat(verb, "d");
    } else if (length >= 2 && verb[length - 2] == 's' && verb[length - 1] == 's') {
        strcat(verb, "ed");
    } else if (length >= 2 && verb[length - 2] == 'g' && verb[length - 1] == 'h') {
        strcat(verb, "ed");
    } else {
        printf("The past tense of '%s' may have an irregular past tense\n", verb);
        return;
    }
    printf("The past tense is %s\n", verb);
}

int main(){
    char verbs[][20] = {"smile", "discuss", "confess", "declare",
                        "laugh", "run", "cough", "teach", "buy"};
    int verbsCount = 9;     // or use sizeof(verbs)/sizeof(verbs[0])

    for (int i = 0; i < verbsCount; i++) {
        past(verbs[i]);
    }
    return 0;
}

The past tense is smiled
The past tense is discussed
The past tense is confessed
The past tense is declared
The past tense is laughed
The past tense of 'run' may have an irregular past tense
The past tense is coughed
The past tense of 'teach' may have an irregular past tense
The past tense of 'buy' may have an irregular past tense

Exercise 6: Split a Sentence Using strtok

Task: Write a C program that takes a sentence and prints each word on its own line using strtok (split by spaces).

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

int main() {
    char sentence[] = "Hello world from C";
    char *token = strtok(sentence, " ");

    while (token != NULL) {
        printf("%s\n", token);
        token = strtok(NULL, " ");
    }
    return 0;
}

Hello
world
from
C

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