Strings in C

# CHAPTER 11

Strings in C #

1. Introduction #

2. Learning Objectives #

• Declare and initialize strings as character arrays.

• Understand the role of the Null Terminator (\0).

• Read and print strings using scanf, gets, and printf.

• Use the <string.h> library functions (strlen, strcpy, strcmp, strcat).

3. Declaring and Initializing Strings #

// Method 1: Using a string literal (Null terminator added automatically)
char greeting[6] = "Hello"; 

// Method 2: Character by character (Must manually add '\0')
char name[] = {'A', 'l', 'i', 'c', 'e', '\0'};

4. The Null Terminator (\0) #

Memory layout of "Hello":
+---+---+---+---+---+----+
| H | e | l | l | o | \0 |
+---+---+---+---+---+----+
  0   1   2   3   4   5

5. Input and Output of Strings #

#include <stdio.h>

int main() {
    char name[50];
    
    printf("Enter your first name: ");
    scanf("%s", name); // No '&' needed for strings!
    
    printf("Welcome, %s!\n", name);
    return 0;
}

*Warning:* scanf("%s") stops reading when it hits a space. To read a full line (like "John Doe"), use fgets():

fgets(name, sizeof(name), stdin);

6. The <string.h> Library #

#### 1. strlen() - String Length Returns the number of characters (excluding \0).

char str[] = "Apple";
int len = strlen(str); // len is 5

#### 2. strcpy() - String Copy You CANNOT use str1 = str2 in C. You must copy the contents.

char source[] = "Hello";
char destination[20];
strcpy(destination, source); // Copies "Hello" into destination

#### 3. strcat() - String Concatenation Joins two strings together.

char str1[20] = "Hello ";
char str2[] = "World";
strcat(str1, str2); // str1 is now "Hello World"

#### 4. strcmp() - String Compare Compares two strings. Returns 0 if they are identical.

char pwd1[] = "password123";
char pwd2[] = "password123";
if (strcmp(pwd1, pwd2) == 0) {
    printf("Passwords match!\n");
}

7. Mini Project: Password Validator #

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

int main() {
    char correct_pwd[] = "admin123";
    char input_pwd[50];

    printf("=== LOGIN SYSTEM ===\n");
    printf("Enter password: ");
    scanf("%s", input_pwd);

    if (strcmp(correct_pwd, input_pwd) == 0) {
        printf("Access Granted.\n");
    } else {
        printf("Access Denied.\n");
    }

    return 0;
}

8. Common Mistakes #

• Forgetting \0 room: Declaring char word[5] = "Hello"; causes an overflow because there's no room for the 6th character (\0).

• Using == to compare strings: if (str1 == str2) compares their *memory addresses*, not their text. Always use strcmp().

• Using & in scanf for strings: scanf("%s", &name) is wrong. Use scanf("%s", name).

9. Exercises #

1. 1. Write a program that asks for the user's first and last name separately, concatenates them using strcat, and prints the full name.

1. 2. Write a program to manually calculate the length of a string using a loop (without using strlen).

1. 3. Write a program to reverse a string manually.

10. MCQ Quiz with Answers #

11. Interview Questions #

• Q: Explain why C doesn't have a String data type, and how it handles text instead.

• Q: How does strlen() work internally?

• Q: What is a buffer overflow, and how can strings cause it?

12. Summary #

13. Next Chapter Recommendation #