String Functions, Library Functions

String Functions and Library Functions in C

In C programming, string manipulation is essential for handling text-based data. C doesn't have a built-in string data type but instead uses character arrays, which are arrays of char terminated with a null character '\0'. To help manage strings, C provides a range of string functions in the <string.h> library. These functions allow you to perform common operations like copying, concatenating, comparing, and searching strings.

Let's explore the key string functions provided by the <string.h> library.

1. strlen() - Length of a String

The strlen() function returns the number of characters in a string, excluding the null-terminating character '\0'.

Syntax:

size_t strlen(const char *str);

Example:

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

int main() {
    char str[] = "Hello, World!";
    
    printf("Length of string: %zu\n", strlen(str));  // Output: 13
    
    return 0;
}

Explanation:

• strlen(str) returns 13, which is the number of characters in the string "Hello, World!", excluding the null terminator.

2. strcpy() - Copying Strings

The strcpy() function copies the content of one string into another. The destination string must be large enough to hold the copied string and the null-terminator.

Syntax:

char *strcpy(char *dest, const char *src);

Example:

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

int main() {
    char src[] = "Hello";
    char dest[20];
    
    strcpy(dest, src);  // Copies "Hello" into dest
    
    printf("Destination string: %s\n", dest);  // Output: Hello
    
    return 0;
}

Explanation:

• strcpy(dest, src) copies the string from src to dest, including the null-terminating character.

3. strcat() - Concatenating Strings

The strcat() function appends the source string to the destination string, starting from the null character '\0' in the destination string.

Syntax:

char *strcat(char *dest, const char *src);

Example:

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

int main() {
    char str1[20] = "Hello";
    char str2[] = " World!";
    
    strcat(str1, str2);  // Concatenates " World!" to "Hello"
    
    printf("Concatenated string: %s\n", str1);  // Output: Hello World!
    
    return 0;
}

Explanation:

• strcat(str1, str2) appends the content of str2 to str1 and the result is "Hello World!".

4. strcmp() - Comparing Strings

The strcmp() function compares two strings lexicographically (character by character).

• It returns:
  • 0 if the strings are equal.
  • A negative value if the first string is lexicographically less than the second.
  • A positive value if the first string is lexicographically greater than the second.

Syntax:

int strcmp(const char *str1, const char *str2);

Example:

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

int main() {
    char str1[] = "Apple";
    char str2[] = "Banana";
    
    int result = strcmp(str1, str2);
    
    if (result == 0) {
        printf("Strings are equal.\n");
    } else if (result < 0) {
        printf("'%s' is less than '%s'.\n", str1, str2);
    } else {
        printf("'%s' is greater than '%s'.\n", str1, str2);
    }
    
    return 0;
}

Explanation:

• strcmp(str1, str2) compares "Apple" and "Banana", and since "Apple" is lexicographically less than "Banana", it returns a negative value.

Output:

'Apple' is less than 'Banana'.

5. strchr() - Searching for a Character in a String

The strchr() function searches for the first occurrence of a character in a string. It returns a pointer to the first occurrence of the character or NULL if the character is not found.

Syntax:

char *strchr(const char *str, int c);

Example:

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

int main() {
    char str[] = "Hello, World!";
    char *ptr = strchr(str, 'W');  // Find the first occurrence of 'W'
    
    if (ptr != NULL) {
        printf("Character found: %c at position: %ld\n", *ptr, ptr - str);
    } else {
        printf("Character not found.\n");
    }
    
    return 0;
}

Explanation:

• strchr(str, 'W') finds the first occurrence of 'W' in the string "Hello, World!". It returns a pointer to that character, and the program prints its position.

Output:

Character found: W at position: 7

6. strstr() - Searching for a Substring

The strstr() function searches for the first occurrence of a substring in a string. It returns a pointer to the first character of the found substring or NULL if the substring is not found.

Syntax:

char *strstr(const char *haystack, const char *needle);

Example:

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

int main() {
    char str[] = "Hello, World!";
    char *result = strstr(str, "World");
    
    if (result != NULL) {
        printf("Substring found at: %s\n", result);
    } else {
        printf("Substring not found.\n");
    }
    
    return 0;
}

Explanation:

• strstr(str, "World") finds the first occurrence of the substring "World" in "Hello, World!" and returns a pointer to the beginning of the substring.

Output:

Substring found at: World!

7. strtok() - Tokenizing a String

The strtok() function is used to split a string into tokens based on delimiters. It modifies the original string by replacing delimiters with null characters.

Syntax:

char *strtok(char *str, const char *delim);

Example:

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

int main() {
    char str[] = "Hello, World! Welcome to C.";
    char *token = strtok(str, " ,.!");  // Delimiters: space, comma, dot, exclamation mark
    
    while (token != NULL) {
        printf("Token: %s\n", token);
        token = strtok(NULL, " ,.!");  // Get the next token
    }
    
    return 0;
}

Explanation:

• strtok(str, " ,.!") breaks the string "Hello, World! Welcome to C." into tokens separated by spaces, commas, dots, and exclamation marks.
• The function continues to tokenize the string in the loop until all tokens are processed.

Output:

Token: Hello
Token: World
Token: Welcome
Token: to
Token: C

8. sprintf() - Formatting Strings

The sprintf() function formats data and stores it as a string. It works similarly to printf(), but instead of printing to the console, it stores the result in a string.

Syntax:

int sprintf(char *str, const char *format, ...);

Example:

#include <stdio.h>

int main() {
    char str[50];
    int age = 25;
    
    sprintf(str, "I am %d years old.", age);  // Format and store in str
    
    printf("%s\n", str);  // Output: I am 25 years old.
    
    return 0;
}

Explanation:

• sprintf(str, "I am %d years old.", age) stores the formatted string "I am 25 years old." in str.

Summary of Key String Functions in C

• strlen(): Returns the length of a string.
• strcpy(): Copies one string to another.
• strcat(): Appends one string to another.
• strcmp(): Compares two strings.
• strchr(): Searches for a character in a string.
• strstr(): Searches for a substring in a string.
• strtok(): Tokenizes a string into smaller parts (tokens).
• sprintf(): Formats and stores a formatted string.

The <string.h> library is a powerful tool for manipulating strings in C, and it provides essential functions for handling text efficiently. Understanding these functions will allow you to perform a wide variety of string operations in your programs.