**Course Title:** Mastering C: From Fundamentals to Advanced Programming **Section Title:** Arrays and Strings **Topic:** Working with strings: string functions in C. ### Introduction In the previous topics, we covered arrays and their applications in C programming. Now, we'll focus on strings, which are essentially arrays of characters. Strings are a fundamental data type in C programming and are used to store and manipulate text data. In this topic, we'll explore string functions in C, which are used to perform various operations on strings. ### String Functions in C C programming provides several string functions that can be used to perform various operations on strings. These functions are defined in the `string.h` header file. Here are some of the most commonly used string functions in C: #### 1. `strcpy()`: Copying a String The `strcpy()` function is used to copy the contents of one string to another. ```c char str1[] = "Hello, World!"; char str2[20]; strcpy(str2, str1); printf("%s\n", str2); ``` Output: ``` Hello, World! ``` #### 2. `strcat()`: Concatenating Strings The `strcat()` function is used to concatenate two strings. ```c char str1[] = "Hello, "; char str2[] = "World!"; strcat(str1, str2); printf("%s\n", str1); ``` Output: ``` Hello, World! ``` #### 3. `strcmp()`: Comparing Strings The `strcmp()` function is used to compare two strings. ```c char str1[] = "Apple"; char str2[] = "Banana"; if (strcmp(str1, str2) == 0) { printf("Strings are equal\n"); } else { printf("Strings are not equal\n"); } ``` Output: ``` Strings are not equal ``` #### 4. `strlen()`: Getting the Length of a String The `strlen()` function is used to get the length of a string. ```c char str[] = "Hello, World!"; printf("Length of the string: %d\n", strlen(str)); ``` Output: ``` Length of the string: 13 ``` #### 5. `strupr()`, `strlwr()`, `strupr_l()`, and `tolower()`: Converting Strings to Uppercase and Lowercase The `strupr()` and `strlwr()` functions are not standard in C programming and the usage should be avoided. However, GNU version of libc library supports them. The standard functions to convert strings to lowercase and uppercase are `tolower()` and `toupper()` respectively (for individual characters), but you need to implement the corresponding functions for whole strings. ```c #include <ctype.h> void upcase(char *str) { while (*str) { *str = toupper(*str); str++; } } void downcase(char *str) { while (*str) { *str = tolower(*str); str++; } } int main() { char str[] = "HeLlO, WoRlD!"; upcase(str); printf("%s\n", str); downcase(str); printf("%s\n", str); return 0; } ``` Output: ``` HELLO, WORLD! hello, world! ``` #### 6. `strrev()`: Reversing a String There is no standard C function to reverse a string. But you can implement it yourself. ```c #include <stdio.h> #include <string.h> void reverse(char *str) { int len = strlen(str); char *p1 = str, *p2 = str + len - 1; while (p1 < p2) { char temp = *p1; *p1 = *p2; *p2 = temp; p1++; p2--; } } int main() { char str[] = "Hello, World!"; reverse(str); printf("%s\n", str); return 0; } ``` Output: ``` !dlroW ,olleH ``` For more information on string functions in C, refer to [cppreference.com: String functions in C](https://en.cppreference.com/w/c/string). ### Key Takeaways * C programming provides several string functions that can be used to perform various operations on strings. * `strcpy()` is used to copy the contents of one string to another. * `strcat()` is used to concatenate two strings. * `strcmp()` is used to compare two strings. * `strlen()` is used to get the length of a string. * The standard functions to convert strings to lowercase and uppercase are not directly provided, but the usage `tolower()` and `toupper()` for individual characters has to be implemented within loops in C. ### Comment/Help Please comment below for more clarification or ask for help in case you encounter any difficulty.