strcpy - Copy Strings

• Header: #include <string.h>
• Syntax: strcpy(dest, src)
• Purpose: Copies the string src into the character array dest
• Returns: Pointer to the start of dest
• Notes:
  • dest must be large enough to hold the copied string
  • The null terminator '\0' is also copied

strcat - Concatenate Strings

• Header: #include <string.h>
• Syntax: strcat(dest, src)
• Purpose: Appends the string src to the end of dest
• Returns: Pointer to the start of dest
• Notes:
  • dest must have enough space to accommodate the result
  • The null terminator from dest is removed and a new one added at the end

strcmp - Compare Strings

• Header: #include <string.h>
• Syntax: strcmp(str1, str2)
• Purpose: Compares two strings character by character
• Returns:
  • Negative if str1 is less than str2
  • Positive if str1 is greater than str2
  • Zero if they are equal
• Comparison is based on ASCII values until a mismatch or null terminator is found

isalpha - Check Alphabetic Characters

• Header: #include <ctype.h>
• Syntax: isalpha(c)
• Purpose: Checks if a character is an alphabetic letter
• Returns:
  • Non-zero if it's a letter
  • Zero otherwise

isupper - Check Uppercase Letters

• Header: #include <ctype.h>
• Syntax: isupper(c)
• Purpose: Determines if a character is an uppercase letter
• Returns:
  • Non-zero if it's uppercase
  • Zero otherwise

toupper - Convert to Uppercase

• Header: #include <ctype.h>
• Syntax: toupper(c)
• Purpose: Converts a lowercase character to uppercase
• Returns:
  • Uppercase version if input is lowercase
  • Original character otherwise

tolower - Convert to Lowercase

• Header: #include <ctype.h>
• Syntax: tolower(c)
• Purpose: Converts an uppercase character to lowercase
• Returns:
  • Lowercase version if enput is uppercase
  • Original character otherwise

strncpy - Copy Fixed-Length Strings

• Header: #include <string.h>
• Syntax: strncpy(dest, src, n)
• Purpose: Copies up to n characters from src to dest
• Notes:
  • If src is shorter than n, the rest of dest is padded with nulls
  • If src is longer than or equal to n, dest won't be null-terminated

strncat - Concatenate Limited Strings

• Header: #include <string.h>
• Syntax: strncat(dest, src, n)
• Purpose: Appends up to n characters from src to dest
• Notes:
  • The null terminator is preserved at the end of the result
  • If src is shorter than n, only its contents are appended

strncmp - Compare Limited Strings

• Header: #include <string.h>
• Syntax: strncmp(str1, str2, n)
• Purpose: Compares up to n characters from two strings
• Returns:
  • Zero if the first n characters are equal
  • Positive or negative depending on the first differing character

strchr - Find Character in String

• Header: #include <string.h>
• Syntax: strchr(str, c)
• Purpose: Finds the first occurrence of a character in a string
• Returns:
  • Pointer to the first occurrence of c
  • NULL if not found

Custom Implementation of strchr

char *myStrchr(char *str, int c) {
    while (*str != '\0') {
        if (*str == c) return str;
        str++;
    }
    return NULL;
}

strstr - Find Substring in String

• Header: #include <string.h>
• Syntax: strstr(str1, str2)
• Purpose: Locates the first occurrence of a substring within a string
• Returns:
  • Pointer to the start of the substring
  • NULL if not found

Custom Implementation of strstr

char* my_strstr(const char* str1, const char* str2) {
    const char* p = str1;
    while (*p != '\0') {
        const char* s1 = p;
        const char* s2 = str2;
        while (*s1 != '\0' && *s2 != '\0' && *s1 == *s2) {
            s1++;
            s2++;
        }
        if (*s2 == '\0') return (char*)p;
        p++;
    }
    return NULL;
}