Beyond isalnum: Exploring Alternative Approaches for Character Classification in C Strings

• The function returns a non-zero value (usually 1) if the character is alphanumeric. It returns 0 if the character is not alphanumeric.
• The function takes an int argument, which is typically obtained by casting a character to an integer using (int)ch. This is because isalnum internally uses the ASCII value of the character for comparison.
• You include the ctype.h header file in your C program.

#include <stdio.h>
#include <ctype.h>

int main() {
  char ch = 'a';

  if (isalnum(ch)) {
    printf("%c is alphanumeric\n", ch);
  } else {
    printf("%c is not alphanumeric\n", ch);
  }

  return 0;
}

This program will print "a is alphanumeric" because 'a' is a lowercase letter.

While isalnum doesn't directly deal with strings, it's often used in conjunction with loops to iterate through characters in a string and perform checks based on whether they are alphanumeric or not. This can be useful for tasks like:

• Removing non-alphanumeric characters from a string.
• Parsing text files that contain alphanumeric data.
• Validating user input to ensure it only contains letters and digits.

---

Checking if a string is alphanumeric

#include <stdio.h>
#include <ctype.h>
#include <stdbool.h>

bool is_alphanumeric_string(const char* str) {
  while (*str) {
    if (!isalnum(*str)) {
      return false;
    }
    str++;
  }
  return true;
}

int main() {
  char str1[] = "Hello123";
  char str2[] = "Hello@World";

  if (is_alphanumeric_string(str1)) {
    printf("%s is alphanumeric\n", str1);
  } else {
    printf("%s is not alphanumeric\n", str1);
  }

  if (is_alphanumeric_string(str2)) {
    printf("%s is alphanumeric\n", str2);
  } else {
    printf("%s is not alphanumeric\n", str2);
  }

  return 0;
}

This code defines a function is_alphanumeric_string that takes a constant character pointer (const char*) as input. It iterates through the string using a while loop until it reaches the null terminator (\0). Inside the loop, it checks if the current character is alphanumeric using isalnum. If any character is not alphanumeric, the function returns false. Otherwise, it keeps iterating. If the loop completes without finding any non-alphanumeric characters, the function returns true.

In the main function, two strings are declared: str1 containing only alphanumeric characters and str2 containing a non-alphanumeric character (@). The is_alphanumeric_string function is called for both strings, and the results are printed.

Removing non-alphanumeric characters from a string

#include <stdio.h>
#include <ctype.h>

void remove_non_alphanumeric(char* str) {
  char *out = str;
  while (*str) {
    if (isalnum(*str)) {
      *out = *str;
      out++;
    }
    str++;
  }
  *out = '\0'; // Add null terminator at the end of the modified string
}

int main() {
  char str[] = "Hello!@World123";
  printf("Original string: %s\n", str);

  remove_non_alphanumeric(str);

  printf("String after removing non-alphanumeric characters: %s\n", str);

  return 0;
}

This code defines a function remove_non_alphanumeric that takes a character pointer as input. It uses two pointers:

• str: This pointer iterates through the original string.
• out: This pointer points to the position where the modified string will be built.

The function iterates through the original string. If the current character is alphanumeric (isalnum returns true), it's copied to the out position, and out is incremented. Otherwise, the character is skipped.

After the loop finishes iterating through the original string, the out pointer will be pointing to the position where the null terminator should be added. The code explicitly adds the null terminator (\0) to mark the end of the modified string.

---

1. Using isalpha and isdigit
   

These are separate functions defined in <ctype.h> that check for specific character classes:

• isdigit(ch): Returns true if ch is a digit (0-9).
• isalpha(ch): Returns true if ch is an alphabetic character (a-z, A-Z).

You can combine them with logical operators (&& for AND, || for OR) to achieve similar functionality as isalnum:

bool is_alphanumeric(char ch) {
  return isalpha(ch) || isdigit(ch);
}

This approach offers more control if you need to differentiate between alphabetic and numeric characters separately.

1. Character Ranges
   

You can leverage character ranges in your conditions to check for alphanumeric characters. This is less portable across different character sets but might be suitable for specific use cases:

bool is_alphanumeric(char ch) {
  return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z') || (ch >= '0' && ch <= '9');
}

1. Lookup Tables
   

For specific character sets or more complex checks, you can create lookup tables. This involves creating an array with flags indicating whether a character is alphanumeric or not. This approach can be less efficient for large character sets but offers customization.

1. Regular Expressions (Advanced)
   

C libraries like regex.h provide functions for working with regular expressions. While more complex to set up, regular expressions can be powerful for pattern matching, including checking for alphanumeric characters within a string.

Choosing the alternative depends on your specific needs and the context of your program.

• Regular expressions are powerful but require more advanced knowledge and setup.
• Lookup tables offer customization but might be less efficient with large character sets.
• If portability is a concern, character ranges might be less suitable.
• If you just need a simple check for alphanumeric characters, using isalpha and isdigit combined can be a good choice.