C Library - isalnum() function

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The C ctype library isalnum() function is used to check if a given character is alphanumeric, which means it is either a letter (uppercase or lowercase) or a digit.

In order to use isalnum() function we have to include the header file <ctype.h> that contains the function.

Syntax

Following is the C library syntax of the isalnum() function −

int isalnum(int c);

Parameters

This function accepts a single parameter −

• c − This parameter represents the character to be checked. It is passed as an int, but it is expected to be a character that can be represented as an unsigned char or the value of EOF

Return Value

Following is the return value −

• The function returns a non-zero value (true) if the character is alphanumeric.

• It returns 0 (false) if the character is not alphanumeric.

Example 1: Checking if a Character is Alphanumeric

In this example, the character a is checked using the isalnum() function. Since a is an alphabetic letter, the function returns a non-zero value, indicating it is alphanumeric.

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

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

   if (isalnum(ch)) {
      printf("The character '%c' is alphanumeric.\n", ch);
   } else {
      printf("The character '%c' is not alphanumeric.\n", ch);
   }
   return 0;
}

Output

The above code produces following result −

The character 'a' is alphanumeric.

Example 2: Checking a Special Character

Now, the special character @ is checked. Since it is neither a letter nor a digit, the isalnum function returns 0, indicating that @ is not alphanumeric.

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

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

   if (isalnum(ch)) {
      printf("The character '%c' is alphanumeric.\n", ch);
   } else {
      printf("The character '%c' is not alphanumeric.\n", ch);
   }
   return 0;
}

Output

The output of the above code is as follows −

The character '@' is not alphanumeric.