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.