C isalnum() Function

The isalnum() function in C is a standard library function that checks if a given character is alphanumeric. It is part of the C standard library (ctype.h). This function is useful for determining if a character is a letter or a digit.

Table of Contents

1. Introduction
2. isalnum() Function Syntax
3. Examples
   • Checking if a Character is Alphanumeric
   • Using isalnum() with User Input
4. Real-World Use Case
5. Conclusion

Introduction

The isalnum() function checks if a given character is alphanumeric, meaning it is either a letter (uppercase or lowercase) or a digit. This function is useful in various scenarios, such as validating user input or parsing text.

isalnum() Function Syntax

The syntax for the isalnum() function is as follows:

#include <ctype.h>
int isalnum(int c);

Parameters:

• c: The character to be checked, which is passed as an int.

Returns:

• The function returns a non-zero value (true) if the character is alphanumeric; otherwise, it returns 0 (false).

Examples

Checking if a Character is Alphanumeric

To demonstrate how to use isalnum() to check if a character is alphanumeric, we will write a simple program.

Example

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

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

    // Check if the character is alphanumeric
    if (isalnum(ch)) {
        printf("'%c' is alphanumeric.\n", ch);
    } else {
        printf("'%c' is not alphanumeric.\n", ch);
    }

    return 0;
}

Output:

'A' is alphanumeric.

Using isalnum() with User Input

This example shows how to use isalnum() to check if a character provided by the user is alphanumeric.

Example

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

int main() {
    char ch;

    // Get user input for the character
    printf("Enter a character: ");
    scanf("%c", &ch);

    // Check if the character is alphanumeric
    if (isalnum(ch)) {
        printf("'%c' is alphanumeric.\n", ch);
    } else {
        printf("'%c' is not alphanumeric.\n", ch);
    }

    return 0;
}

Output (example user input '7'):

Enter a character: 7
'7' is alphanumeric.

Real-World Use Case

Validating User Input

In real-world applications, the isalnum() function can be used to validate user input, ensuring that only alphanumeric characters are accepted.

Example: Validating a Username

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

int main() {
    char username[100];
    int i, is_valid = 1;

    // Get user input for the username
    printf("Enter a username: ");
    scanf("%s", username);

    // Check if the username contains only alphanumeric characters
    for (i = 0; username[i] != '\0'; i++) {
        if (!isalnum(username[i])) {
            is_valid = 0;
            break;
        }
    }

    // Print the result
    if (is_valid) {
        printf("The username '%s' is valid.\n", username);
    } else {
        printf("The username '%s' is invalid. Only alphanumeric characters are allowed.\n", username);
    }

    return 0;
}

Output (example user input username "user123"):

Enter a username: user123
The username 'user123' is valid.

Conclusion

The isalnum() function is essential for checking if a character is alphanumeric in C. It is useful in various applications, particularly in fields like data validation and text processing, where it is necessary to ensure that characters belong to the alphanumeric set.