isalnum() function in C Language

isalnum() function in C programming language checks whether the given character is alphanumeric or not. isalnum() function defined in ctype.h header file. Alphanumeric: A character that is either a letter or a number. Syntax:

int isalnum(int x);

Examples:

Input : 1
Output : Entered character is alphanumeric
Input : A
Output : Entered character is alphanumeric
Input : &
Output : Entered character is not alphanumeric

// C code to illustrate isalphanum()
#include <ctype.h>
#include <stdio.h>

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

    // checking is it alphanumeric or not?
    if (isalnum(ch))
        printf("\nEntered character is alphanumeric\n");
    else
        printf("\nEntered character is not alphanumeric\n");
}

Output:

Entered character is alphanumeric

isalnum() function returns some value. It always returns '8' for all characters except special characters like (#,@,% etc.). It will always return '0' for special characters.

Input: 'A'                                                                                                                                                                                                                

Output: 8

Input: 'b'

Output:8

Input: '4'

Output: 8

Input: '#'

Output: 0

Input: '?'

Output: 0

#include <iostream>
#include <cctype>    // header file for function isalnum()
using namespace std;

int main() {
char ch;
  cin>>ch;
    cout <<" function will return: "<<isalnum(ch);
    return 0;
}

Application: isalnum() function is used to find out number of alphanumeric in a given sentence(or any input). Example:

Input: abc123@
Output: Number of alphanumerics in the given input is : 6
Input: a@#
Output: Number of alphanumerics in the given input is : 1
Input: ...akl567
Output: Number of alphanumerics in the given input is : 6

// C code to illustrate isalphanum()
#include <ctype.h>
#include <stdio.h>

int ttl_alphanumeric(int i, int counter)
{
    char ch;
    char a[50] = "www.geeksforgeeks.org";
    ch = a[0];

    // counting of alphanumerics
    while (ch != '\0') {
        ch = a[i];
        if (isalnum(ch))
            counter++;

        i++;
    }

    // returning total number of alphanumerics
    // present in given input
    return (counter);
}

int main()
{
    int i = 0;

    int counter = 0;
    counter = ttl_alphanumeric(i, counter);

    printf("\nNumber of alphanumerics in the "
           "given input is : %d", counter);
    return 0;
}

Output:

Number of alphanumerics in the given input is : 19