isalnum() function in C

The isalnum() function is a library function declared in the <ctype.h> header. It is used to determine whether a given character is alphanumeric, which means it is either a letter (a-z or A-Z) or a digit (0-9). Here’s the function signature:

int isalnum(int c);

• c: The integer representation of the character to be tested.

The function returns a non-zero (true) value if the character is alphanumeric and 0 (false) if it’s not.

Here’s an example of how to use isalnum():

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

int main() {
    char ch1 = 'A';
    char ch2 = '7';
    char ch3 = '$';

    if (isalnum(ch1)) {
        printf("%c is alphanumeric.\n", ch1);
    } else {
        printf("%c is not alphanumeric.\n", ch1);
    }

    if (isalnum(ch2)) {
        printf("%c is alphanumeric.\n", ch2);
    } else {
        printf("%c is not alphanumeric.\n", ch2);
    }

    if (isalnum(ch3)) {
        printf("%c is alphanumeric.\n", ch3);
    } else {
        printf("%c is not alphanumeric.\n", ch3);
    }

    return 0;
}

In this example, isalnum() is used to check whether three characters (ch1, ch2, and ch3) are alphanumeric. The function returns true for 'A' and '7' since they are alphanumeric characters, and it returns false for '$' since it’s not alphanumeric.

Output:

A is alphanumeric.
7 is alphanumeric.
$ is not alphanumeric.

You can use isalnum() to validate and process input where you expect alphanumeric characters or to filter out characters that don’t meet this criterion.