Given a character, we have to check whether the given character is alphanumeric or not without using the library function.

Program:

The source code to check a given character is alphanumeric or not without using the library function is given below. The given program is compiled and executed using GCC compile on UBUNTU 18.04 OS successfully.

// C program to check a given character is
// alphanumeric or not without using library function

#include <stdio.h>

int isAlphaNumeric(char ch)
{
    if ((ch >= '0' & ch <= '9') || (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'))
        return 1;

    return 0;
}

int main()
{
    char ch = 0;

    printf("Enter character: ");
    scanf("%c", &ch);

    if (isAlphaNumeric(ch))
        printf("Given character is an alphanumeric character\n");
    else
        printf("Given character is not an alphanumeric character\n");

    return 0;
}

Output:

RUN 1:
Enter character: G
Given character is an alphanumeric character

RUN 2:
Enter character: 8
Given character is an alphanumeric character

RUN 3:
Enter character: +
Given character is not an alphanumeric character

Explanation:

In the above program, we created two functions isAlphaNumeric() and main(). The isAplhaNumeric() function is used to check the given character is alphanumeric or not.

In the main() function, we read a character from the user and check given character is alphanumeric or not by calling the isAlphaNumeric() function and print the appropriate message on the console screen.