isalpha() and isdigit() always return 0

Question

I have no idea why isdigit() and isalpha() keep doing this. No matter how I use them they always return a 0. Am I using them incorrectly or is my compiler acting up?

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

//example of isdigit not working, it ALWAYS returns 0 no matter what the input is.
int main()
{
  int num=0;
  int check=0;

  printf("Enter a number: ");
  fflush(stdin);
  scanf("%d",&num);
  //Just checking what value isdigit has returned
  check=isdigit(num);
  printf("%d\n",check);
  if(check!=0)
  {
       printf("Something something");
       system("pause");
       return 0;
       }
  else
  {
       system("pause");
       return 0;
       }
}

Answer 1

isdigit acts on an int, but it is supposed to be char extended to an int. You are reading a number and convert it to a binary representation, so unless your inout happens to be a value matching 0 - ´9` (i.e. 0x30 - 0x39) you will get false as a result.

If you want to use isdigit() you must use it on the individual characters from the string the user enters as the number, so you should use %s, or %c for single digits, and loop through the string.

Example:

char c;
scanf("%c",&c);
check=isdigit(c);

or (quick and dirty example)

char buffer[200];
if (scanf("%s", buffer) > 1)
{
    int i;
    for(i = 0; buffer[i] != 0; i++)
    {
        check=isdigit(buffer[i]);
    }
}