Here is the code:

import re
s = "             -42"
s = re.split(r'\s+', s)
s = list(filter(None,s))
print(s[0].isdigit())

This prints out False. I don't understand why because if I go to simply print it without the isdigit it gives back 42.

Original Question

I just started my first attempt at learning to program. I'm currently working through "Learn C Programming for the Absolute Beginner" and for the life of me I can't understand why this code does not work as expected:

//1. Build a number-guessing game that uses input validation (isdigit() function) to verify that the user has entered 
//   a digit and not a nondigit (letter). Store a random number between 1 and 10 into a variable each time the program 
//   is run. Prompt the user to guess a number between 1 and 10 and alert the user if he was correct or not.

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

int main() 
{
    int iRandomNum = 0;
    int iGuess = 0;

    srand(time(NULL));
    iRandomNum = (rand() % 10) + 1;

    printf("\nNumber Guessing Game, Chapter 3 Challenge");
    printf("\nGuess a number between 1 and 10: ");
    scanf("%d", &iGuess);
   
    if (isdigit(iGuess))
    {
        if ( iGuess > 0 && iGuess < 11)
        {
            printf("You guessed %d", iGuess);
            printf("The correct answer was %d", iRandomNum);

            if ( iGuess == iRandomNum)
            {
                printf("Congratulations! You guessed correctly!");
            }
            else
            {
                printf("Sorry! You guessed incorrectly...");
            }
            
        }
        else
        {
            printf("Invalid Response: You did not choose a number between 1 and 10");
            
        }
    }
    else
    {
        printf("\nInvalid Response: You did not select a number");
        
    }
    return 0;
}

No matter what my input, whether it is a number 1 - 10, or some other character, the code returns:

"Invalid Response : you did not select a number"

Edit:

All, thanks for your help. I understand now that isdigit only tests whether a single character is a digit.

To fix my code, (if isdigit returns true), I convert the character to a number like so:

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

int main() 
{
    int iRandomNum = 0;
    int iGuess = 0;
    char cResponse = '\0';

    srand(time(NULL));
    iRandomNum = (rand() % 10) + 1;

    printf("\nNumber Guessing Game, Chapter 3 Challenge");
    printf("\nGuess a number between 1 and 10: ");

    scanf("%c", &cResponse);

    if (isdigit(cResponse) == 0)
    {
        printf("\nInvalid Response, you did not input a number!");
    }   
    else 
    {
        //if iResponse is a digit, convert it to integer type by subtracting '0'
        iGuess = cResponse - '0';

        if ((iGuess < 1) || (iGuess > 10))
        {
            printf("\nInvalid Response: You did not choose a number between 1 and 10");
            printf("\nYou guessed %d", iGuess);
            printf("\nThe correct answer was %d", iRandomNum);  
        }
        else 
        {
            if ( iGuess == iRandomNum)
            {
                printf("\nCongratulations! You guessed correctly!");
            }
            else{
                printf("\nSorry! You guessed incorrectly...");
            }
        }        

    }
    return 0;
}

Edit 2:

Welp, looks like I still have some debugging to do, but that's another issue, unrelated to isdigit and more so catching a response that is longer than a single character. Not looking for help on that, just wanted to add this note in case someone tries to rely on this subpar codeblock for learning purposes. Back to it. Thanks again everyone.

Edit 3: (last one I promise)

Once again, I stand corrected. Isdigit does accept integers. As u/CodeTinkerer so kindly pointed out, I missed an explanation of isdigit posted by u/strcspn in an earlier reply on this thread. For the sake of correcting some misinformation in my post (above), here it is:

The function actually takes an int, but the value of that int is supposed to represent a character.

It's basically to allow passing in EOF, which can't be represented by unsigned char.

Hopefully, someone can take something useful away from my (mis)adventures in scanf().