Why does isdigit(argv[1]) return segmentation fault?

Question

Rewriting my question, because I've finally figured out which function was returning segfault, but I still don't understand why.

The first part of my code:

#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

int main (int argc, string argv[]) 

{

    //Checks that the key exists.
    if (argc!=2)
    {
        printf("Please enter a valid numerical key after program name.\n");
        return 1;
    }

    //Checks that the key is a non-negative number.
    else if (argv[1] < 0 || isdigit(argv[1])
    {
        printf("Please enter a non-negative numerical key after program name.\n");
        return 1;
    }
}

I've figured out that it's the isdigit function that keeps returning segfault, and removing it results in the program working perfectly. My question is: why? Must I turn argv[1] into an int before checking that it's a digit or..?

Answer 1

Doesn't isdigit only work on chars, rather than argv[1] which is a string?

Answer 2

If you don't provide a key, you will segfault when you try to call atoi(argv[1]). Are you verifying that argv[1] exists? If not, you must.

Answer 3

Looks like you also forgot to close a parenthesis at the end of:

else if (argv[1] < 0 || isdigit(argv[1])