CS50 PSET-2 Caesar segmentation fault

Question

#include <cs50.h>
#include <stdio.h>
#include <stdlib.h> //covert str into int atoi()
#include <ctype.h> // isalpha(), isdigit() etc.
#include <string.h> //strlen() etc.


int main(int argc, string argv[])
{
    // convert argv[1] into int
    int key = atoi(argv[1]);
    // if input is 2 command lines and argv > 0 and there is no alphabet n shit in argv[1] we good to go
    if (argc == 2 && key > 0 && isdigit(argv[1])) 
    {
        //ask for input to chiper
        string plain = get_string("Plaintext: ");
        int len_plain = strlen(plain);
        // check each char in string plain
        for (int i = 0; i < len_plain; i++)
        {
            // if its from 'a' to 'z' add number of key
            if (plain[i] >= 'a' && plain[i] <= 'z')
            {
                char cipher = (plain[i] + key) % 26;
                printf("%c", cipher);
                return 0;
            }
        }

    }
    else
    {
        printf("Usage: ./caesar kay");
        return 1;
    }
}

When running this code, I'm faced with a segmentation fault. What did I do wrong? I have been working a few days on this code but I cannot make it work.

Answer 1

This isdigit(argv[1]) is giving a seg fault. The function signature of isdigit (from the man page):

int isdigit(int c);

But, argv[1] (if it exists.) is a string. Suggest you follow the spec by

1. verifying the right number of command line arguments is entered
2. checking that each char in said argument is a decimal digit

Only then are you assured that atoi will give the desired result.