CS50x Caesar - Segmentation fault when inserting isalpha(argv[1]) - c

Question

For the CS50x Problem Caesar I have created a program that encrypts messages using Caesar’s cipher.
For this, the program must recognize that a command-line argument is only a number. So no two or more numbers, no number below zero, and no text.
But as soon as I add the check if it is a text with || isalpha(argv[1]), the program does not work anymore.
The terminal prints the following when I try to run the program:
Segmentation fault
Can anyone tell me what is the problem with the code
#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
int main(int argc, string argv[])
{
int kkey = 0;
// Check if correct command-line arguments
if (argc != 2 || atoi(argv[1]) < 0 || isalpha(argv[1])) //segfault here
{
printf("./caesar key\n");
return 1;
}
else
{
kkey = atoi(argv[1]);
}
// Ask for Plaintext to encrypt
string plaintext = get_string("plaintext: ");
for (int i = 0, n = strlen(plaintext); i < n; i++)
{
if (isalpha(plaintext[i]) && islower(plaintext[i]))
{
plaintext[i] = (plaintext[i] - 'a' + kkey) % 26 + 97;
}
else if (isalpha(plaintext[i]) && isupper(plaintext[i]))
{
plaintext[i] = (plaintext[i] - 'A' + kkey) % 26 + 65;
}
printf("%c", plaintext[i]);
}
printf("\n");
return 0;
}
Thank you very much for your help.

Answer 1

As said by #Gerhardh, you can't use strings as argument of isalpha, you need a loop to check each character of the string.
In any case that is not the best approach, using a negated isdigit would be a better option, because it accounts for all the other non numeric characters.
//...
// Check if correct command-line arguments
if (argc != 2 || atoi(argv[1]) < 0)
{
printf("./caesar key\n");
return 1;
}
for(size_t i = 0; i < strlen(argv[1]); i++){
if(!isdigit(argv[1][i])){ //if one of the characters is not a digit 0-9
puts("./caesar key\n");
return 1;
}
}
kkey = atoi(argv[1]); //no else needed
//...
Note that atoi will invoke undefined behavior if the converted value is not representable by an int.
You can use strtol for a more robust alternative.
The link is for Linux man page which I find quite nice, but this is cross-platform.
Again, as stated by #Gerhardh, using character codes may backfire, in this case you are using ASCII encoding, but there are others, this makes your code less portable, use the character instead, 26 + 'a' and 26 + 'A'.