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

int main(int argc, string argv[])
{
    // Print error message if more/less than one command line argument is given
    if (argc != 2)
    {
        printf("Error\n");
        return 1;
    }

    // If one command line argument is given, turn the string into an int to give key k
    int k = atoi(argv[1]);

    // Error message k is a negative integer
    if (k < 0)
    {
        printf("Error\n");
        return 1;
    }
    else
    {
        // Prompt user for the plaintext string
        printf("plaintext:  ");
        string s = get_string();

        // Transform plaintext string by size 'k'
        printf("ciphertext: ");
        for (int i = 0, n = strlen(s); i < n; i++)
        {
            if (isupper(s[i]))
            {
                printf("%c", (((s[i] + k) - 65) % 26) + 65);
            }
            else if (islower(s[i]))
            {
                printf("%c", (((s[i] + k) - 97) % 26) + 97);
            }
            else
            {
                printf("%c", s[i]);
            }
        }
    }
    printf("\n");
    return 0;
}