Caesar Cipher

// Osman Zakir
// 10 13 2016
// Introduction to Computer Science
// Problem Set 2, caesar.c
// Take the key for the cipher from the user as a command-line argument and use it to encrypt a message

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

int main(int argc, string argv[])
{
    if (argc == 1 || argc > 2)
    {
        printf("Error: Please input one number to use as key for cipher!\n");
        return 1;
    }
    printf("Enter plaintext to be shifted by %d positions\n", atoi(argv[1]));
    string plaintext = GetString();
    int key = atoi(argv[1]);
    const int size = strlen(plaintext);
    char ciphertext[size];
    if (key >= pow(2, 31) - 26 || key <= 0)
    {
        printf("The value for the key is invalid!\n");
    }
    else
    {
        for (int i = 0, n = size; i < n; i++)
        {
            if (isalpha(plaintext[i]))
            {
                //int c = plaintext[i] + key;

                ciphertext[i] = plaintext[i] + key;
                if (plaintext[i] >= 'Z' && isupper(plaintext[i]))
                {
                    ciphertext[i] = ((plaintext[i] + key) - 'A') % 26;
                }
                else if (plaintext[i] >= 'z' && islower(plaintext[i]))
                {
                    ciphertext[i] = ((plaintext[i] + key) + 'a') % 26;
                }
                /*if (plaintext[i] >= 'z')
                {
                    ciphertext[i] = 'a' + (c % ('a' + 26));
                }
                else if (plaintext[i] >= 'Z')
                {
                    ciphertext[i] = 'A' + (c % ('A' + 26));
                }*/
            }
            else
            {
                ciphertext[i] = plaintext[i];
            }
        }
    }
    printf("%s\n", ciphertext);
    return 0;
}