CS50/PSET2/vigenere

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

int main(int argc, string argv[])
{
    string keyString;
    if (argc == 2)
    {
        keyString = argv[1];
        if (!isalpha(keyString[0]))
        {
            return 1;
        }
    }
    else
    {
        return 1;
    }

    int keyLength = strlen(keyString);
    string pt = get_string("Enter text: ");
    char ct[strlen(pt) + 2];

    char chaesarChar (char alpha, char key);

    int keyIndex = 0;


    for (int i = 0, n = strlen(pt); i < n; i++)
    {
        //index
        int index = keyIndex % keyLength;
        //taken, plaintext is a word
        if (isalpha(pt[i]))
        {
            if (isupper(pt[i]))
            {
                ct[i] = chaesarChar(pt[i], keyString[index]);
                if (ct[i] > 90)
                {
                    ct[i] -= 26;
                }
            }
            else if (islower(pt[i]))
            {
                ct[i] = chaesarChar(pt[i], keyString[index]);
                if (ct[i] > 122)
                {
                    ct[i] -= 26;
                }
            }
        }
        else
        {
            ct[i] = pt[i];
        }

        keyIndex += 1;

    ct[strlen(pt) + 1] = '\0';

    printf("%s\n", ct);

    return 0;
}

char chaesarChar (char alpha, char key)
{
    char x = key;
    if (islower(key)){
        x = key - 97;
    }
    else if (isupper(key))
    {
        x = key - 'A';
    }
    char res = alpha + x;

    return res;
}