vigenere

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

int main(int argc, string argv[])
{
    if (argc != 2 || argv[1] == NULL )
    {
        return 1;
    }
    string keyword = argv[1];

    for (int j = 0, n = strlen(keyword); j < n; j++)
    {
        if ( !isalpha(keyword[j]) && argv[1] = NULL)
        {
            printf("Keyword must only contain letters A-Z and a-z\n");
            return 1;
        }
        else
        {
                string plaintext = GetString();

                for (int i = 0, n = strlen(plaintext); i < n; i++)
                {

                    if (isalpha(plaintext[i]))
                    {

                        int code_lower = (keyword[j] - 'a');
                        int code_upper = (keyword[j] - 'A');
                        int letter = plaintext[i];
                        int result_lower = (letter - 'a' + (code_lower %26)) + 'a';
                        int result_upper = (letter - 'A' + (code_upper %26)) + 'A';

                        if (islower(plaintext[i]))
                        {
                            if (result_lower > 'z')
                            {
                                int result_alphlower = (result_lower - 'z') + 96;
                                printf("%c", result_alphlower);
                            }
                            else
                            {
                                printf("%c", result_lower);
                            }
                        }
                        if (isupper(plaintext[i]))
                        {
                            if (result_upper > 'Z')
                            {
                                int result_alphupper = (result_upper - 'Z') + 64;
                                printf("%c", result_alphupper);
                            }
                            else
                            {
                                printf("%c", result_upper);
                            }
                        }
                    }
                    else
                    {
                        printf("%c", plaintext[i]);
                    }
                }
                printf("\n");
                return 0;
        }
    }
}