vigenere

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

int main(int argc, string argv[])
{

//verify that have the correct number of arguments
    if (argc!=2){
        printf("Please dont forget to add ONE alphabetical argument\n");
        return 1;
        }
    string plaintext;
    string key = argv[1];

//verify that the key is alphabetical
    if (isalpha(key)) {
        plaintext = GetString();
         }
    else {
        printf("Please use am alphabetical key argument\n");
        return 1;
        }


//encipher here
    int nonalpha = 0;
    int keylength = strlen(key);
    int keynum;
    for (int i = 0, n=strlen(plaintext); i < n; i++)
    {
        int keyposition = (i - nonalpha) % keylength;
        if (islower(key[keyposition])){
                keynum = (int)(key[keyposition] -97);
            }
        else {
                keynum = (int)(key[keyposition] -65);
            }

        if (isalpha (plaintext[i])){
            if (islower(plaintext[i])){
                int c = (((plaintext[i] - 97 + keynum) %26) +97);
                printf("%c", c);
                }
            else {
                int c = (((plaintext[i] - 65 + keynum) %26) +65);
                printf("%c", c);
                }
            }
        else {
            printf("%c", plaintext[i]);
            nonalpha = nonalpha + 1 ;
            }
    }

    printf("\n");

}