vigenere

/*
*
* Vigenere.c for Cs50
* By Ricky Notaro
* ci = (pi + kj) % 26
*/

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
int main(int input, string argv[])
{
    // check if the number of argv is ok
    if (input == 2)
    {
        // set the arg to passkey
        string passkey = argv[1];
        // This loop check if the pass key is a-z or A-Z        
        for (int currentchar = 0, n = strlen(passkey); currentchar < n; currentchar++)
        {
            if (!isalpha(passkey[currentchar]))
            {
                printf("<passkey> must only contain letters A-Z and a-z.\n");
                return 1;
            }
        }
        // Get plaintext to encrypt
        string plaintext = GetString();
        // Begin of the encryption process
        int i = 0;
        int l = strlen(passkey);
        for (int currentchar = 0,n = strlen(plaintext); currentchar < n; currentchar++)
            {
                // if the current char is alpha the char will be encrpted
                if (isalpha(plaintext[currentchar]))
                {
                    if (i < l)
                    {
                        i++;
                    }
                    else
                    {
                        i = 1;
                    }                    
                    int a = plaintext[currentchar];
                    char b;
                    int c;
                    if (isupper(a))
                    {
                        b = 'A';
                    }
                    else 
                    {                                        
                    b = 'a';
                    }
                    plaintext[currentchar] = ((((plaintext[currentchar] + tolower((passkey[i-1])) + 7) - b) % 26)+b);
                }
                printf("%c",plaintext[currentchar]);
            }
            printf("\n");
    }
    else
    {
        printf("Usage: vigenere <passkey>\n");
        return 1;
    }
}