vigenere.c

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

int main(int argc, string argv[])
{
    //CHeck to see if the correct number of arguments have been entered.
    if (argc != 2)
    {
        printf("Please enter the correct number of arguments\n");
        return 1;
    }
    else
    {
        for (int i = 0, n = strlen(argv[1]); i<n; i++)
        {
            if(!isalpha(argv[1][i]))
            {
                printf("Please enter the correct argument. Key must be alphabetic characters only.");
                return 1;
            }
        }
    }

    //Get key from argument as a string and get the length
    string key = argv[1];
    int key_length = strlen(key);

    //Get user input for plaintext
    printf("plaintext: ");
    string plaintext = get_string();
    if (plaintext != NULL)
    {
        printf("ciphertext: ");
        //Iterate through the string and convert each letter
        for (int i = 0, j = 0, n = strlen(plaintext); i < n; i++)
        {
            //Get the key for this letter
            int letter_key = tolower(key[j % key_length])-97;

            //Check for lowercase
            if (islower(plaintext[i]))
            {
                printf("%c", ((((plaintext[i] - 97) + letter_key) % 26) + 97));
                j++;
            }
            //check for uppercase
            else if(isupper(plaintext[i]))
            {
                printf("%c", ((((plaintext[i] - 65) + letter_key) % 26) + 65));
                j++;
            }
            //Print non-alpha characters as they are
            else
            {
                printf("%c", plaintext[i]);
            }
        }
    }
    //Print new line to end the deciphered text.
    printf("\n");
    //Exit the program
    return 0;
}