Still struggling with vigenere

Question

I have been futzing with this for a couple of weeks. I actually put it away while I did pset3 but, having come back to it, I'm still stuck. I spent an entire day on it this weekend and tried adding a loop, moving the loop, restructuring the if statements, etc. I'm pretty sure I went in circles and I can't figure out how to create an index for the key. I feel like this should work and it compiles but it doesn't print out any letters for the ciphertext. When I type "hello" I get "".

Can someone help me understand why this doesn't work and give me a tip on how to establish an index for the key so it will iterate one letter at a time for only alpha characters?

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

int main(int argc, string argv[])

{
    //Ensure that user enters 2 command line arguments (argv[0] - program name and argv[1] - the key number)
if (argc != 2)
{
    printf("Enter the file name and a key.\n");
    return 1;
}

 //Increment across key, letter by letter
string key = argv[1];

for (int j = 0, o = strlen(key); j < o; j++)

   //Ensure argv[1] is alpha
while (!(isalpha (key[j]) != '\0'))
{
    printf("Key must be alpha.\n");
    return 1;
}

    //Get plaintext string that user wants converted to ciphertext
    string plaintext = get_string("plaintext:  ");
    printf("ciphertext:  ");

    //Increment across user's plaintext entry, letter by letter.
    for (int i = 0, n = strlen(plaintext); i < n; i++)
    {
        if (isalpha(plaintext[i]))
        {
            for (int k = key[0], l = strlen(key); k < l; k++)

            //Ensure capital letters stay capital and lowercase stay lower.
            //encipher, shifting each letter of plaintext string one at a time by the key.
            if (isupper(plaintext[i]))
            {
                printf("%c", (((plaintext[i] - 'A' + (toupper(key[k % l])) - 'A')) % 26) + 'A');
            }
            else if (islower(plaintext[i]))
            {
                printf("%c", (((plaintext[i] - 'a' + (tolower(key[k % l])) - 'a')) % 26) + 'a');
            }
            // Ensure non alpha characters stay as they are.
            else
            {
                printf("%c", plaintext[i]);
            }
        }
        else
        {
            printf("%c", plaintext[i]);
        }
    }
printf("\n");
return 0;

}

Answer 1

I'm still not 100% sure why the other thing didn't work (I have a couple of theories, still interested to see what people say here) but I futzed with it some more and this seems to have done it. Quite frustrating since it seems obvious now why this works. :/

    //Increment across user's plaintext entry, letter by letter.
    for (int i = 0, m = 0, n = strlen(plaintext); i < n; i++)

        if (isalpha(plaintext[i]))

            //Ensure capital letters stay capital and lowercase stay lower.
            //encipher, shifting each letter of plaintext string one at a time by the key.
            if (isupper(plaintext[i]))
            {
                printf("%c", (((plaintext[i] - 'A' + (toupper(key[(m++) % k])) - 'A')) % 26) + 'A');
            }
            else if (islower(plaintext[i]))
            {
                printf("%c", (((plaintext[i] - 'a' + (tolower(key[(m++) % k])) - 'a')) % 26) + 'a');
            }
            else
            {
                printf("%c", plaintext[i]);
            }

Answer 2

What does this loop do?

for (int k = key[0], l = strlen(key); k < l; k++)

And with the following:

printf("%c", (((plaintext[i] - 'A' + (toupper(key[k % l])) - 'A')) % 26) + 'A');

What is (k % l)?

If you want to cycle through your key, you just need to divide the length of the key into some value that is constantly increasing but which doesn't increase if

isalpha(plaintext[i]) == false

You seem to have done this with your updated example.