Vigenere - can't stop key from iterating when plaintext is non-alphabetical

[GK77]

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


int shift(char c);


int main(int argc, string argv[])
{
    // Checks to make sure user enters correct number of command-line arguments
    if (argc != 2)
    {
        printf("Usage: ./vigenere keyword\n");
        return 1;
    }
    else
    {
        
        // Checks that user has only entered letters as a second command-line argument (the keyword)
        string key = argv[1];
        for (int i = 0, n = strlen(key); i < n; i++)
        {
            if (isalpha(key[i]) == 0)
            {
                printf("Invalid keyword\n");
                return 1;
            }
        }
        // Gets plaintext from user
        string plaintext = get_string("plaintext: ");
        printf("ciphertext: ");
        
        // Converts plain text to cipher text
        for (int l = 0, o = strlen(plaintext); l < o; l++)
        {
            int m = (l % strlen(key));
            if (islower(plaintext[l]))
            {
                char cipheredletter = (plaintext[l] + shift(key[m]) - 97) % 26 + 97;
                printf("%c", cipheredletter);
            }
            else if (isupper(plaintext[l]))
            {
                char cipheredletter = (plaintext[l] + shift(key[m]) - 65) % 26 + 65; 
                printf("%c", cipheredletter);
            }
            else
            {
                char cipheredletter = plaintext[l];
                printf("%c", cipheredletter);
            }
        } 
    } 
    printf("\n");
    return 0;
}
    
int shift(char c)
// Converts alphabetical character to a numerical key
    
{  
    if (islower(c))
    {
        c = c - 97;
    }
    else
    {
        c = c - 65;
    }
    return c;
}

[laserlight]

What exactly is the problem you're facing with the key for non-alphabetic characters?

By the way, l isn't a good variable name: too easily mixed up with 1 in some fonts, and it's inconsistent with your other loop index names. Just use i like you did in your first loop, or use a more descriptive name (unneccessary in this case for a loop index). [b]o[/] is also a poorly named variable, but for the more common reason that a more descriptive variable name would be better, e.g., plaintext_len.

You should also avoid magic numbers like 97 and 65. Either name them as constants, or in these cases you could just use 'a' and 'A'.

I note that you aren't making good use of cipheredLetter: if you had declared the variable at the start of the inner loop then assigned to it, you could just print it once rather than in three places.

[Salem]

When I asked you in your other thread to post a self-contained example, I meant something like this.

//!!#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>

int shift(char c);

int main()
{
    // Examples from https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
    char key[] = "LEMON";
    char plaintext[] = "ATTACKATDAWN";
    char result[] = "LXFOPVEFRNHR";

    // Converts plain text to cipher text
    for (int l = 0, o = strlen(plaintext); l < o; l++)
    {
        int m = (l % strlen(key));
        if (islower(plaintext[l]))
        {
            char cipheredletter = (plaintext[l] + shift(key[m]) - 97) % 26 + 97;
            printf("%c", cipheredletter);
        }
        else if (isupper(plaintext[l]))
        {
            char cipheredletter = (plaintext[l] + shift(key[m]) - 65) % 26 + 65;
            printf("%c", cipheredletter);
        }
        else
        {
            char cipheredletter = plaintext[l];
            printf("%c", cipheredletter);
        }
    }
    printf("\n");
    return 0;
}

int shift(char c)
// Converts alphabetical character to a numerical key

{
    if (islower(c))
    {
        c = c - 97;
    }
    else
    {
        c = c - 65;
    }
    return c;
}

No guessing as to what to type in.
No need for mysterious header files like cs50.h

Now, starting from this, make an example which demonstrates the problem.

But I kinda know already, so

        int m = (l % strlen(key));
        if (islower(plaintext[l]))
        {
            char cipheredletter = (plaintext[l] + shift(key[m]) - 97) % 26 + 97;
            printf("%c", cipheredletter);
        }

to

        if (islower(plaintext[l]))
        {
            char cipheredletter = (plaintext[l] + shift(key[m]) - 97) % 26 + 97;
            printf("%c", cipheredletter);
            m = (m+1) % strlen(key);
        }