I can't get my Vigenere code to skip spaces and punctuation, I don't know why

Question

#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);
                m++;
            }
            else if (isupper(plaintext[l]))
            {
                char cipheredletter = (plaintext[l] + shift(key[m]) - 65) % 26 + 65; 
                printf("%c", cipheredletter);
                m++;

            }
            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;
}

Answer 1

The index var for the key and the index var for the plaintext must be completely independent of each other. The key index must only be incremented when a letter is processed and never when a non-alpha is processed, while the plaintext index var is incremented for every char processed.

This code has these two index variables, m and l, very tightly coupled! I'm surprised it works as well as it does!

BTW, I'll restate what I said before. Single letter vars bad. Descriptive vars good. Specifically, instead of l and m, how about keyIndex or plainIndex.

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