pset2 vigenere: Only one error in check50: :( encrypts "world, say hello!" as "xoqmd, rby gflkp!" using "baz" as keyword

Question

I cannot figure out why I received this error. Only the word 'say' in the "world, say hello!" check50 is not ciphering correctly. My current code is below. Any advice will be greatly appreciated!

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

int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Try again!\n");
        return 1;
    }

    string kw = argv[1];
    int nkw = strlen(kw);
    for (int pkw = 0; pkw < nkw; pkw++)
    {
        if (!isalpha (kw[pkw]))
        {
            printf("Try again!\n");
            return 1;
        }
    }

    for (int pkw = 0; pkw < nkw; pkw++)
    {
        if (islower (kw[pkw]))
        {
            kw[pkw] = kw[pkw] - 32;
        }
    }

    string s = GetString();
    int n = strlen(s);
    int pkw = 0;
    for (int p = 0; p < n; p++, pkw++) 
    {
        if (isalpha (s[p]))
        {
            if (isupper (s[p]))
            {
                s[p] = ((s[p] - 'A') + (kw[pkw % nkw] - 'A')) % 26 + 'A';
                printf("%c", s[p]);
            }

            if (islower (s[p]))
            {
                s[p] = ((s[p] - 'a') + (kw[pkw % nkw] - 'A')) % 26 + 'a';
                printf("%c", s[p]);
            }
        }    
        else
        {
            printf("%c", s[p]);
        }
    }
    printf("\n");
}

Answer 1

Common error. The code increments the indexes for both the key and the plain text for every character in the plain text string that is processed. The key index should only be incremented when a letter is encoded and not when a non-letter is processed. When the spaces and the comma in the plain text are printed, the index for the key is being incremented, putting it out of synch. Since there are 3 non-alphas in the plain text and 3 letters in the key, they come back into synch for the last word. That particular test was designed to check for this error.

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