Pset2 Vigenere very close but errors on Check50

Question

this was very useful last time when I was struggling with an unseen error. I'm just after a nudge in the right direction again, My code compiles and passes most of the check50 tests bar a few, the check50 report is as follows;

:) vigenere.c exists

:) vigenere.c compiles

:) encrypts "a" as "a" using "a" as keyword

:( encrypts "world, say hello!" as "xoqmd, rby gflkp!" using "baz" as keyword

\ expected output, but not "ciphertext: xoqmd, szz gflkp!\n"

:( encrypts "BaRFoo" as "CaQGon" using "BaZ" as keyword

\ expected output, but not "ciphertext: CaQGoh\n"

:) encrypts "BARFOO" as "CAQGON" using "BAZ" as keyword

:) handles lack of argv[1]

:) handles argc > 2

:) rejects "Hax0r2" as keyword

It's strange how close the rejected answers are to the correct ones. I don't have anywhere near as much code as other examples I've seen of this problem which worries me, my core code for calculating the ciphertext is as follows;

for (int i = 0, n = strlen(p); i < n; i++)

{
    char c = p[i];
    char kc = k[i % strlen(k)]; 

    if isalpha(c)                           
    {

        if isupper(c)                       
        {
            c = c - 65;
            kc = kc- 65;
            printf("%c", (c + kc)%26 + 65);  
        }

        else if islower(c)                  
        {
            c = c - 97;
            kc = kc - 97;
            printf("%c", (c + kc)%26 + 97);
        }
    }    
        else
        {
            printf("%c", c);               
        }

}

I've been trying to avoid redundancy and keep it as neat as possible, but am I missing something? I spent a fair while jotting down pseudocode but feel that I may have messed up a loop somewhere with relation to the k[ i % strlen(k) ], any help would be greatly appreciated.

Cheers!

Answer 1

The index inside the keyword should be incremented only when encrypting a letter. It has to be kept separate from the index in the plaintext, which advances on any character.

Case of character to be encrypted is independent of case in keyword. For simplicity, you could ignore the case in the keyword by writing kc = tolower(kc) - 97; or similar.