Vigenere Encryption Problem

Question

I've been working on vigenere.c for awhile but I can't get the encryption right for the life of me. This is the closest I've gotten, but I keep getting this error when I use check50:

:) 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 "xoqmd, szz gflkp!\n"
:( encrypts "BaRFoo" as "CaQGon" using "BaZ" as keyword
\ expected output, but not "CaQGoh\n"
:) encrypts "BARFOO" as "CAQGON" using "BAZ" as keyword
:) handles lack of argv[1]
:) handles argc > 2
:) rejects "Hax0r2" as keyword

I can't figure out why some of it encrypts properly and some of it doesn't, or why \n is showing up at the end of some encryptions, so a nudge in the right direction would be awesome. Here's the relevant part of my code:

// declare key variable
string key = argv[1];

// get text to encrypt from user
string text = GetString();

// encryption
for (int i = 0, n = strlen(text); i < n; i++)
{
    // check if alphabetical
    if (isalpha(text[i]))
    {
        // check if lowercase
        if (islower(text[i]))
        {
            int keyletter = key[i % strlen(key)] - 97;
            int cipher = ((((text[i] - 97) + keyletter) % 26) + 97);
            printf("%c", cipher);     
        }
        // check if uppercase
        if (isupper(text[i]))
        {
            int keyletter = key[i % strlen(key)] - 65;
            int cipher = ((((text[i] - 65) + keyletter) % 26) + 65);
            printf("%c", cipher);
        }
    }
    // print non-alphabetical characters
    else
        printf("%c", text[i]);
}

// print new line
printf("\n");

return 0;

Thanks!

Answer 1

You have the two most common problems with this pset.

First, you can't index through the key using the same index as you use for the plaintext. The index for the key should only be incremented when a letter is encoded, not when a non-alpha is processed. It needs a separate index variable.

Second, your code assumes that the key and the plaintext are the same case. It won't properly handle the situation where one is upper case and the other is lower case. You need to add this to the code.

As for the '\n', it's supposed to be at the end of your output. I think it looks different to you because when it's right, the test spec shown is a canned string that doesn't have it, while the actual output that your program produces contains it. I think that you're correct with the \n. However, once everything else is fixed, if you still have errors, then look into it.

If this answers your question, please click the check mark to accept this and remove the question from the unanswered pool. Let's keep up on forum maintenance. ;-)