Vigenere code still rotates for nonalpha

Question

I've been trying to get my vigenere straight for some time now, managed to get rid of most of the bugs, but all I can see is that it still rotates the code for me even when I have a non-alpha characters. Could you please help me? Probably I declared j inside or outside of some loop i shouldn't have, but honestly, I have no idea. Any hint, help is highly appreciated. So this is what I managed to work out so far:

    string text = GetString();
    string keyword = argv[1];
    for (int j = 0; j < strlen(keyword); j++)
        {
            keyword[j] = toupper(keyword[j]);
        }
        // i want the key to code with lowercase only. does that work?

    for (int i = 0; i < strlen(text); i++)

    {
        if (isalpha (text[i])) 
        // for letters
        {   
            int j = i % strlen(keyword);
            if (isupper(text[i]))
            {
                int result = ((text[i] + keyword[j] - 2*'A') % 26) + 'A';
                printf("%c", result);

            }
            else 
            // for lower
            {
                int result = (text[i] + keyword[j] -'A' - 'a') % 26 + 'a';
                printf("%c", result);

            }
        }   
        else   

        {
            printf ("%c", text[i]);
           // should not proceed with the code letters but it still does 
        }
    }
        printf("\n");
// probably it shouldnt but prettier
        return 0;

Answer 1

You're on the right track that something is wrong with J. The problem is that you've tied the index for the key, j, to the index for text, i. int j = i % strlen(keyword); You are probably seeing the encryption fail when you process a non-alpha in text.

When a non-alpha is processed, such as a space, number or punctuation, the index for j must not be changed. J must be set independently and should only be incremented when a letter is encoded and should reset to 0 each time the last letter in the key is used.

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