My vigenere program is outputting extra characters

Question

I declared one variable to track the position of the text and a variable to track the position of key. But my program is giving extra characters.

This is my For Loop

    for(int i = 0, n = strlen(text); i < n; i++)
{
    for(int j = 0; j < i; j++)
    {
        if(isalpha(text[i]))
        {
            if(isupper(text[i]))
            {
                if(isupper(argv[1][j]))
                {
                    printf("%c",((text[i] - 65) + (argv[1][j] - 65)%26 + 65));
                }
                else if(islower(argv[1][j]))
                {
                    printf("%c",((text[i] - 65) + (argv[1][j] - 97)%26 + 65));
                }
            }
            else if(islower(text[i]))
            {
                if(isupper(argv[1][j]))
                {
                    printf("%c",((text[i] - 97) + (argv[1][j] - 65)%26 + 97));
                }
                else if(islower(argv[1][j]))
                {
                    printf("%c",((text[i] - 97) + (argv[1][j] - 97)%26 + 97));
                }
            }

        }
        else
        {
            printf("%c", text[i]);
        }
    }
}

Key: AAAA

text: ABCDEF

Output: ABBCCCDDDDEEEEFFFFF

Answer 1

The problem is that you are using two loops, trying to control the index on the key (j) with a loop. You can't control the indexing of the key with a loop, there's just no way to do so.

Remember that when you have one loop inside the other, the inner loop is going to execute i * j times! You don't want to cycle through the key more times than necessary.

This is a case where you want to keep it simple. Think about how the key is used. When you encode a letter of plaintext, you increment the index of the key once. When you process a non-letter in the plaintext, you don't do anything with the index of the key. The simple way to do so is to just create an integer variable and only increment it when you encode a letter. When you get to the end of the key, reset the var to 0. No need for a loop.

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

Answer 2

I can see your inner for loop is iterating i no. of times which in case incrementing by 1 on each iteration of outer for loop, that's why you're getting one A on first iteration, two B on second iteration, three C on third iteration... on each iteration of outer for loop. Hope this makes sense.