Vigenere loop problems

Question

    //Iterate over each character in plaintext(i)
int m = strlen(keyword);
for (int i = 0, j = 0, l = strlen(plain); i < l; i++)
{//Iterate over each character in keyword(j) only if plaintext[i] is alphabetic
    if (islower(plain[i]))
    {
        printf("%c", (((plain[i] = plain[i] - 'a') + (keyword[j]%m) % 26) + 'a'));
        j++;
    }
    else if (isupper(plain[i]))
    {
        printf("%c", (((plain[i] = plain[i] - 'A') + (keyword[j]%m) % 26) + 'A'));
        j++;
    }
    else
    {
        printf("%c", plain[i]);
    }
}
printf("\n");
return 0;

Hey, hoping someone can give me some advice. My program compiles but doesn't output the correct cyphertext. For example, an input of 'michael' with a key of ABC results in 'nkchcgn. I think the problem is somewhere within the loop but I can't figure out where.

I tried to read some other common loops problems people had with vigenere and I made sure to track the [i]th character over the plaintext separately from the [j]th character of the keyword. I also made sure to increment [j] only if the plaintext received is alphabetic. I'm not very confident with my use of loops and the modulo function, so I would appreciate any advice.

My full code can be found here

Answer 1

This:

keyword[j]%m

should be:

keyword[j%m]

Also the assignments shouldn't be inside the printf().

Try:

printf("%c", (((plain[i] - 'A') + (keyword[j%m]) % 26) + 'A'));

There are other ways to make your code faster and better overall, like transforming at the beginning the keyword to its integer representation so you don't have to do keyword[j%m]) % 26 on every iteration.

You can also assign j the value j = (j + 1) % 26 instead of j++ to make sure (even thought it's highly improbable) that it won't overflow, for example if the plaintext had a length larger that what can fit in an int.