Pset2 Caesar - Fails on "barfoo" only

Question

After many many hours of trying to accomplish Caesar, check50 only yeilds one error:

• :) caesar.c exists.
• :) caesar.c compiles.
• :) encrypts "a" as "b" using 1 as key
• :) encrypts "barfoo" as "yxocll" using 23 as key
• :) encrypts "BARFOO" as "EDUIRR" using 3 as key
• :) encrypts "BaRFoo" as "FeVJss" using 4 as key
• :( encrypts "barfoo" as "onesbb" using 65 as key expected "ciphertext: one...", not "ciphertext: ihy..."
• :) encrypts "world, say hello!" as "iadxp, emk tqxxa!" using 12 as key
• :) handles lack of argv[1]

I'm not sure why it's the only error. My code is:

int main(int argc, string argv[]) {

if (argc != 2)
{
    printf("Error -102: Invalid User Input\n");
    return 1;
}
else
{

    string message;

    do
    {
        message = get_string("plaintext: ");
    }
    while (!message);
    printf("ciphertext: ");

    int key = atoi(argv[1]);

    char fin;

    for (int i = 0; i < strlen(message); i++) {

        char ch = message[i];
        int c = message[i];

        int add = c;
        if (add > 90)
        {
            add = add - 32;
        }
        add = add - 65;






       if (add < 0 || add >= 155)
       {
           fin = message[i];
           printf("%c", fin);
       }
       else
       {


            add = add + key;

            if ((add + 65) > 90) {
                add = add + 39;

            }
            else {
                add = add + 65;
            }

            fin = (char) add;
            if (islower(ch)) {
                printf("%c", tolower(fin));
            }
            else {
                printf("%c", toupper(fin));
            }




       }



    }
    printf("\n");

}

}

Thank you for any and all help you can offer!

Answer 1

When the key is sufficiently large enough, the algorithm that was designed here won't work. Imagine if the key were 1000. The results generated will not even be a valid ASCII code. A key of 65 is large enough to expose this problem but small enough to still generate letters as a result.

A rethink is in order. When adding the key, it's necessary to wrap around from z to a for every 26 count.

I'd recommend going back and reviewing the lectures, walkthroughs and other material related to this pset.

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