pset2 Vigenere close but not all characters correct

Question

as the title says, I have most of the characters correct in the output compared to what it says in the instructions for pset2 Vigenere but a few of them are off, any help would be appreciated!

#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

int main(int argc, string argv[])
{
if(argc != 2)
{
    printf("IDIOT!\n");
    return 1;
}    

string key = argv[1];

for (int n = 0, keylength = strlen(key); n < keylength; n++)
{
    if(!isalpha(key[n]))
    {
        printf("You must enter a keyword with only letters, no numbers.\n");
        return 1;
    }
}

string p = GetString();
int l = strlen(key);

for(int i = 0, j = 0, n = strlen(p); i < n; i++)
{
    if(isupper(p[i]))
    {
        char upper = (p[i] - 65) + (key[j%l] - 97) %26 + 65;
        printf("%c", upper);
        j++;
    }
    else if(islower(p[i]))
    {
        char lower = (p[i] - 97) + (key[j%l] - 97) %26 + 97;
        printf("%c", lower);
        j++;
    }
    else if(isspace(p[i]))
    {
        printf(" ");
    }
}
printf("\n");
return 0;
}

when I input this with the keyword "bacon"

Meet me at the park at eleven am

I get this:

Neg� zf av �uf pc�x bt gzrwep oz

Which is close except for the unknown unicode characters

(Note: edited for clarity of the problem) Suggestions?

Answer 1

Simple. Your code assumes that the key is all lower case. check50 tests against mixed case keys and text.

BTW, your code handles letters and spaces. What happens if there's punctuation or numbers in the plaintext. You should print any non-alpha characters, not just spaces.

[EDIT] And after looking deeper, there's a simple oversight. Look at the following:

 char lower = (p[i] - 97) + (key[j%l] - 97) %26 + 97;

The idea of the %26 is to wrap back around when the encoding goes past z. BUT, because of the placement (or lack of placement) of parentheses, the % 26 only applies to the key and not p[i]. Here's the fix:

char lower = (  (p[i] - 97) + (key[j%l] - 97)  ) %26 + 97;

This cleans up the encoding.

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