What's wrong with this code for the Caesar Cipher?

Question

I created this Caesar Cipher program, but for some reason, I can't find its bug. Here is the code. It compiles, but it just prints out the same thing I enter, for any key.

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

string encryptor(string encryptString, int key);

int main(int argc, string argv[]) {
    if (argc >= 2) {
        int k = atoi(argv[1]);
        string stringToBeEncrypted = GetString();
        printf("%s\n", encryptor(stringToBeEncrypted, k));
    } else {
        printf("Please add a key argument\n");
        return 1;
    }
}

string encryptor(string encryptString, int key) {
    for (int i = 0, n = strlen(encryptString); i < n; i++) {
        int ascii = (int) encryptString[i];
        key = key % 26;
        if (isalpha(encryptString[i]) == true) {
            if (islower(encryptString[i]) == true) {
                ascii = ((ascii-96+key) % 26) + 96;
            }else {
                ascii = ((ascii-64+key) % 26) + 64;
            }
            // ascii += key;
            // if (ascii > 122) {
            //     //ascii = ascii - 122 + 96;
            //     ascii = (ascii % 26) + 97;
            // } else if (ascii > 90 && ascii < 97) {
            //     //ascii = ascii - 90 + 64;
            //     ascii = (ascii % 26) + 65;
            // }

        }

        char newChar = (char) ascii;
        encryptString[i] = newChar;
    }
    return encryptString;
}

Answer 1

The problem lies with your use if isalpha() and islower(). Your code is:

if (isalpha(encryptString[i]) == true)

Surprisingly unintuitively, if the character is an alpha, this will NOT evaluate as a true statement.

If you read the documentation, it says that it returns 0 if false and a non-zero if true. This number varies, depending on the architecture, OS, and other factors. (In the cloud9 IDE, it's 1024.) The fail occurs when you compare that number to true. However, all architectures treat 0 as false and non-0 as true, even though it doesn't compare equally to true. So, either of the following will work, although the first is preferred.

if ( isalpha(encryptString[i]) )

if (isalpha(encryptString[i]) != false)

Same logic applies to isupper and islower.

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