C++ ::toupper not allowing equality comparison? - c++

Question

I am trying to convert a string to uppercase so I can manipulate it, but while I can successfully manipulate natural uppercase strings, as well as convert lowercase to uppercase, using this method of conversion fails to allow the manipulation.
For example, if I pass "hello" through the encryption, my encrypted string becomes "HELLO", but when I pass "HELLO" through (naturally capitalized), it correctly shifts.
Is there a different way of forcing uppercase that I need to be using or am I doing something wrong?
int Caesar::encrypt (const std::string &message, std::string &emessage) {
int count = 0;
emessage = message;
std::transform(emessage.begin(), emessage.end(), emessage.begin(), ::toupper);
for (std::string::size_type i = 0; i < message.size(); i++) {
for (int j = 0; j < 26; j++) {
if (emessage[i] == std_alphabet[j]) {
std::replace(emessage.begin(), emessage.end(), message[i], c_alphabet[j]);
}
}
count++;
}
return count;
}
constructor:
Caesar::Caesar (int shift) {
// loop to populate vector with 26 letters of English alphabet
// using ASCII uppcase letter codes
for (int i = 0; i < 26; i++) {
std_alphabet.push_back(i + 65);
}
// fills Caesar alphabet with standard generated alphabet
c_alphabet = std_alphabet;
// shifts Caesar alphabet based off the constructor parameter
std::rotate(c_alphabet.begin(), c_alphabet.begin() + shift, c_alphabet.end());
}
test file:
void testCaesar() {
Caesar test(4);
std::string original = "HELLO";
std::string encrypted = "";
test.encrypt(original,encrypted);
std::cout << encrypted << std::endl;
std::cout << original << std::endl;
}
int main() {
testCaesar();
return 0;
}
Obviously there is a header and includes and stuff but that is the basic code
the header file includes the two private vectors

Answer 1

The specific issue you are seeing is that you're replacing the wrong thing here:
std::replace(emessage.begin(), emessage.end(), message[i], c_alphabet[j]);
If message was lowercase, then emessage will be all upper-case letters - none of which will be message[i]. so that replacement won't do anything. You meant:
std::replace(emessage.begin(), emessage.end(), emessage[i], c_alphabet[j]);
^^^^^^^^^^^
That said, your algorithm is totally wrong as HELLO encrypts as BCBBA with a shift of 4. There is a 1-1 mapping on letters, so H and L cannot both go to B. What you want to do is shift each letter as you go by just replacing it with what its next letter should be. That is:
for (std::string::size_type i = 0; i < emessage.size(); ++i) {
emessage[i] = c_alphabet[emessage[i] - 'A'];
}
With which you don't actually need the initial transformation step:
emessage = message;
for (std::string::size_type i = 0; i < emessage.size(); ++i) {
emessage[i] = c_alphabet[::toupper(emessage[i]) - 'A'];
}
The whole thing can be abridged quite a bit by just dropping your count (which is just the size anyway, so is redundant) and taking the message by-value:
std::string encrypt(std::string from) { // intentionally copying
for (char& c : from) {
c = c_alphabet[::toupper(c) - 'A'];
}
return from;
}