Simple Encryption program array - c++

Question

Building a simple program that multiplies the ASCII value of chars in a string by 3 to encrypt and then divide by 3 to decrypt. So far I got the encryption part down but whenever I enter what the encryption gave and try to decrypt it doesn't work. I think it has something to do with the buffer stream but I could be wrong if anyone could help.
#include<iostream>
using namespace std;
int main()
{
string message;
int charValue;
int counter;
int encrypt;
char choice;
char quit = 'N';
while (quit == 'N')
{
cout << "Enter E to encrypt or D to Decrypt\n";
cin >> choice;
toupper(choice);
cout << "Enter text no spaces: ";
cin >> message;
int messagelen = message.length();
string stringArray[255];
if (choice == 'E')
{
for (counter = 0; counter < messagelen; counter++) //*3 to ascii val
{
stringArray[counter] = message[counter] * 3;
}
for (counter = 0; counter < messagelen; counter++)
{
cout << stringArray[counter];
}
}
else
{
for (counter = 0; counter < messagelen; counter++) // divide 3 to ascii val
{
stringArray[counter] = message[counter] / 3;
}
for (counter = 0; counter < messagelen; counter++)
{
cout << stringArray[counter];
}
}
cout << "\nY to go again N to quit";
cin >> quit;
}
return 0;
}

Answer 1

This is a working implementation, although I agree with the other answer that you should use encrypt and decrypt functions. I found quite a few other bugs with your code working through it. You should enable all warnings with -Wall -Werror and fix them:
#include <iostream>
#include <vector>
#include <sstream>
int main()
{
// removed some unused variables
std::string message;
size_t counter;
char choice;
char quit;
// use vector of int instead of array of strings
std::vector<int> encryptArray;
// change to do while loop. Not particularly necessary, but I think
// it makes more sense in this case. Your condition is broken. If the
// user enters 'Y' at the end to go again, then the quit == 'N'
// condition is false and the program terminates.
do
{
std::cout << "Enter E to encrypt or D to Decrypt\n";
std::cin >> choice;
// toupper returns a value, you need to assign it to choice.
// Not capturing the return value makes this a noop.
choice = toupper(choice);
if (choice == 'E')
{
std::cout << "Enter text no spaces: ";
std::cin >> message;
size_t messagelen = message.length();
// initialize vector to input message length size
encryptArray = std::vector<int>(messagelen);
for (counter = 0; counter < messagelen; counter++) //*3 to ascii val
{
encryptArray[counter] = message[counter] * 3;
}
// Note, this 2nd loop is more work than you need, you could
// simply put the std::cout line in the loop above below the
// assignment
for (counter = 0; counter < messagelen; counter++)
{
// added the separator just for clarity. You could also print
// hex bytes
std::cout << encryptArray[counter] << "|";
}
}
else
{
// all the data we care about is in the vector now
for (counter = 0; counter < encryptArray.size(); counter++) // divide 3 to ascii val
{
// you don't want to /3 what's in the message here, you want
// to /3 the encrypted values, which are in the vector
encryptArray[counter] = encryptArray[counter] / 3;
}
// plenty of ways to convert the vector to a string, this is not
// a "modern" way.
// Note, you could skip this loop entirely, and in the one
// above, simply do ss << (char)(encryptArray[i] / 3);
// Not necessary to write the data back to encryptArray first.
std::stringstream ss;
for (size_t i=0; i<encryptArray.size(); ++i)
{
ss << (char)encryptArray[i];
}
std::cout << "decrypted string: " << ss.str() << std::endl;
}
std::cout << "\nY to go again N to quit: ";
std::cin >> quit;
} while(quit != 'N'); // loop until quit == N
return 0;
}
Finally, I removed using namespace std;, here's why
Things get squirrely working with stdin on godbolt, but here's a working demonstration, at least initially.

Answer 2

It would genuinely help you to break this down into smaller problems. Let's "encrypt" a std::string into a std::vector<int>:
std::vector<int> encrypt_msg(std::string s) {
std::vector<int> v;
for (auto ch = s.begin(); ch != s.end(); ch++) {
v.push_back(static_cast<int>(*ch) * 3);
}
return v;
}
Then let's "decrypt" a message, performing the transformation in reverse.
std::string decrypt_msg(std::vector<int> v) {
std::string s;
for (auto i : v) {
s += static_cast<char>(i / 3);
}
return s;
}
Now you can test and see that your individual functions that do one thing work, and putting together the overall program should be much easier.