Question

bool choose() {
    int answer = 0;
    while(answer != 1 || answer != 2) {
        cout << endl << "Do you want to encrypt(enter 1) or decrypt(enter 2)?" << endl;
        cin >> answer;
    }
    if(answer == 1) return true;
    return false;
}

What is the best way to read from line and compare input with integers?
I know ways cin , gets , getline(cin, answer) .
Which should I use and why?

At the moment, this way is not working, because when i enter 1 or 2, it still stays in while.

Answer 1

This code is a very good candidate for do-while loop, and if you use that you’re not required to initialize the variable answer.

int answer; //= 0; no need to initialize!
do {

  cout<<"Do you want to encrypt(enter 1) or decrypt(enter 2)?"<<endl;
  cin >> answer;

}while(answer != 1 && answer != 2);

And of course, you need to use && as others has already pointed out. 🙂

By the way, what would happen if user entered any non-integer input such as hgjkhg? cin>>answer would fail to read the input and it will remain there forever, and the flag of cin will be set failure, and cin will not be able to read futher input. That means, the loop will never exit!

To avoid this, and to make the code more robust and complete you should write it as:

int answer = 0; 
while(answer != 1 && answer != 2) {

  cout<<"Do you want to encrypt(enter 1) or decrypt(enter 2)?"<<endl;
  if ( !(cin >> answer) )
  {
    cin.clear(); //clear the failure flag if there is an error when reading!
    std::string garbage;
    std::getline(cin, garbage); //read the garbage from the stream and throw it away
  }
}

Yes. You can use your while loop instead of do-while. All that you need to add the if(!(cin>>answer)) { ... } in your code. 🙂