CS 10 SI Classroom Session 4 - Palindrome Testing

main.cpp

#include <iostream>

using namespace std;

int main() {
  // In programming, a sentinel value is a special value that guarantees
  // loop termination when seen. In this example, we use 'q' to represent quit.
  const string SENTINEL = "q";
  string userInput = "";

  cout << "Enter a string to test if it is a palindrome. Enter 'q' to quit: ";
  cin >> userInput;

  // We want to keep testing user inputted string to see if they are palindromes
  // until they choose to quit.
  while (userInput != SENTINEL) {
    // Let's skip doing work if we already know it's a palindrome.
    // I.E. It has one or less characters.
    if (userInput.size() <= 1) {
      cout << "Of course one letter or less words are palindromes!" << endl;
    } else {
      // Now, we actually need to figure out if the string is a palindrome.
      // So, first of all, a palindrome is a word that is spelled the same
      // way backwards and forwards like 'bob' or 'racecar'.
      
      // So let's look through half of the string, and compare the front half
      // to the back half.
      
      // Let's keep a boolean variable to help us keep track of whether or
      // not everything matches on the front and back.
      bool isMatching = true;
      for (int i = 0; i < userInput.size() / 2 && isMatching; ++i) {
	// If the front half and back half at this position don't match,
	// update boolean to reflect discovery.

	// Note that we do `userInput.size() - i - 1` because we count from 0.
	if (userInput.at(i) != userInput.at(userInput.size() - i - 1)) {
	  isMatching = false;
	}
      }

      // If the loop finishes and everything matches,
      // you can tell it's a palindrome.
      if (isMatching) {
	cout << "Cool, " << userInput << " is a palindrome!" << endl;
      } else {
	cout << "Hmm, it doesn't look like " << userInput << " is a palindrome!"
	     << endl;
      }
    }

    // Now, remember that we the user to enter in new input or else this will
    // loop forever.
    cout << "Enter a string to test if it is a palindrome. Enter 'q' to quit: ";
    cin >> userInput;    
  }

  return 0;
}