Last Updated: February 25, 2016
·
186
· sukima

Rotating a number / array index

This is a super simple concept but often missed. Example problem set: You have a game with two or more players and the currentPlayer holds the state as an int. Normally when it's time to go to the next player a currentPlayer + 1 would do till you reached the last player and you get an index out of bounds error (currentPlayer => 3 when there are only two players). Normally a conditional to check out of bounds is do:

currentPlayer++;
if (currentPlayer >= numerOfPlayers) {
  currentPlayer = 0;
}

However, there is an easier way with the modulus. Since math is our friend, a modulus will convert your out of bounds into a rotating function. This is often the case for determining even odd values with an incrementing counter:

var i, isEven, results = [];
for (i = 0; i < 20; i++) {
  isEven = (i % 2) === 0;
  results.push(isEven);
}

If you reassign the counter to the result it's resetting that counter. This is equivalent to the first example:

currentPlayer = (currentPlayer + 1) % numberOfPlayers;
// Or for golf competitions:
c = ++c % n;

Using this technique and the poer of having an int for an array index the following code:

function playTurn(domElTile) {
  if (currentPlayer === 0) {
    domElTile.className = 'white';
    currentPlayer = 1;
  } else {
    domElTile.className = 'black';
    currentPlayer = 0;
  }
}

Can be simplified down to:

function playTurn(domElTile) {
  domElTile.className = ['white', 'black'][currentPlayer];
  currentPlayer = (currentPlayer + 1) % 2;
}