Question

I need to add an element to an array only if it is not already there in Javascript. Basically I’m treating the array as a set.

I need the data to be stored in an array, otherwise I’d just use an object which can be used as a set.

I wrote the following array prototype and wanted to hear if anyone knew of a better way. This is an O(n) insert. I was hoping to do O(ln(n)) insert, however, I didn’t see an easy way to insert an element into a sorted array. For my applications, the array lengths will be very small, but I’d still prefer something that obeyed accepted rules for good algorithm efficiency:

Array.prototype.push_if_not_duplicate = function(new_element){
    for( var i=0; i<this.length; i++ ){
        // Don't add if element is already found
        if( this[i] == new_element ){
            return this.length;
        }
    }
    // add new element
    return this.push(new_element);
}

Answer 1

If I understand correctly, you already have a sorted array (if you do not have a sorted array then you can use Array.sort method to sort your data) and now you want to add an element to it if it is not already present in the array. I extracted the binary insert (which uses binary search) method in the google closure library. The relevant code itself would look something like this and it is O(log n) operation because binary search is O(log n).

function binaryInsert(array, value) {
  var index = binarySearch(array, value);
  if (index < 0) {
    array.splice(-(index + 1), 0, value);
    return true;
  }
  return false;
};

function binarySearch(arr, value) {
  var left = 0;  // inclusive
  var right = arr.length;  // exclusive
  var found;
  while (left < right) {
    var middle = (left + right) >> 1;

    var compareResult = value > arr[middle] ? 1 : value < arr[middle] ? -1 : 0;
    if (compareResult > 0) {
      left = middle + 1;
    } else {
      right = middle;
      // We are looking for the lowest index so we can't return immediately.
      found = !compareResult;
    }
  }
  // left is the index if found, or the insertion point otherwise.
  // ~left is a shorthand for -left - 1.
  return found ? left : ~left;
};

Usage is binaryInsert(array, value). This also maintains the sort of the array.