Question

i was wondering if somebody could help me on my diff function. its supposed to find the diff between 2 inputs. the function takes in 2 arrays so if array 1 has.. “word1”, “word2” and array 2 has “word1”, “word2”, and “word3”, it should just return “word3”. it works for the most part, but with the certain input below, it doesn’t work.

function diff(a1, a2) {

if (a1.length > a2.length
       || a1.length == a2.length) {
    return [""];
}
a1 = a1.slice(0);
a2 = a2.slice(0);


for (i = 0; i < a1.length; i++) {
    for (k = 0; k < a2.length; k++) {
        if (a1[i] == a2[k]) {
            a1.splice(i, 1);
            a2.splice(k, 1);
        }
    }
}

for (j = 0; j < a2.length; j++) {
    for (p = 0; p < a1.length; p++) {
        if (a2[j] == a1[p]) {
            a2.splice(p, 1);
            a1.splice(j, 1);
        }
    }
}


a1 = a1.concat(a2);
return a1;
}

var s1 = ["one", "two", "three", "four",];
var s2 = ["one", "two", "three", "four", "five"];


document.write(diff(s1, s2));

the answer should just return “five”, but its returning “four”, “four”, “five”. help would be much appreciated. im super stuck. thanks a bunch!

Answer 1

As far as I understand, you want to return an array containing only differing elements from both. Try it this way, without complex double loops and splices and whatnot:

function diff(a1, a2) {

  var aa1 = {}, aa2 = {}, res = [];

  // create an object from a1, containing value:array-index pairs:
  for (var i = 0; i < a1.length; i++) {
      aa1[a1[i]] = i;
  }
  // create an object from a2 like the one from a1
  for (var i = 0; i < a2.length; i++) {
      aa2[a2[i]] = i;
  }
  /**
    * loop through the first object (from a1)
    * if a value in this object is not found in 
    * the object from a2, push that value to 
    * the result array (res)
   */
  for (var l in aa1){
    if (aa1.hasOwnProperty(l) && !(l in aa2)) {
        res.push(a1[aa1[l]]);
    }
  }
  /**
    * but the second array can also contain
    * values differing from the values in the first.
    * So, repeat the above for the object created 
    * from a2
   */
  for (var l in aa2){
   if (aa2.hasOwnProperty(l) && !(l in aa1)) {
        res.push(a2[aa2[l]]);
   }
 }
 /**
   * now res contains all different values
   * note: double values are not counted
   * so ['one','two','tree'] vs ['one','two','four','four']
   * will return ['tree','four']
   * note2: if you want to find the first difference,
   * return res[0]
   */

 return res;
}

If it’s just the first difference you want to find, this (array only) method will do:

function diff(a1, a2) {
  var firstdiff = null, i = -1;

  /**
    * local function to check if value [val] 
    * exists in array [arr]. Called from
    * within the loops
    */
  function check(val,arr){
      var i = -1;
      while(++i<arr.length){
          if(val === arr[i]){ return true;}
      }
      return false;
  }
  // loop a1 and check values vs a2-values
  while (++i<a1.length){
    if (!check(a1[i],a2)) {
      return firstdiff = a1[i];
    }
  }
  // no difference found, continue a2 vs a1
  i = -1;
  while (++i<a2.length){
    if (!check(a2[i],a1)) {
      return firstdiff = a2[i];;
    }
  }
  // there are no differences, firstdiff = null
  return firstdiff;
}