Javascript checking whether string is in either of two arrays

Question

I'm pulling my hair out over this one. I have two arrays, likes & dislikes, both filled with about 50 strings each.

I also have a JSON object, data.results, which contains about 50 objects, each with an _id parameter.

I'm trying to check find all the objects within data.results that aren't in both likes and dislikes.

Here's my code at present:

var newResults = []
      for(var i = 0; i<data.results.length; i++){
        for(var x = 0; x<likes.length; x++){
          if(!(data.results[i]._id == likes[x])){
            for(var y = 0; y<dislikes.length; y++){
                if(!(data.results[i]._id == dislikes[y])){
                  newResults.push(data.results[i]);
                  console.log("pushed " + data.results[i]._id);
                }
                else
                {
                   console.log("They already HATE " + data.results[i]._id + " foo!"); //temp
                }
            }
          }
          else
          {
             console.log(data.results[i]._id + " is already liked!"); //temp
          }
        }
      }

As you can see, I'm iterating through all the data.results objects. Then I check whether their _id is in likes. If it isn't, I check whether it's in dislikes. Then if it still isn't, I push it to newResults.

As you might expect by looking at it, this code currently pushes the result into my array once for each iteration, so i end up with a massive array of like 600 objects.

What's the good, simple way to achieve this?

@rps: This is probably *[not a good idea](http://stackoverflow.com/questions/500504/why-is-using-for-in-with-array-iteration-such-a-bad-idea)*. — Scott Sauyet, Jul 30 '13 at 11:29
@ScottSauyet thanks , never thought about the `Array.prototype.foo = 1;` but I have always used it so far without problems, Guess I better stop now. — , Jul 30 '13 at 11:41
@rps btw, using an `for ... in` loop to iterate over an array, is not only a bad idea, but also, way slower than using a standard `for` loop — Moritz Roessler, Jul 30 '13 at 11:44
@C5H8NNaO4 because it checks whether any value is there or not at each index and returns value alone? — , Jul 30 '13 at 11:46
Enumerating the properties of an object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; (http://es5.github.io/#x12.6.4) In other words, it has to look up the prototype chain of the array, before iterating, which is not the case in a normal for loop — Moritz Roessler, Jul 30 '13 at 11:56

score 3 · Accepted Answer · answered Jul 30 '13 at 11:21

3

for (var i = 0; i < data.results.length; i++) {
    isInLiked = (likes.indexOf(data.results[i]) > -1);
    isInHated = (dislikes.indexOf(data.results[i]) > -1);
    if (!isInLiked && !isInHated) {
        etc...
    }
}

answered Jul 30 '13 at 11:21

Moos Hueting

650
4
14

Note, though, that if you have to support IE < 9, you will probably need a *[shim](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf#Compatibility)*. Or you could write a simpler *[contains](http://stackoverflow.com/questions/237104/array-containsobj-in-javascript)* function. – Scott Sauyet Jul 30 '13 at 11:26
@AndréDion They aren't the same length. – JVG Jul 30 '13 at 11:26

Moritz Roessler · Answer 2 · 2013-07-30T11:48:41.450

When checking whether an Array contains an element, Array.prototype.indexOf (which is ECMAScript 5, but shimmable for older browsers), comes in handy. Even more when combined with the bitwise NOT operator ~ and a cast to a Boolean !

Lets take a look how this could work.

Array.prototype.indexOf returns -1 if an Element is not found.

Applying a ~ to -1 gives us 0, applying an ! to a 0 gives us true.

So !~[...].indexOf (var) gives us a Boolean represantation, of whether an Element is NOT in an Array. The other way round !!~[...].indexOf (var) would yield true if an Element was found.

Let's wrap this logic in a contains function, to simply reuse it.

function contains (array,element) {
    return !!~array.indexOf (element);
}

Now we only need an logical AND && to combine the output, of your 2 arrays, passed to the contains function.

var likes = ["a","b","f"] //your likes
var dislikes = ["c","g","h"] //your dislikes

var result = ["a","c","d","e","f"]; //the result containing the strings
var newresult = []; //the new result you want the strings which are NOT in likes or dislikes, being pushed to

for (var i = 0,j;j=result[i++];) //iterate over the results array
     if (!contains(likes,j) && !contains (dislikes,j)) //check if it is NOT in likes AND NOT in dislikes
           newresult.push (j) //if so, push it to the newresult array.

console.log (newresult) // ["d","e"]

Here is a Fiddle

_{Edit notes:

1. Added an contains function, as @Scott suggested}

I might add a `contains` function for readability: `var contains = function(haystack, needle) {return !!~haystack.indexOf(needle);};` and change the test to `if (!contains(likes, j) && !contains(dislikes, j))`. I think that would make the code cleaner, and `contains` is clearly reusable. — Scott Sauyet, Jul 30 '13 at 11:43
@Scott yep, that sounds like a good idea =), updated the answer — Moritz Roessler, Jul 30 '13 at 11:50

score 0 · Answer 3 · answered Jul 30 '13 at 11:20

0

Use likes.indexOf(data.results[i]._id) and dislikes.indexOf(data.results[i]._id).

if (likes.indexOf(data.results[i]._id) != -1)
{
// they like it :D
}

answered Jul 30 '13 at 11:20

Valentin D

705
7
14

Beware that [`Array.indexOf`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf) isn't supported by all browsers. – André Dion Jul 30 '13 at 11:22
Right, forgot about old browsers :( here is the implementation for those old browsers. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf?redirectlocale=en-US&redirectslug=JavaScript%2FReference%2FGlobal_Objects%2FArray%2FindexOf#Compatibility – Valentin D Jul 30 '13 at 11:28

score 0 · Answer 4 · answered Jul 30 '13 at 11:30

Try first creating an array of common strings between likes and dislikes

var commonStrAry=[];
for(var i = 0; i<likes.length; i++){
for(var j=0; j<dislikes.length; j++){
    if(likes[i] === dislikes[j]){
        commonStrAry.push(likes[i] );
    }
}
}

then you can use this to check against data.results and just remove the elements that don't match.

Javascript checking whether string is in either of two arrays

4 Answers4