Can someone explain what is wrong with my code that is trying to compare 2 arrays?

Question

I am trying to compare 2 arrays and return a new array with any items only found in one of the two given arrays. So here is what I got:

function diffArray(arr1, arr2) {
  var newArr = [];
  var max;
  var test;
  (arr1.length > arr2.length) ? (max = arr1, test = arr2) : (max = arr2, test = arr1);
  for (let i = 0; i < test.length; i++) {
    if (max.indexOf(test[i]) === -1) {
      newArr.push(test[i])
    }
  }
  return newArr;
}

console.log(diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]));

However, when I run it, newArr returns an empty array. Can someone point out the error?

what if the arrays are the same size but have different data? — Nikki9696, Jul 09 '20 at 19:17
Because you are only testing one array's values to the other, you arent checking if the other has any in the first. Ie your code never checks to see if `4` in `arr2` is in `arr1` — Patrick Evans, Jul 09 '20 at 19:18
What patrick said: you're testing the smaller against the larger. Swap that. — Nikki9696, Jul 09 '20 at 19:23
I really dislike oh-so-clever code like using the comma operator with ternaries. Just write some `if`s. — Heretic Monkey, Jul 09 '20 at 19:29
For a much better solution thank mine please see Luis Sieira's answer [here](https://stackoverflow.com/questions/1187518/how-to-get-the-difference-between-two-arrays-in-javascript) — W-B, Jul 09 '20 at 19:41

fro · Answer 1 · 2020-07-09T19:35:03.490

0

function diffArray(arr1, arr2) {
  var newArr = [];
  let checkArr = [];
  for (const val of arr1) {
    checkArr[val] = 0
  }
  for (const val of arr2) {
    checkArr[val] = checkArr[val] !== undefined ? checkArr[val] + 1 : 0
  }
  for (const val of arr1) {
    if (checkArr[val] === 0) {
        newArr.push(val)
    }
  }
  for (const val of arr2) {
    if (checkArr[val] === 0) {
        newArr.push(val)
    }
  }
  return newArr;
}

console.log(diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]));

edited Jul 09 '20 at 19:35

answered Jul 09 '20 at 19:22

fro

402
3
8

While is does solve one of the problems it doesn't solve the following cases: 1) Arrays are the same length and have different values. 2) the shorter array has values the longer one doesn't – W-B Jul 09 '20 at 19:26
Just added another solution. That will be better, I think – fro Jul 09 '20 at 19:35

score 0 · Accepted Answer · answered Jul 09 '20 at 19:35

The error is that you are only checking the values in one array. You have to check for if values in arr1 are in arr2 and if values of arr2 are in arr1.

note: I added extra values to the arrays for testing

function diffArray(arr1, arr2) {
    var newArr = [];
    arr1.forEach(element => {
        if(arr2.indexOf(element) === -1){
            newArr.push(element)
        }
    });

    arr2.forEach(element => {
        if(arr1.indexOf(element) === -1){
            newArr.push(element)
        }
    });
    return newArr
  }
  
  console.log(diffArray([1, 2, 3,6, 5,7], [1, 2, 3, 4,10,23,11,123, 5]));
  console.log(diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]));

For a much better solution please see Luis Sieira's answer [here](https://stackoverflow.com/questions/1187518/how-to-get-the-difference-between-two-arrays-in-javascript) — W-B, Jul 09 '20 at 19:41

Can someone explain what is wrong with my code that is trying to compare 2 arrays?

2 Answers2