Help with the Symmetric Differece

1

Hy, I’m on the Symmetric Difference Challenge. This is the code I wrote:

function removeFromArr(arr, val) {
  arr.forEach(function (el, i) {
    if (el === val)
      arr.splice(arr.indexOf(el), 1);
  });
}

function symmetricDiff(a, b) {
  var arr = [];
  
  arr = a.concat(b);
  
  a.forEach(function (val, i) {
    b.forEach(function (val1, j) {
      if (val1 === val) removeFromArr(arr, val);
    });
  });
  
  return arr;
}

function sym(args) {
  args = Array.prototype.slice.call(arguments);
  
  return args.reduce(function (a, b) {
    console.log(a);
    return symmetricDiff(a, b);
  });
}

But it only passes for two arrays.
The removeFromArr() is a helper function i created to remove elements from the array.
Thanks in advance.

2

I think there are at least two problems with your code.

  1. removeFromArr doesn’t work as intended: when you remove something from an array while iterating over it you will skip items.
    Why? Let’s say you are in a for loop iterating over an array and your loop counter i is currently 2. Now you remove array[2], thus array[3] becomes the new array[2]. Then the loop continues and i is incremented. Now array[3] is checked, the value that is now in array[2] has been skipped.
    That’s the reason why the case sym([1, 2, 5], [2, 3, 5], [3, 4, 5]) fails: sym([1, 2, 5], [2, 3, 5]) becomes [1, 3]. Then when you calculate sym([1, 3], [3, 4, 5]), you get the concatenated array [1, 3, 3, 4, 5]. You remove the first occurance of 3 while iterating over the array, effectively skipping the second 3 that comes next :slight_smile:
    That’s why you should never modify objects you iterate over, work with a copy instead.

  2. You don’t handle the case where one array contains duplicates. The result should be a proper set meaning it should only contain unique values, i. e. [1, 2, 3] instead of [1, 1, 2, 3].

2 Likes
3

Thank you so much, now it makes sense.

4

Hi there,

I wrapped your code in spoiler tags. It’s unlikely that anyone looking at this topic wouldn’t already have completed the task, but it’s good practice to blur challenge solutions anyways.

5

Hi just wondering have you got this sorted or do you still need some help

6

Yeah I finished the algorithm, all I did was modify the removeFromArray() helper function and filter the end result of the main function so there are no duplicates. Thanks again.

7

How do I do that, please?

8 
[spoiler]
// your code here (wrapped in backticks)
[/spoiler]