Comparing array elements against the rest of the array

Question

The question might be a bit vague, but I'll explain the result I'm expecting to get with an example.

Say I have the following array made out of objects with the following shape:

[
   {
      id: 1,
      value: 10
   },
   {
      id: 2,
      value: 100
   },
   {
      id: 3,
      value: 10
   },
   {
      id: 4,
      value: 10
   },
   {
      id: 5,
      value: 1000
   },

]

This array might contain hundrends, maybe thousands of entries, but for simplicity, I'll keep it small.

What I'm trying to achieve is compare the value property of every object with the other value properties and assign a new property duplicate with a boolean value to that specific object.

Given the example above, I would expect to receive an array with the following members:

[
  {
     id: 1,
     value: 10,
     duplicate: true
  },
  {
     id: 2,
     value: 100
  },
  {
     id: 3,
     value: 10,
     duplicate: true
  },
  {
     id: 4,
     value: 10,
     duplicate: true
  },
  {
     id: 5,
     value: 1000
  },

]

Whats the most optimal way I could implement this behavior ?

Thank you.

What defines if an entry is a duplicate? The id? The value? Or id and value? — Christian Baumann, Sep 25 '20 at 07:51
Please visit the [help], take the [tour] to see what and [ask]. Do some research, search for related topics on SO; if you get stuck, post a [mcve] of your attempt, noting input and expected output using the `[<>]` snippet editor. — mplungjan, Sep 25 '20 at 07:55

T.J. Crowder · Accepted Answer · 2020-09-25T08:14:52.697

I'd do a single pass through the array remembering the first seen entry with a given value in a Map, marking that first entry (and any others) as duplicates if it's present, like this:

const map = new Map();
for (const entry of array) {
    const previous = map.get(entry.value);
    if (previous) {
        previous.duplicate = entry.duplicate = true;
    } else {
        map.set(entry.value, entry);
    }
}

Live Example:

const array = [
   {
      id: 1,
      value: 10
   },
   {
      id: 2,
      value: 100
   },
   {
      id: 3,
      value: 10
   },
   {
      id: 4,
      value: 10
   },
   {
      id: 5,
      value: 1000
   },

];

const map = new Map();
for (const entry of array) {
    const previous = map.get(entry.value);
    if (previous) {
        previous.duplicate = entry.duplicate = true;
    } else {
        map.set(entry.value, entry);
    }
}

console.log(array);

@b9s - Yes, it does -- note the `previous` part where it marks duplicates. :-) That's why it's a `Map` and not just a `Set`. — T.J. Crowder, Sep 25 '20 at 07:55
@b9s click on *"Show code snippet"* to check the runnable snippet. — adiga, Sep 25 '20 at 07:57

score 1 · Answer 2 · answered Sep 25 '20 at 07:53

1

You can do this by first determining which are the duplicates, and then setting the 'duplicate' attribute.

counts = items.reduce((counter, item) => {
    if (counter[item.value] != null) {
        counter[item.value] += 1;
    } else {
        counter[item.value] = 1;
    }
    return counter;
}, {});

After this, you can go over your items, and if the count is >=2, set the 'duplicate' attribute.

items.forEach((item) => {
    if (counter[item.value] > 1) {
        item['duplicate'] = true;
    }
});

answered Sep 25 '20 at 07:53

b9s

517
4
13

Could use `counter[item.value] = (counter[item.value] || 0) + 1` to simplify the conditional. – Masklinn Sep 25 '20 at 07:55
There's no need for two passes through the array to do this. (But this is still a viable approach.) – T.J. Crowder Sep 25 '20 at 07:55

Magiczne · Answer 3 · 2020-09-25T07:59:07.093

1

You can use Array.map and Array.filter for that.

const input = [
  { id: 1, value: 10 },
  { id: 2, value: 100 },
  { id: 3, value: 10 },
  { id: 4, value: 10 },
  { id: 5, value: 1000 }
]

const output = input.map(entry => {
  if (input.filter(x => x.value === entry.value).length > 1) {
    return {
      duplicate: true,
      ...entry
    }
  }
  
  return entry
})

console.log(output)

edited Sep 25 '20 at 07:59

answered Sep 25 '20 at 07:55

Magiczne

1,586
2
15
23

Nina Scholz · Answer 4 · 2020-09-25T08:15:34.620

A copy from myself with a single loop and an object for storing seen values.

This approach returns a new array and does not mutate the given data.

var data = [{ id: 1, value: 10 }, { id: 2, value: 100 }, { id: 3, value: 10 }, { id: 4, value: 10 }, { id: 5, value: 1000 }],
    result = data.map((seen => ({ ...o }) => {
        if (o.value in seen) {
            o.duplicate = true;
            if (seen[o.value]) {
                seen[o.value].duplicate = true;
                seen[o.value] = false;
            }
        } else seen[o.value] = o;
        return o;
    })({}));

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

score 0 · Answer 5 · answered Sep 25 '20 at 07:57

0

I would create a map with value as the key, and a list of ids as the values, than after iterating over the whole map and creating the new mapping, unpack it back tothe desired form, and add duplicated for keys with more than one value.

answered Sep 25 '20 at 07:57

Uriya Harpeness

628
1
6
21

score 0 · Answer 6 · answered Sep 25 '20 at 07:58

0

I think this will help you. arr is your array.

arr.forEach(e=> {
  const dublicatedDataLenth = arr.filter(a => a.value == e.value).length;
  if(dublicatedDataLenth > 1){
    e.dublicate = true;
  }
})

It should be what you are looking for.

answered Sep 25 '20 at 07:58

Hasip Timurtas

983
2
11
20

Comparing array elements against the rest of the array

6 Answers6