Pushing items to an object key's value array

Question

I have the array of objects:

[ { pair_id: 1, exchange_pair_id: 183 }, { pair_id: 1, exchange_pair_id: 2}, ... ]

I want to rebuild this array as

[ { pair_id: 1, exchange_pair_id: [183, 2] }, ... ]

Here is the code I have written (the code which brings me closest to the desired result anyway):

var array = []; rows.forEach(function(row) { var obj = {}; obj.pair_id = [row.pair_id]; obj.exchange_pair_id = [row.exchange_pair_id] array.push(obj); });

Which results in:

[ { pair_id: 1, exchange_pair_id: [183] }, { pair_id: 1, exchange_pair_id: [2] }, ... ]

This seems like a very simple problem with a simple solution, but I've been wracking my brains and can't figured it out.

All you do in obj.exchange_pair_id = [row.exchange_pair_id] is wrap the value into an array but not check for similar pair ids. The simplest idea would be using keys in your array. — Paper
– Paper, Commented Sep 22, 2017 at 13:15
A simple if statement of if pair_id: == 1, etc. and then pushing would suffice in my opinion — Robert
– Robert, Commented Sep 22, 2017 at 13:17

Suren Srapyan · Accepted Answer · 2017-09-22 13:29:45Z

Try this solution. I iterate over the array by Array#forEach and try to find an element in the groupedArray by pair_id using Array#find function. If the element is found I push the exchange_pair_id into the array of that element. If not I create a new item in the array according to the item's values.

const array = [ { pair_id: 1, exchange_pair_id: 183 }, { pair_id: 1, exchange_pair_id: 2}, { pair_id: 2, exchange_pair_id: 7}, { pair_id: 3, exchange_pair_id: 988}, { pair_id: 2, exchange_pair_id: 8}, { pair_id: 3, exchange_pair_id: 98} ]; const groupedArray = []; array.forEach(item => { const found = groupedArray.find(x => x.pair_id === item.pair_id); if(found) { found.exchange_pair_id.push(item.exchange_pair_id); } else { groupedArray.push({ pair_id: item.pair_id, exchange_pair_id: [item.exchange_pair_id]}); } }); console.log(groupedArray);

Thank you, excellent answer; will research find() further. I actually feel that I could skip this step by performing a better MySQL query, if you do have a moment of your time to see this it may be a way to skip this step. Really appreciated. stackoverflow.com/questions/46365942/…

Nina Scholz · Accepted Answer · 2017-09-22 13:18:37Z

You could take a hash table for same pair_id.

var array = [{ pair_id: 1, exchange_pair_id: 183 }, { pair_id: 1, exchange_pair_id: 2}], hash = Object.create(null), result = array.reduce(function (r, o) { if (!hash[o.pair_id]) { hash[o.pair_id] = []; r.push({ pair_id: o.pair_id, exchange_pair_id: hash[o.pair_id] }); } hash[o.pair_id].push(o.exchange_pair_id); return r; }, []); console.log(result);

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

Duncan Thacker · Accepted Answer · 2017-09-22 13:23:15Z

What you're trying to do is aggregation or reduction, so I'd use Array.reduce() here. Take each element in turn and build up a new set of elements, like this:

const rows = [ { pair_id: 1, exchange_pair_id: 183 }, { pair_id: 1, exchange_pair_id: 2}, { pair_id: 4, exchange_pair_id: 6} ]; const result = rows.reduce( function( aggregate, nextElement ) { const pair_id = nextElement.pair_id; //create a new row in the aggregate if one doesn't exist let aggregatedRow = aggregate.find( r => r.pair_id === pair_id ); if ( !aggregatedRow ) { aggregatedRow = { pair_id, exchange_pair_id: [] }; aggregate.push( aggregatedRow ); } //add the new exchange pair id to the aggregate row aggregatedRow.exchange_pair_id.push( nextElement.exchange_pair_id ); return aggregate; }, []); //start with an empty array console.log( result );

Faly · Accepted Answer · 2017-09-22 13:30:38Z

Try this:

var arr1 = [ { pair_id: 1, exchange_pair_id: 183 }, { pair_id: 1, exchange_pair_id: 2}, ]; var arr2 = []; arr1.forEach(el => { var obj = arr2.find(it => it.pair_id === el.pair_id); if (!obj) { el.exchange_pair_id = [el.exchange_pair_id]; arr2.push(el); } else { obj.exchange_pair_id.push(el.exchange_pair_id); } }); console.log(arr2);

Alejandro C. · Accepted Answer · 2017-09-22 13:32:51Z

You're close, you just need to check if there's previous results. Using a map is more time efficient than looking through the list with something like find every time you see an item.

var results = new Map(); rows.forEach(function(row) { previous_result = results.get(row.pair_id) if (previous_result != null) { previous_result.exchange_pair_id.push(row.exchange_pair_id); } else { var obj = {}; obj.pair_id = row.pair_id; obj.exchange_pair_id = [row.exchange_pair_id] results.set(row.pair_id, obj); } }); Array.from(results.values())

This solution didn't work I'm afraid: Map { 1=> { pair_id: [1], exchange_pair_id: [183] }, ...
@Nick Happy to help. I tested locally and it seemed to work. What didn't work for you?

Collectives™ on Stack Overflow

Pushing items to an object key's value array

5 Answers 5

1 Comment

Comments

Comments

1 Comment

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

1 Comment

Comments

Comments

1 Comment

2 Comments

Linked

Related