Breaking a single array into many arrays based on values within array. JavaScript

Question

This is a continuation from a previous question in regards to grouping. I have a file of structure below and what I want to do rather than simply isolating one id I want to loop through the entire array (imagine 1000+ ids) and create separate arrays for each which I can then do further work on. So in the example below id1 would be group together in one array and id2 would be grouped together in another array. Once I had separated each ID into a separate array I would then continue to filter each further based on a set of conditions.

[{col1: 'id1', col2: '123', col3: '12/01/12'},
{col1: 'id1', col2: '100', col3: '12/01/12'},
{col1: 'id2', col2: '-100', col3: '12/01/12'},
{col1: 'id2', col2: '123', col3: '13/01/12'}]

.

Any advice on the best way to break the array down and then how to call individual ID arrays would be much appreciated.

Thanks in advance

How would the desired result look like? Any advice on the best way to break the array down... seems a bit philosophical? Best in what? Performance? Readability? — ibrahim mahrir
– ibrahim mahrir, Commented Apr 29, 2017 at 18:02
@ibrahimmahrir by best I meant actually gets the job done! Will be sure to frame my question better next time @ Pankaj - just different arrays that are associated with an individual ID - in the real data set a ID may have 100's of rows associated with it which I then want to do further manipulation on. Both solutions offered below do the job nicely. — GoodCat
– GoodCat, Commented Apr 29, 2017 at 22:50

Nina Scholz · Accepted Answer · 2017-04-29 18:21:27Z

3

You could use the given id as key for an object and collect the items in own array.

var data = [{ col1: 'id1', col2: '123', col3: '12/01/12' }, { col1: 'id1', col2: '100', col3: '12/01/12' }, { col1: 'id2', col2: '-100', col3: '12/01/12' }, { col1: 'id2', col2: '123', col3: '13/01/12' }],
    object = data.reduce(function (r, o) {
        r[o.col1] = r[o.col1] || [];
        r[o.col1].push(o);
        return r;
    }, Object.create(null));
    
console.log(object);

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

edited Apr 29, 2017 at 18:21

answered Apr 29, 2017 at 18:09

Nina Scholz

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1 Comment

GoodCat Over a year ago

Thanks - did exactly what I needed - if you have a moment and could explain the method it would be much appreciated just for my own learnings

vinayakj · Accepted Answer · 2017-04-29 18:19:50Z

1

If you want to have the array of arrays of same ids grouped together, try below code.

var a  = [{col1: 'id1', col2: '123', col3: '12/01/12'},
{col1: 'id1', col2: '100', col3: '12/01/12'},
{col1: 'id2', col2: '-100', col3: '12/01/12'},
{col1: 'id2', col2: '123', col3: '13/01/12'}];
var currentID = a[0].col1;
var group = [];
var collectionOfIDs = [];
a.forEach(function(v, i) {
  //console.log(i,v);
  if (currentID == v.col1) {
    collectionOfIDs.push(v);
  } else {
    group.push(collectionOfIDs);
    currentID = v.col1;
    collectionOfIDs = [v];
  }
});
group.push(collectionOfIDs);
console.log(group)

answered Apr 29, 2017 at 18:19

vinayakj

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6 Comments

vinayakj Over a year ago

The above code is useful if you want to have control over structure of grouped data & if its needed then use it else go with the solution given by @Nina if that structure is ok for the further solution.

GoodCat Over a year ago

Thanks - both work really well - can you explain how this is different from Ninas? Both appear to give me control over the newly group data.

vinayakj Over a year ago

By 'control over structure', I meant while creating the grouped data itself, you can tweak the structure to whatever you want, like having array of arrays or object of objects with keys being ids. for more info about Ninas answer refer reduce API - developer.mozilla.org/en/docs/Web/JavaScript/Reference/…

GoodCat Over a year ago

Thanks again - really appreciate the help

GoodCat Over a year ago

@Vinayaki - as a follow up: If I try and access the data sitting at id1 I can access all data associated with that id but if I want to get just get all col2 values associated with that id I get undefined. I tried the following: console.log(group[1].col2); The alternative is: console.log(group[1][1].col2) But this then only gives me one element of the grouped array. When I would like to log all data associated with that col. Is there an effective way to do this or would I have to create a loop which looped through each ID and then each array within that ID array? Thanks!

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