Reorder an array around an index

Question

My issue is very simple to explain, but figuring out the most efficient way of doing it is proving difficult.

I am trying to reorder an array starting at a particular index in the array.

What I have:

var array = [1,2,3,4,5];

What I want:

array = [3,4,5,1,2];

What I've done:

var array = [1,2,3,4,5];
var arr1 = array.slice(array.indexOf(3), array.length);
var arr2 = array.slice(0, array.indexOf(3));

arr1.push(arr2);

var newArray = arr1.join(); // [3,4,5,1,2];

As you can see, there is quite a bit going on for what is seemingly a simple task. If you have any suggestions on how to improve this I'd be grateful. Either vanilla JS or Underscore/Lodash is fine.

Duplicate of http://stackoverflow.com/questions/1985260/javascript-array-rotate — aadarshsg, Jun 23 '15 at 10:02
`var newArray = array.slice(array.indexOf(3), array.length).concat(array.slice(0, array.indexOf(3))` ? — moonwave99, Jun 23 '15 at 10:05

score 3 · Accepted Answer · answered Jun 23 '15 at 10:03

3

How about one line:

var array = array.slice(2, array.length).concat(array.slice(0, 2));

DEMO

answered Jun 23 '15 at 10:03

Andy

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score 1 · Answer 2 · answered Jun 23 '15 at 10:03

1

This will not get you a flat array at the end :

arr1.push(arr2);
var newArray = arr1.join();

what you need to do is to call the method Array.prototype.concat

var newArray = arr1.concat(arr2);

answered Jun 23 '15 at 10:03

Khalid

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score 1 · Answer 3 · answered Jun 23 '15 at 10:07

1

More functional-like approach:

var reorder = function(array, index) {
  return _.flatten(_.partition(array, function(v, i) {return i < index}).reverse())
};
reorder([1,2,3,4,5], 3);

Or instead of _.flatten, you can use _.union.apply(null, ...).

answered Jun 23 '15 at 10:07

andrusieczko

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score 1 · Answer 4 · answered Jun 23 '15 at 22:28

A nice easy way with underscore is to use the first and rest functions and concatenate the results. Here's a mixin that does just that:

    _.mixin( { reorder: function(list, index){
        return _.rest(list,index).concat(_.first(list,index));
    }});

    var result = _.reorder(array, 3)

Reorder an array around an index

4 Answers4