I have an array like

var arr = [12, 13, 14, 17, 18, 19, 20]

I was wondering how I could iterate through the array so that I can split this array into 2 arrays based on the sequences ? i.e. basically if i+1 != true make it a new array

var arr = [12, 13, 14]
var arr2 = [17,18,19,20]

I am using lodash and have tried a number of for statements using splice but Im getting a bit confused ? Appreciate any help

I have an array like

var arr = [12, 13, 14, 17, 18, 19, 20]

I was wondering how I could iterate through the array so that I can split this array into 2 arrays based on the sequences ? i.e. basically if i+1 != true make it a new array

var arr = [12, 13, 14]
var arr2 = [17,18,19,20]

I am using lodash and have tried a number of for statements using splice but Im getting a bit confused ? Appreciate any help

• javascript
• for-loop
• underscore.js
• lodash

• 1 little more information please how you want to split - the split after 3 elements is pretty easy, is that all you need? – luk2302 Commented Jun 2, 2013 at 18:57
• 2 arrays with equals half? – iraycd Commented Jun 2, 2013 at 18:57
• I need the split to recognize when the sequence has stopped ? i.e. 12, 13, 14 - then there is a break and the sequence doesnt start again until 17, 18, 19. i.e. i+1 != true split to new array ? – Andy Commented Jun 2, 2013 at 18:58

4 Answers 4

Example on jsFiddle

var a = [1, 2, 3, 5, 6, 7];

var r = [];
var t = [];

for (var i = 0; i < a.length; ++i)
{
    if (i == 0)
    {
        t.push(a[i]); // add the first element and continue
        continue;
    }
    if (a[i - 1] != (a[i] - 1))
    {
        // if the current is not sequential
        // add the current temporary array to arrays result
        r.push(t);

        // clear the temporary array and start over
        t = [];
    }

    t.push(a[i]);
}
r.push(t);

r will contain all your arrays

Minified version

function seq(e,t,n,r){t=[];n=[];for(r=0;r<e.length;++r){if(!r){n.push(e[r]);continue}if(e[r-1]!=e[r]-1){t.push(n);n=[]}n.push(e[r])}t.push(n);return t}

var result = seq([1, 2, 3, 5, 6, 7]);

Here's a another, more dense approach, which uses underscore's quite convenient groupBy and values methods:

var origin = [12,13,14,15,17,18,19,21,22,23];
var c = 0, result = _.values( _.groupBy(origin, function(el, i, arr) { 
  return i ? c+= (1 !== el - arr[i-1]) : 0; }) );

As a result, result archive will contain all the sequences as elements. Here's the JSFiddle to play with.

Explanation: groupBy groups the source array with help of the callback (which returns a new sequence number each time the difference between the currently processed element (el) and the previous one (arr[i-1]) is bigger than 1. It returns an object, though, so I have to put it through _.values; you may or may not this step.

I wonder is that possible to request something like groupByInArray function? Should be trivial to implement, but might be very useful in situations like this.

Do you mean like this?

function split(arr) {
    var res = [];
    var subres = [];
    for (var i = 0; i < arr.length; i++) {
        var length = subres.length;
        if (length === 0 || subres[length - 1] === arr[i] - 1) {
            subres.push(arr[i]);
        } else {
            res.push(subres);
            subres = [arr[i]];
        }
    }
    res.push(subres);
    return res;
}

try this :)

var array = [12,13,14,4567,789,0]; //e.g.
var index;
var previous = array[0];
for (var index = 1; index++; index < array.length) {
  if (previous + 1 == array[index]) {
    previous = array[index]; //++
  } else {
    break;
  }
}
var firstPart = array.slice(0, index + 1);
var secondPart = array.slice(index + 1);

http://jsfiddle/zajnH/