How do I select N items from array with starting index higher than "array.length"?

Question

If I have an array of n number of adverts which have class 'ad' . For example; when n = 5:

<div class="ad hide">...</div>
<div class="ad hide">...</div>
<div class="ad hide">...</div>
<div class="ad hide">...</div>
<div class="ad hide">...</div>

I want to removeClass('hide') 3 items the array starting with an index i , which is a counter, but treating $('.ad') as an infinite loop:

if i = 0 :

<div class="ad">...</div>
<div class="ad">...</div>
<div class="ad">...</div>
<div class="ad hide">...</div>
<div class="ad hide">...</div>

if i = 1 :

<div class="ad">...</div>
<div class="ad hide">...</div>
<div class="ad hide">...</div>
<div class="ad">...</div>
<div class="ad">...</div>

if i = 2 :

<div class="ad hide">...</div>
<div class="ad">...</div>
<div class="ad">...</div>
<div class="ad">...</div>
<div class="ad hide">...</div>

NB There is a "business" reason random ordering the array of adverts isn't a solution. They must be displayed in sequential order.

How do I create a selector that selects the three items as above?

This doesn't achieve what I want, but shows what I mean - eg :

var adIndex = 8;

adIndex = adIndex / $('.ad').length;

$('.ad').slice(adIndex, adIndex+3).removeClass('hide');

Many thanks in advance for your help :)

Answer 1

You can use

N % length

To rotate the index.

8 % 5 = 3

Then pick 3 items from that index, since it rotates you can create an array like this to pick the indexes it should show.

var arr = [0, 1, 2, 3, 4, 0, 1, 2, 3, 4];

// for N
// var arr = Array.from({ length: $('.ad').length }, (n, i) => i);
// arr = arr.concat(arr);

Knowing the indexes that should be shown, filter the elements and show them.

$('.ad').filter(i => indexes.indexOf(i) !== -1);

I'm calculating the index based on "i" ( time ).

var time = 2;
var adIndex = (time * 3) % $('.ad').length;

Full example:

 var time = 2; var adsToDisplay = 3; var adIndex = (time * adsToDisplay) % $('.ad').length; var arr = Array.from({ length: $('.ad').length }, (n, i) => i); arr = arr.concat(arr); var indexes = arr.slice(adIndex, adIndex + 3); var showAds = $('.ad').filter(i => indexes.indexOf(i) !== -1); var hideAds = $('.ad').filter(i => indexes.indexOf(i) === -1); showAds.removeClass('hide'); hideAds.addClass('hide');

 .hide { display: none }

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <div class="ad hide">0</div> <div class="ad hide">1</div> <div class="ad hide">2</div> <div class="ad hide">3</div> <div class="ad hide">4</div>

Answer 2

Try this solution as it will consider your $('.ad') as an infinite loop exactly as you mentioned:

 var ads = $('.ad'); var len = ads.length; var adIndex = 8; if (adIndex > len) { adIndex = adIndex % len; } for (var i = adIndex; i < adIndex + 3; i++) { if (i >= len) { ads.eq(i - len).removeClass('hide'); } else { ads.eq(i).removeClass('hide'); } }

 .hide { display: none; }

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <div class="ad hide">1</div> <div class="ad hide">2</div> <div class="ad hide">3</div> <div class="ad hide">4</div> <div class="ad hide">5</div>

Answer 3

If I got it right and I still think example is contradictory with text, resolution for core of problem (without JQuery and HTML) would be this:

function getPositions(i, arr) {
  for (pos = 0; pos < 3; pos++) {
    var item = (pos+i) < arr.length ? arr[pos+i] : arr[pos+i-arr.length];
    console.log(item);
  }
}

Function takes i and array as input and outputs array indexes. To apply this in HTML and JQuery only console.log needs to be replaced with some action on array item.

In case i means "i groups of 3" then small change does it:

function getPositions(i, arr) {
  i = i*3 % arr.length;
  for (pos = 0; pos < 3; pos++) {
    var item = (pos+i) < arr.length ? arr[pos+i] : arr[pos+i-arr.length];
    console.log(item);
  }
}