Javascript: take every nth Element of Array

Question

I get an Array with an unknown Number of data. But I only have an predefined amount of data to be shown/store. How can I take every nth Element of the initial Array and reduce it in JavaScript?

Eg.: I get an Array with size=10000, but are only able to show n=2k Elements.

I tried it like that: delta= Math.round(10*n/size)/10 = 0.2 -> take every 5th Element of the initial Array.

for (i = 0; i < oldArr.length; i++) {   arr[i] = oldArr[i].filter(function (value, index, ar) {     if (index % delta != 0) return false;     return true;   }); } 

With 0.2 it´s always 0, but with some other deltas (0.3) it is working. Same for delta=0.4, i works, but every second Element is taken with that. What can I do to get this to work?

Answer 1

Maybe one solution :

avoid filter because you don't want to loop over 10 000 elements ! just access them directly with a for loop !

   var log = function(val){document.body.innerHTML+='<div></pre>'+val+'</pre></div>'}     var oldArr = [0,1,2,3,4,5,6,7,8,9,10]  var arr = [];    var maxVal = 5;    var delta = Math.floor( oldArr.length / maxVal );    // avoid filter because you don't want  // to loop over 10000 elements !  // just access them directly with a for loop !  //                                 |  //                                 V  for (i = 0; i < oldArr.length; i=i+delta) {    arr.push(oldArr[i]);  }      log('delta : ' + delta + ' length = ' + oldArr.length) ;  log(arr);

Answer 2

Filter itself returns an array. If I'm understanding you correctly, you don't need that surrounding loop. So:

newArr = oldArr.filter(function(value, index, Arr) {     return index % 3 == 0; }); 

will set newArr to every third value in oldArr.