Question

152

Javascript: take every nth Element of Array

rated 0 times [ 157] [ 5] / answers: 1 / hits: 110161 / 9 Years ago, mon, november 2, 2015, 12:00:00

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?

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jarettajb

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answered 9 Years ago finn · Accepted Answer

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);