[removed] 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 redu

Answer 1

Try

arr = oldArr.filter(function (value, index, ar) {
    return (index % ratio == 0);
} );

where ratio is 2 if you want arr to be 1/2 of oldArr, 3 if you want it to be 1/3 of oldArr and so on.

ratio = Math.ceil(oldArr.length / size); // size in the new `arr` size

You were calling filter() on each element of oldAdd inside a loop and you're supposed to call filter() on the whole array to get a new filtered array back.

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.

Answer 3

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 4

Borrowing from @anonomyous0day's solution, generate a new Array with the desired indices from the given array:

(Take every 3 items)

Array.prototype.take = function(n) {
  if (!Number(n) && n !== 0) {
    throw new TypeError(`Array.take requires passing in a number.  Passed in ${typeof n}`);
  } else if (n <= 0) {
    throw new RangeError(`Array.take requires a number greater than 0.  Passed in ${n}`);
  }

  const selectedIndicesLength = Math.floor(this.length / n);
  return [...Array(selectedIndicesLength)].map((item, index) => this[index * n + 1]);
};

[1, 2, 3, 4, 5, 6, 7, 8].take(2); // => 2, 4, 6, 8