CODEWITHSUNDEEP
javascriptarrays
EsoMoa
EsoMoa

Reputation: 717

Javascript: take every nth Element of Array

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?

Upvotes: 43

Views: 126577

Answers (7)

Aram M Yunis
Aram M Yunis

Reputation: 1

To separate array every 5 element need to make another 2d array to store every 5 element separately it's my solution if can help to your problem

const arrayParts = [];
var separateEach = 5;
var startLen = 0;

var partCount = Math.ceil(infos.length / separateEach);
console.log('partCount', partCount);

for (var i = 1; i <= partCount; i++) {
  arrayParts.push(infos.slice(startLen, separateEach * i));
  startLen = separateEach * i;
}

console.log('Parts ', arrayParts);

Upvotes: 0

Adam Foody
Adam Foody

Reputation: 19

may help!

 const myFunction = (a, n) => {

   let array = []


    for(i = n; i <= a.length; i += n){
      array.push(a[i-1]);

   }

  return array;

}

Upvotes: 0

shunryu111
shunryu111

Reputation: 6553

this also works by using map to create the new array without iterating over all elements in the old array..

// create array with 10k entries
const oldArr = [ ...Array( 10000 ) ].map( ( _el, i ) => i );
const max = 10;
const delta = Math.floor( oldArr.length / max );

const newArr = [ ...Array( max ) ].map( ( _el, i ) => (
  oldArr[ i * delta ]
) );

console.log( newArr );

Upvotes: 1

user1429980
user1429980

Reputation: 7158

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

Upvotes: 0

ericbn
ericbn

Reputation: 11008

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.

Upvotes: 12

Anonymous0day
Anonymous0day

Reputation: 3042

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);

Upvotes: 63

nicholas
nicholas

Reputation: 14593

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.

Upvotes: 33

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