JavaScript: a better way to extract every 5th item from an array

Question

I have an array of tuples of coordinates like

[ [ 1, 1 ],
  [ 2, 2 ],
  [ 3, 3 ],
  [ 4, 4 ],
  [ 5, 5 ],
  [ 6, 6 ],
  [ 7, 7 ],
  [ 8, 8 ],
  [ 9, 9 ],
  [ 10, 10 ]
  ...

I want to extract every 5th coordinates out of them into a new array

Here is my implementation

 const coordinates = Array.from({ length: 30 }, (_, i) => [i + 1, i + 1]) const { filteredCoordinates } = coordinates.reduce( (accu, currCoordinate) => { if (accu.count === 5) { accu.filteredCoordinates.push(currCoordinate) accu.count = 1 } else { accu.count += 1 } return accu }, { count: 1, filteredCoordinates: [], } ) console.log(filteredCoordinates);

This works fine but I am wondering if there is a better way to do this?

Answer 1

You could take an index and increment it by the wanted count.

This approach does not iterate the complete array, only the wanted indices.

 const coordinates = Array.from({ length: 30 }, (_, i) => [i + 1, i + 1]), filtered = []; for (let i = 4; i < coordinates.length; i += 5) filtered.push(coordinates[i]); console.log(filtered);

 .as-console-wrapper { max-height: 100% !important; top: 0; }

Answer 2

You can filter the array based on the modulo of five given the index.

 const coordinates = Array.from({ length: 30 }, (_, i) => [i + 1, i + 1]) const everyFithCoordinate = coordinates.filter((_, i) => (i+1) % 5 === 0); console.log(everyFithCoordinate);

Answer 3

There is no need to loop over completely, You can just use traditional for loop and increment it with step ie 5

 const coordinates = Array.from({ length: 30 }, (_, i) => [i + 1, i + 1]); const result = []; const step = 5; for (let i = step - 1; i < coordinates.length; i += step) { result.push(coordinates[i]); } console.log(result);

 /* This is not a part of answer. It is just to give the output full height. So IGNORE IT */ .as-console-wrapper { max-height: 100% !important; top: 0; }

Answer 4

I want to extract every 5th coordinates out of them into a new array

Depending on your definition of "extract" you may be wanting to move those coordinates from the first to the second array. This example uses a for statement . I cache the length, and then splice out the coordinate from the array at the given index, and push it into the new array. Then decrement both the length and the index because they have changed before the next iteration.

 const coordinates = Array.from({ length: 32 }, (_, i) => [i + 1, i + 1]); const out = []; const count = 5; let { length } = coordinates; for (let i = count - 1; i < length; i += count) { out.push(coordinates.splice(i, 1)[0]); --length; --i; } console.log(JSON.stringify(out)); console.log(JSON.stringify(coordinates));

Answer 5

The proper and efficient functional way to achieve this task is done by using Array.from() . However as far as i can see nobody has implemented it properly so far. Lets try.

 var coords = Array.from({ length: 30 }, (_, i) => [i + 1, i + 1]), period = 5, result = Array.from( {length : ~~(coords.length / period)} , (_,i) => coords[i*period+period-1] ); console.log(result);