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);

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

CodePudding user response：

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);

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CodePudding user response：

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);

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CodePudding user response：

There is no need to loop over completely, You can just use traditional for loop and increment it with step i.e 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);

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CodePudding user response：

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));

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