Retrieve an evenly distributed number of elements from an array

Question

I know how to extract every nth item in an array, but what I have difficulties with is the following:

How can I extract every nth item from an array of 1800 elements, always including the first and the last element, up to a total of 256 elements?

Example:

array = [1,2,3,4,5,6,7,8,9,10];

Extract 5 elements:

extract = [1,3,5,7,10];

Answer 1

Like this?

/**
 * Retrieve a fixed number of elements from an array, evenly distributed but
 * always including the first and last elements.
 *
 * @param   {Array} items - The array to operate on.
 * @param   {number} n - The number of elements to extract.
 * @returns {Array}
 */
function distributedCopy(items, n) {
    var elements = [items[0]];
    var totalItems = items.length - 2;
    var interval = Math.floor(totalItems/(n - 2));
    for (var i = 1; i < n - 1; i++) {
        elements.push(items[i * interval]);
    }
    elements.push(items[items.length - 1]);
    return elements;
}

Example:

// Set up an array for testing purposes
var items = [];
for (var i=1; i<= 1800; i++) {
    items.push(i);
}

var extracted = distributedCopy(items, 256);

console.log(extracted);

Answer 2

I slightly improved previous responses

function evenlyPickItemsFromArray<T>(allItems: T[], neededCount: number) {
  // if we want more items than avaliable, return all items
  if (neededCount >= allItems.length) {
      return [...allItems];
  }
  // buffer for collecting picked items
  const result: T[] = [];
  const totalItems = allItems.length;
  // default interval between items (might be float)
  const interval = totalItems/neededCount;

  for (let i = 0; i < neededCount; i++) {
    // always add half of interval, so 'picking area' is 'aligned' to the center
    // eg evenlyPickItemsFromArray([0...100], 1); // [50] instead of [0]
    const evenIndex = Math.floor(i * interval + interval / 2);

      result.push(allItems[evenIndex]);
  }
  return result;
}

// TESTING

// helper to create 0...n array
function createNLongArray(n: number) {
  return Array.from({length: n}).map((_, i) => i);
}

console.log(evenlyPickItemsFromArray(createNLongArray(100), 20)) // [2, 7, 12, 17, 22, 27, 32, 37, 42, 47, 52, 57, 62, 67, 72, 77, 82, 87, 92, 97]

console.log(evenlyPickItemsFromArray(createNLongArray(200), 3)) // [33, 100, 166]

note it's not picking first/last elements every time as it's actually quite complex - eg. if I'm picking 1 elem - should I pick first or last etc. It's just finding even distribution and pick them. 1st and last will be picked only when you want all or almost all elements from the input array