How can I fill a big array from small array in Javascript

Question

Suppose I have an array :

const A = [1,2,3]

and I want to fill another array, say B, with the elements of A; B.length >> A.length.

I am interested in any function that can help create array B of the form:

[1,1,1,2,2,2,2,2,3,3,3,3,2,2,2,1,3,1]

The placements of the elements of A into B depends on specified index limits. Element 1 can be placed in several specified location/interval in B. The length of B is known; N is the length of B.

In my trial, I tried to use the array.fill function, but don't know how to get the start and end indexes. Any help will be appreciated.

const A = [1,2,3];
const B = [];
for (let k = 0; k < A.length; k++){
  for (let j = 0; j < N; j++){
    B.fill(A[k], start_index[j], end_index[j]);
  }
}

I want to use array.fill function. So to use the fill function, I need to provide the beginning and end of the index location (in array B) of the parameter from A. The start and end can be arbitrary. — Isaac
– Isaac, Commented Nov 1, 2018 at 18:06
How are your "specified index limits" specified? It's unclear how you want to place elements of A into B without that information. — slider
– slider, Commented Nov 1, 2018 at 18:20
Thank you Slider. As at now I don't know how to code the limits, but you can make it arbitrary or use the description specified in B above. — Isaac
– Isaac, Commented Nov 1, 2018 at 19:12

Barmar · Accepted Answer · 2018-11-01 19:22:50Z

1

Since you're filling multiple ranges of B with the same element of A, you need a data structure that has multiple start/end/index combinations.

.fill() won't extend the length of an array by itself, so you need to adjust the length when appending new elements.

const params = [
    {index: 0, start: 0, end: 3},
    {index: 1, start: 3, end: 8},
    {index: 2, start: 8, end: 12},
    {index: 1, start: 12, end: 15},
    {index: 0, start: 15, end: 16},
    {index: 2, start: 16, end: 17},
    {index: 0, start: 17, end: 18}
];
const A = [1, 2, 3];
const B = [];
params.forEach(({index, start, end}) => {
  if (B.length < end) {
    B.length = end;
  }
  B.fill(A[index], start, end)
});

console.log(B);

answered Nov 1, 2018 at 19:22

Barmar

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3 Comments

Isaac Over a year ago

Thank you Barmar. Is it possible to have the values of the keys of params as arrays? I mean something like this: params = [{index: [0,1,2,1,0,2,0],start:[0,3,8,12,1,16,17],end:[3,8,12,15,1,17,18]}]. How can I construct this form of array for params. And it seems the array.forEach will be affected.

Barmar Over a year ago

Sure, you can do that, it's just not as clear when you separate related items. You loop over one of the arrays, and then use the index to access the corresponding elements of the other 2 arrays.

Barmar Over a year ago

params.index.forEach((index, i) => { let start = params.start[i], end = params.end[i]; ... })

Isaac · Accepted Answer · 2018-11-03 05:52:29Z

0

To condense Barmar's suggestion:

const params = [
    {index: [0,1,2,1,0,2,0], start: [0,3,8,12,15,16,17], end: [3,8,12,15,16,17,18]}
];
const A = [1, 2, 3];
const B = [];
for (let j=0; j<7;j++){
params.forEach(({index, start, end}) => {
  if (B.length < end[j]) {
    B.length = end[j];
  }
  B.fill(A[index[j]], start[j], end[j])
});
}

console.log(B);

answered Nov 3, 2018 at 5:52

Isaac

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