CODEWITHSUNDEEP
javascriptarrays
Isaac
Isaac

Reputation: 396

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

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]);
  }
}

Upvotes: 0

Views: 882

Answers (2)

Isaac
Isaac

Reputation: 396

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);

Upvotes: 0

Barmar
Barmar

Reputation: 780818

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);

Upvotes: 1

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