Create JS array of repeated value

Question

I would like to ask for efficient way to create such array:

[
    1, 1, 1, 1, 1, 1,
    2, 2, 2, 2, 2, 2,
    3, 3, 3, 3, 3, 3
    ...
    n, n, n, n, n, n
]

Every 6 items the number is added 1++.

function createFaces(n){
    var array = [];
    var l = 1; 
    while(n > 0){
        for(var i = 0; i < 6; i++){
            array.push(l)
        }
        l++;
        n--;
    }
    return array;
}

The way you have written traverses through each target element array exactly once, with minimal overhead. You could make it slightly more efficient (maybe) by making it a single loop from i=6 through i=6*n+5, and pushing i/6 to the array. Similarly, you could eliminate one of your outer indices. That's all nearly pointless micro-optimization though. — James K
– James K, Commented Jan 30, 2018 at 20:51

Nina Scholz · Accepted Answer · 2018-01-30 20:54:02Z

2

You could use Array.from with a function for the value.

function createFaces(n) {
    return Array.from({ length: 6 * n }, (_, i) => Math.floor(i / 6) + 1);
}

console.log(createFaces(7));

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

answered Jan 30, 2018 at 20:54

Nina Scholz

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1 Comment

Ele Over a year ago

nice approach with Array.from

James · Accepted Answer · 2018-01-30 20:55:20Z

1

To create an array of size n filled with value v, you can do

Array(n).fill(v);

In the context of your function:

function createFaces(n){
    var array = [];
    for (var i=1; i <= n; i++) {
      array = array.concat(Array(6).fill(i));
    }
    return array;
}

edited Jan 30, 2018 at 20:55

answered Jan 30, 2018 at 20:50

James

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2 Comments

Herohtar Over a year ago

This also creates an array of arrays, which isn't quite what the OP's code does. To do the exact same thing, the line inside the for loop should be array = array.concat(Array(6).fill(i));

James Over a year ago

Oops, I saw an array of arrays where one didn't exist. Yes to concat.

I wrestled a bear once. · Accepted Answer · 2018-01-30 20:55:07Z

0

If you want a flat array...

function createFaces(n, x){
    for(var i=0, a=[]; i<n; a.push(...Array(x).fill(i)) && i++){}
    return a;
}

console.log(createFaces(7, 6));

answered Jan 30, 2018 at 20:55

I wrestled a bear once.

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Comments

Ele · Accepted Answer · 2018-01-30 21:31:59Z

0

Using Array.prototype.map() function

let fill = function(length, threshold) {
  let i = 1;
  return new Array(length * threshold).fill().
            map((_, idx) => 
                (idx + 1) % threshold === 0 ? i++ : i);
};

console.log(fill(7, 6));

edited Jan 30, 2018 at 21:31

answered Jan 30, 2018 at 21:19

Ele

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