What is the fastest way to copy an array?

Question

I decided to make a game, but I feel that Array.filter is too slow, so I created a method:

Array.prototype.removeIf = function(condition: Function): any[] {
    var l: any[] = Array.from(this);
    var __begidx = 0;
    for (var i: number = l.length; --i; ) {
        /** i; */
        if (condition(l[i])) {
            l[i] = l[l.length - 1];
            __begidx += 1;
        }
    }
    l.length -= __begidx;
    return l;
}

if without Array.from, this method takes only 130 - 295ms to remove 99999999 elements, because it just put element to remove at end of list, and slice out. Even filter takes 670 - 1330 ms to remove.

but when I test Array.from:

var timeit = new TimeIt(); TimeIt.set();
var range = Array.from(arr) // 'arr' is a Array contains 99999999 elements
timeit.print(); // 1320 - 1634ms

Why is Array.from so slow? It's 10 times slower than 'removeIf', and slower than Array.filter. Are there any faster way to copy list?

I have a hard time believing that your function which uses further array methods is faster than the engine's built in implementation of filter(). — pilchard, May 27 '22 at 18:07
@pilchard because this method do not have order, after remove, order will be disrupted — Cflowe Visit, May 27 '22 at 18:18
@pilchard and also, my question is are there any way to copy list faster? — Cflowe Visit, May 27 '22 at 18:19
You might want to test the output of your function fairly certain it's not doing what you think it is. (possibly why it's testing faster). — pilchard, May 27 '22 at 18:26
Why did you accept the answer? It doesn't contain`Array.slice` — jabaa, May 27 '22 at 18:26
@jabaa In fact, Array.slice is fast but still takes 200ms, still a bit slow — Cflowe Visit, May 27 '22 at 18:28
You still didn't answer whether you need a deep or shallow copy. — jabaa, May 27 '22 at 18:31
Nothing of this creates a deep copy. The dupe target is wrong. The first method in the answer creates a shallow copy. `Array.from` creates a shallow copy. Now this is a different question. — jabaa, May 27 '22 at 18:33
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/245111/discussion-between-cflowe-visit-and-jabaa). — Cflowe Visit, May 27 '22 at 18:35
Hmm, in my tests using: a.slice(), a.concat and even [].concat(a) I find no reliable/discernible difference. [a=Array(1e7).fill(3)] each ~24ms. start1 = new Date(); b = a.slice(); console.log('a.slice():', new Date() - start1, b.length); (Chrome 101; MacOS/MacBookPro-2019) — Jack Punt, May 29 '22 at 02:21
For the record: using the tool at https://jsben.ch/MqSBj slice() & concat() alternate being 98-99% slower than the other. I image the internal implementation is identical. and the purported difference is side-effects (GC or JIT or whatever); OR: let's find the source code and check! — Jack Punt, Jun 02 '22 at 00:49

score -3 · Answer 1 · answered May 27 '22 at 18:20

Being completely honest, JS is not a good programming language for game development, and if you have an array that contains that many elements that you need to manipulate in less than 300ish ms then you're probably doing something wrong. However,

You could make it yourself, make a loop that goes through each element and pushes it to a different array. Quick example:

let arr = new Array(99999999); 
let newArr = []; 
for(let i = 0; i < arr.length; i++) {
    newArr.push(arr[i]); 
}

all that does is simply loop through each element, push it to the new array until done

or you could just do:

let arr = new Array(99999999); 
let newArr = arr;

both of those would work, I'm unsure how fast they would be however, But it's the fastest method I could think of. Hopefully this helps :)

The first is a shallow copy without memory preallocation and the second isn't a copy of the array. — jabaa, May 27 '22 at 18:23

What is the fastest way to copy an array?

1 Answers1