Why does array.concat copy the array rather than doing for(e...) array.push(e);

(Kyliathy) #1

I’ve just realized that array.concat creates a copy of the array and returns it. This requires an additional assignment, something I was never fond of.

Isn’t this less efficient than the following code which can be hosted in an Array static extension?

for (newElement in elements) array.push(newElement);

I’m probably missing something because if the above would have been more efficient, I’m pretty sure it would have been in Std since ages ago :smiley:.

In other words, why does array.concat create another copy?


(Juraj Kirchheim) #3

That’s how it’s defined in the ECMAScript standard for ages, so both flash and js implemented it that way. And FWIW even PHP does it that way:

So it’s not really a design choice. That said, you can just make a static extension:

class ArrayTools {
  static public function append<T>(target:Array<T>, toBeAdded:Array<T>) {
    for (x in toBeAdded) target.push(x);
    return target;

Then add using ArrayTools; to a toplevel import.hx and you’re all set :wink:


(Kyliathy) #4

Already did that my sibling :slight_smile:. Well, of course, not with all your bells and whistles of import.hx, mister haxor (yeah, knew about that too, I guess I’m graduating Haxe Noobs hopefully?)

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(Leo Bergman) #5

The reason why concat and many other array operations will return a copy is that methods that modifies the array tends to be an excellent footgun.
I would be more concerned with avoiding side effects than any marginal performance cost.

Since the need for actually doing an append instead is quite rare, having a function for it would encourage bad practices, and I would prefer to be explicit to make it obvious what is happening by simply doing b.forEach(val -> a.push(val)).


(Kyliathy) #6

Yeah, and I just realized that this would seriously annoy people that just want to do a for (element in originalArray.concat(some extra elements)).


(Mike Robinson) #7

“Footgun.”:smiley: … I’m gonna have to remember that one.

I think that everyone rightly assumes that library-calls do not have side-effects: they do not alter the parameters sent to them. If you want to modify the array, then, as Leo says, write a one-liner to do it, right then and there. With a comment to make it even more obvious.

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