Adding on to existing arrays and vectors is one of those really common tasks that sounds dreary. Everyone knows about push() and unshift() for single elements and concat() for lots of elements. But what if you want to add a lot of elements to an existing array or vector without allocating a new array or vector? I have the solution.

If we combine Function.apply() and Array.push()/Vector.push(), the latter of which takes varargs, we can push a whole array of values:

var evens:Array = [2,4,6,8];
var odds:Array = [1,3,5,7];
// you can pass null instead of odds but you can't pass nothing (no crash though!)
odds.push.apply(odds, evens);
trace(odds); // 1,3,5,7,2,4,6,8
trace(odds.length); // 8

It even works with vectors:

var oddsVec:Vector.<int> = Vector.<int>([1,3,5,7]);
oddsVec.push.apply(oddsVec, evens);
trace(oddsVec); // 1,3,5,7,2,4,6,8
trace(oddsVec.length); // 8

Which leads to a semi-generalized function:

private function replaceVector(from:Array, into:Vector.<int>): void
	into.length = 0;
	into.push.apply(into, from);

Unfortunately, due to the lack of templating or generics support available to the AS3 programmer, this function cannot be truly generalized. You will have to write a version of this function for every single type of vector you would like to offer this replacing functionality. Each time, you will need to change the type of the vector. Also, Function.apply() will only accept an array of parameters, not a vector. This will not be caught at compile time, but will result in an error at runtime.

This is a good tip for avoiding allocation, which is slow, and later deallocation, which is also slow. The cost of the dynamic function call is totally worth it. I find that most of the time I do not need to use concat() due to this, as I am most often replacing the vector or array with the concatenated version of itself. I have a hunch that this is true for most people.