Arrays :: Count Only Set Indexes Using Loops

In PHP you can do something like this:
Array1 = 0,0,3,6,6,6,7,8,8,9

Then loop through Array1 and set the keys of Array2
Array2[0] = true;
Array2[3] = true;
Array2[6] = true;
Array2[7] = true;
Array2[8] = true;
Array2[9] = true;
Array2.count = 6;

But doing a similar thing in AS3 would result in a count of 10 (true,,,, true,,,, true, true, true, true) as all of the keys that are not set are counted. Is there a way of doing this without using any more loops? or Functions, simplicity and efficiently are important here.