AS3 - 保持稀疏阵列稀疏

I'm working with an array in AS3, which is sparse by default. I make an array and add a value to it at a given position. I nullify it's contents at that index. It retains that index value, but nullifies the contents. The issue is that the length still traces the same. Is there a way to actually I'm working with an array in AS3, which is spar