Ada indexes have to be continuous ranges. So you'd need a type that was only every 100th number in order to use them as indexes, which I don't think can be made (I've never seen it, and looking it up I'm not seeing any way to make it). But you can select any arbitrary range as your array index, very handy as it neatly resolves the 0-/1-based issue: Choose the natural range for your situation.