Array syntax

[MechanicalMethod <mechanicalmethod@xxxxxxxxx>]

I want to store 3 values in an array. The 9999 is a necessary index because
it's not storing info on every bar, just when I get a new signal.

array: myArray[9999,3](0);

Does that reserve these 3 places?
0
1
2

or does it reserve 4 places?
0
1
2
3

Is it correct to assume that if I define the array larger than I need it will
slow down the processing and consume more memory than necessary?