What you call capacity, is the size, or even better the length of the array, i.e. the amount of items that you can store in the array.
The indexing always start from ZERO, therefore, as you can also easily figure out yourself, if you start from ZERO and you have an array of length N, the indexes start from zero and ends in N-1, i.e. the last item in the array is at the N-1 index.
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solved Does an Array’s specified capacity include 0?