Exercise 2: Key-to-index Mapping
Define a function which
- takes a key value in the range lo..hi
- returns an index value into array of size hi-lo+1
- aborts if the "key" value is outside the range lo..hi
E.g. lo == 12, hi == 27, a[16]
indexOf(12) == 0, indexOf(17) = 5,
indexOf(21) == 9, indexOf(27) == 15
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