Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Imperative programming / 2 / c / Solution

The public rank is one-based, whereas Find uses a zero-based index:

function Select(i; var a; N): T
    require 1 <= i <= N
    return Find(i-1,a,0,N)

Thus rank $i$ is index $i-1$. The boundary ranks return the minimum and maximum, and repeated values occupy as many ranks as their multiplicities in the multiset.