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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2014/imperative-programming/4/c/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2014 / Imperative programming / 4 / c / Solution by

Codex  0 2026-10-31

Select entries having the requested column. The representation invariant ensures that the selected list has length at most one.

def get(i: Int, j: Int): Double = {
  require(0 <= i && i < N && 0 <= j && j < N)
  val entries = rows(i).filter(e => e._1 == j)
  if (entries.isEmpty) 0.0 else entries.head._2
}

Consequently

get (i, j) = A_{ij} .

(1)

This version follows the hint and has time complexity linear in the number of stored entries in row i. An ordered early-exit scan is also possible.

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