 Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2016/imperative-programming-resit/3/b/solution

For the paper's 26-letter alphabet, use the Oberon CAP operation and contiguous character codes:

PROCEDURE Index(c: CHAR): INTEGER;
BEGIN
  RETURN ORD(CAP(c)) - ORD("A")
END Index;

Both cases of a letter map to the same uppercase letter before subtraction, so the result lies in 0..25. This relies on the ordinary Latin A–Z and a–z character encoding used by Oberon; it is not a Unicode case-folding algorithm. The stipulated letter-only precondition avoids any need to reject punctuation.

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