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REVISION AND PRACTICE
Week 6 revision and mixed practice
Try a procedure with different inputs, then answer before revealing the explanation. Name the input representation, comparison key, tie policy and eligible population.
REAL COURSE DATA · SORT, GROUP AND VERIFY
Unpack paragraph → sentence → word
Follow the selected procedure. Change a field, tie rule or weekday and explain why the result changes.
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Predict the next change. Keep track of which list level, field and position convention the procedure uses.
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For a paragraph represented as lists of character-list words inside sentence lists, what does first(first(P)) return?
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The first word’s character list. One more first reaches its first character, provided each relevant list is non-empty.
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Insert 3 into [1,3,3,5] using strict <.
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The new occurrence goes after the old 3s: [1,3,3,3,5]. IDs or labels would show its relative position among equals.
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Which line must run for every old insertion element, even after a match?
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The line appending the old element to the output. Otherwise existing data may be lost.
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Can you call the insert helper on an unsorted list and assume the result is sorted?
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No. Its contract assumes a sorted input.
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What two properties form the insertion-sort invariant?
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The accumulated result is sorted and contains exactly the occurrences processed so far.
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What does the tutorial’s ≤ do to equal-page insertion order?
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It inserts the new equal-page record before old equals; the lecture’s strict < puts it after them.
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Does sorting 30 items by the copying helper avoid all quadratic work on a favourable order?
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No. It still copies 0+1+…+29 = 435 old elements. Key-comparison work is a separate count.
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A sorted birthday run contains four students. How many equal-neighbour links and all unordered pairs?
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Three equal-neighbour links, but six unordered student pairs.
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What is the usual final-step bug in sorted group counting?
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Forgetting to save the final group after the loop.
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Under the displayed zero-based positional grading code, what are 30-student A/B/C/D counts?
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7,7,8,8. This follows the exact comparisons, not a hand-adjusted quarter split.
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Does first(G) return an ID when G=[AIDs,BIDs,CIDs,DIDs]?
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No. It returns the whole AIDs list.
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Maths A and Physics B: which of the two rules confirms it?
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The looser both-A/B rule confirms it; the at-least-as-good rule rejects it.
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Why does found need resetting for each student?
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A match for an earlier student must not affect whether the next student is found.
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Does exitloop mean return from the procedure?
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No. It ends the nearest enclosing loop, then execution continues outside that loop.
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For the library example, what does first(last(books)) mean?
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The ID of the final, maximum-page book pair, not a page count.
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02201: Kolkata 20:00 Day 1 to Bhubaneswar 03:30 Day 2. Duration?
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450 minutes, or 7 h 30 min.
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A Sunday origin departure reaches a station on Day 2. Which weekday?
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Monday, one day later.
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Why can train and station weekdays disagree without an error?
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They may refer to different events or journey days. Align those meanings first; remaining mismatches should be flagged.
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What is insufficient about checking only whether both station names occur in a route?
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Direction and departure availability matter: source must precede destination, and service must run on the relevant source weekday.
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What does a different-next-call junction clue fail to prove?
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It does not prove physical track branching or that a timed transfer is feasible.