Quiz 3 — Aggregation and Strings (Lectures 09–10)

Time: 20 minutes · Closed book · Answer all questions. Show accumulators and initial values explicitly.

Q1. Aggregation (5 marks)

A queue log is a list of waiting times in minutes, e.g. [3, 12, 7, 12, 20]. Write pseudocode or Python that outputs, in one pass: the longest wait, the average wait, and how many waits were over 10 minutes.

(a) List every accumulator with its initial value. (2) (b) Write the loop. (3)

Q2. The zero bug (4 marks)

This line is meant to find the shortest word in a list:

shortest = ""
for w in words:
    if len(w) < len(shortest):
        shortest = w

Give one input for which it produces a wrong (or empty) answer, explain the mechanism, and give the initialisation that fixes it.

Q3. String reasoning (5 marks)

A student ID is valid when it has exactly 8 characters, starts with a letter, and ends with a digit.

(a) Write the check as pseudocode or Python, returning a different verdict message for each failed rule. (3) (b) Give two inputs at the boundaries of the length rule. (2)

Q4. Trace the code (6 marks)

s = "abca"
seen = ""
for ch in s:
    if ch not in seen:
        seen = seen + ch
print(seen, len(s) - len(seen))

(a) Trace: after each iteration, state seen. (4) (b) State the printed output. (1) (c) What does the second printed number count, in words? (1)