Challenges — Decision Making

← Module home · Exercises →

How to work. Challenges are design briefs, not recipes: no starter code, no solution to peek at. For each one — extract the conditions (Lesson 4 §2), sketch the table or ladder, write the code, then build the test table yourself and dry-run the boundaries. Difficulty: ★★ stretch · ★★★ serious. Where a challenge extends a course example, the link tells you what to build on.

C1 — The two-flag discount

A shop gives 10% off when the bill exceeds 2000 or the customer holds a loyalty card; 20% when both are true. Read bill and hasCard (1/0); print the final bill with two decimals. Self-check: 2500/no-card → 2250.00; 2500/card → 2000.00; 1500/card → 1350.00; 1500/no-card → 1500.00.

C2 — Traffic-light state

Read a colour as a character (r, y, g) and print the driver instruction (Stop, Ready, Go); anything else prints Fault and the program exits with status 1. Then extend: y behaves as Stop unless the road is clear — read a second input roadClear (1/0) that only matters for y. Structure matters: which shape (switch? nested? ladder?) keeps the extension honest?

C3 — Password strength gate

Read a password as a single word (std::string). Print Strong when it has at least 8 characters (password.size() >= 8 — one string method, used like a number) and contains a digit somewhere. Checking “contains a digit” needs a scan — you haven’t met loops yet, so approximate: read the password and a second input hasDigit (1/0) that a friend’s checker supplied, and decide on both. Mark with a comment exactly which check the loop-free version can’t do yet — Unit 05 will remove the crutch.

C4 — The grade ladder’s missing rule

Rebuild E6 and add the university’s rule that was silently missing: a mark of 39 gets a “conceded pass” if attendance was at least 75%. Read marks and attendance; print A/B/C/D/Conceded/F. Where does the concession check live — before, inside, or after the ladder? Defend the placement with a dry run of (39, 80), (39, 60), (38, 90).

C5 — Elevator controller

Read currentFloor (1–10) and targetFloor. Print Up, Down, or Already there; reject any floor outside 1–10 with Out of range and status 1. Then add: floors 4 and 13 are skipped by the building (superstition) — pressing them is Out of range too. Does your range check get more complex, or does a second check join it? Which reads better, and why?

Read two double operands and an operator character (+ - * /). Use a switch on the operator; default must reject unknown operators. Division must guard the divisor before dividing (E4 pattern). Print the result with fixed/setprecision(2). Self-check: 9 / 4 → 2.25; 9 / 0 → your message, no crash; 3 ? 4 → rejected.

C7 — Cinema pricing, complete

Rebuild the classic with every rule at once: base price 700; under-12 pays 350; 60+ pays 400; students (read isStudent 1/0) of any age pay 500; Wednesday (read day 1–7) takes 20% off whatever price was decided. Order your decisions so the student rule and the age rules interact correctly (a 10-year-old student pays 350 — which rule wins, and does the requirement say?). Document the precedence you chose.

C8 — The boundary stress-test harness

Write a program that asks the user for a band edge e and then prints the verdict of your ladder from E6 for e-1, e, e+1 in one run — three grades per edge. Run it for every edge in the ladder. You’ll write the ladder once and re-verify edges mechanically. (Yes, this is a loop-shaped idea written without a loop — three explicit calls. Unit 05 will make it one line.)

C9 — Tax slab calculator

Income slabs: first 600,000 → 0%; next 600,000 (to 1.2M) → 5%; next 800,000 (to 2M) → 10%; above 2M → 15% on the portion above 2M only — marginal, like real tax systems. Read income as a double; print tax with two decimals. This is a ladder that computes, not just classifies: each branch adds its slab’s share. Self-check: 1,000,000 → 20,000; 1,500,000 → 50,000; 2,500,000 → 115,000.

C10 — Design the un-testable

A friend writes: if (x * 0 == 0) { ... } and if (x / 2 * 2 == x) { ... }. For each: (1) determine for which values it is true, (2) decide whether it’s a legitimate check or an obfuscated one, and (3) rewrite it in honest terms (or explain why no honest rewrite exists — one of them hides a division-by-zero the condition never guards). Then write a two-sentence rule about when arithmetic-in-conditions is acceptable.

How to know you’re done

Every challenge is finished when:

the decision table or ladder sketch exists on paper before the code
the code compiles clean under -Wall -Wextra
your test table covers every boundary value in the brief — one below, the edge, one above
at least one test exercises the catch-all (else/default)
a classmate could rebuild your logic from your comments alone

That same checklist is the marking rubric for the labs — practice it here where the stakes are zero.