Exercises — Decision Making

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How to work: write your answer and dry-run it before opening any answer. Stars: ★ recall · ★★ apply · ★★★ design. Selected answers (E1–E10, and starred hints for later items) are in a collapsed block at the bottom of this page — the Try-It-Yourself rule from the module home applies to every one.

Setup for code exercises: every program compiles with g++ -std=c++17 -Wall -Wextra eNN.cpp -o eNN (see the toolchain notes).

Part A — One-condition decisions (★)

E1 — Pass/fail. Read marks (0–100). Print PASS when marks are 40 or more, otherwise FAIL. Then print Checked. on its own line either way.

E2 — Adult gate. Read an age. Print Adult if 18 or more, Minor otherwise. Then add: if the age is exactly 17, additionally print One year to go.

E3 — Negative guard. Read a number. If it is negative, print Negative input and stop the program with return 1;. Otherwise print OK: and the number.

E4 — Zero division guard. Read integers total and count. If count is 0, print Cannot divide by zero.; otherwise print the integer quotient and remainder (total / count, total % count — recall Foundations Lesson 3).

E5 — Coin doubles. Read a number. If it is positive, print the number doubled; otherwise print the number unchanged. One decision, two outputs.

Part B — else-if ladders (★–★★)

E6 — Grade ladder. Read marks (0–100) and print the grade with a single ladder: A ≥ 80, B ≥ 70, C ≥ 60, D ≥ 40, else F. Print the letter only, no newline gymnastics needed.

E7 — BMI bands. Read weight (kg) and height (m). Compute bmi = weight / (height * height) as a double and classify: < 18.5 Underweight, < 25 Normal, < 30 Overweight, else Obese. Print the band and the BMI with two decimal places (fixed/setprecision — I/O Lesson 1 §6).

E8 — Day type. Read a day number 1–7 (Mon=1). Print Working day for 1–5, Weekend for 6–7, and Invalid day for anything else. One ladder, three outcomes.

E9 — Ticket bands. Read age. Prices: under 12 → Child 300, 12–17 → Teen 450, 18–59 → Adult 700, 60+ → Senior 350. Print e.g. Adult 700. Then state (in a comment) which boundary values your ladder places in which band, and why 12 and 17 land where they do.

E10 — Temperature advisory. Read a Celsius temperature as a double (integer divisions off!). Print: ≥ 40 Heat warning, ≥ 30 Hot, ≥ 20 Pleasant, ≥ 10 Cool, < 10 Cold.

Part C — Compound conditions (★★)

E11 — Range check. Read a number and print In range when it is between 10 and 20 inclusive, else Out of range. Then answer in a comment: does your condition admit 10? 20? Why?

E12 — Login rule. Read a username (std::string) and a 4-digit pin (int). Print Welcome when username is "admin" and pin is 1234; otherwise Access denied. Note the I/O whitespace rules: >> reads one word for the string.

E13 — Discount rule. Read isMember (1/0) and total. Print Discount applied when the customer is a member and total is more than 500; else Full price. Keep the message decision in one if-else.

E14 — Seasons two ways. Read a month number 1–12. Print the season (12/1/2 Winter, 3–5 Spring, 6–8 Summer, 9–11 Autumn, else Invalid month) twice: once with an else-if ladder using ||, once with a switch using stacked cases. Both must produce identical output for every month 0–13.

E15 — Triangle validity. Read three sides. Print Valid triangle when all three are positive and the sum of any two is greater than the third (three checks joined by &&), else Invalid.

E16 — Leap year, first attempt. Read a year and print Leap when it is divisible by 4, else Not leap. (Save your answer — C12 extends this to the full rule.)

E17 — Overlap check. Read two time slots as start/end hour pairs (s1 e1 s2 e2, each 0–23). Print Overlap when one slot starts before the other ends and vice versa: (s1 < e2 && s2 < e1), else Free. Test with slots that merely touch (one ends where the next begins) — is touching an overlap per your formula?

E18 — XOR, the exclusive or. Read two answers a and b as 1/0 booleans. Print Exactly one when exactly one of them is true, else Both or neither. One comparison, no nested ifs: recall Lesson 2 §6 (a != b when both are bool).

E19 — Military time check. Read hours and minutes as two integers. Print Valid time when hours are 0–23 and minutes 0–59, else Invalid time. Then re-do it with a single compound condition and confirm both versions agree on (24, 0), (23, 60), (0, 0), (-1, 30).

Part D — Validation and boundaries (★★–★★★)

E20 — Three-guard program. Read age, weight, and haemoglobin (the Lesson 4 example — but write it yourself before rereading the worked solution). Reject out-of-range inputs individually (return 1 after each message), then classify exactly as the worked example does. Your output text may differ; the branch structure may not.

E21 — Boundary line-up. A rule says “students with more than 60 marks get a scholarship.” For each of 59, 60, 61: state whether your if (you choose the comparison) admits the student. Then rewrite the condition the opposite way (< vs >= family) without changing which values qualify, and state the flip rule from Lesson 2 §5 you used.

E22 — Band-edge owner. Marks ladder with bands A ≥ 80, B ≥ 70: which band owns exactly 70 in your ladder? Prove it by dry-running 69, 70, 71 through your code and recording the branch each takes. Then move the boundary deliberately (change >= 70 to > 70) and list which test values change verdicts.

E23 — Validation loop, one pass. Read marks; if they are outside 0–100 print Invalid and return 1. Combine this with E6’s ladder so invalid input never reaches the grading ladder. Test: −5, 0, 39, 40, 100, 101 — record each output.

E24 — Weekday menu. Read a day number 1–7 and use a switch to print the day’s schedule: 1–5 Classes, 6 Lab cleanup, 7 Off, default Invalid day. Then convert it to an else-if ladder. In a comment: which version would you extend first if the schedule became per-day different, and why?

E25 — Fee rule stack. A course fee is 15000. Rules: (a) full fee with no discount; (b) 10% discount when the student is a merit scholar or the fee is paid within the first week; (c) an additional 5% (on the already-discounted fee) when both are true. Read isMerit (1/0) and paidEarly (1/0), compute and print the fee with two decimals. Design the decisions so both-true (rule c) really stacks — test with both-false, each-alone, and both-true; the both-true answer is 12825.00.

Selected answers

if (age >= 18) std::cout << "Adult\n"; else std::cout << "Minor\n";
if (age == 17) std::cout << "One year to go.\n";