Debugging Exercises — Decisions

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How to work: read the code, predict the symptom by hand (a dry run on the given test inputs), then check the hint ladder only as needed — H1 nudges, H2 locates, H3 explains the fix. Each program compiles cleanly unless stated: the bug is in the logic, which no compiler can catch. Fix, re-run the tests, and confirm the corrected output.

D1 — The missing =

// Rule: children under 12 eat free.
int age;
std::cin >> age;
if (age = 12)                     // compiles with a warning under -Wall
{
    std::cout << "Free meal\n";
}
else
{
    std::cout << "Pay 500\n";
}

Task: the rule says under-12s eat free, but the program prints Free meal for age 20 and for age 5 alike. Fix it.

D2 — The unbraced body

// Rule: passing students get PASS and a congratulations line.
int marks;
std::cin >> marks;
if (marks >= 40)
    std::cout << "PASS\n";
    std::cout << "Congratulations!\n";

Task: a failing student (marks 25) still sees Congratulations!. Why, and fix it.

D3 — The stray semicolon

// Rule: refuse out-of-range marks.
int marks;
std::cin >> marks;
if (marks < 0 || marks > 100);
{
    std::cout << "Invalid marks\n";
    return 1;
}

Task: the program refuses every mark, including valid ones. Find it.

D4 — The unordered ladder

// Grading: A >= 80, B >= 70, C >= 60, D >= 40, else F.
int marks;
std::cin >> marks;
if (marks >= 40)
{
    std::cout << "D\n";
}
else if (marks >= 60)
{
    std::cout << "C\n";
}
else if (marks >= 70)
{
    std::cout << "B\n";
}
else if (marks >= 80)
{
    std::cout << "A\n";
}
else
{
    std::cout << "F\n";
}

Task: every mark from 40 upward prints D; 85 prints D. Reorder so the intended grading works, and state the ladder property that made the original wrong (Lesson 1 §5).

D5 — The impossible condition

// Rule: warn when temperature is out of the safe band 10..30.
double temp;
std::cin >> temp;
if (temp >= 10 && temp <= 30)
{
    std::cout << "Warning: outside safe band\n";
}
else
{
    std::cout << "OK\n";
}

Task: the warning never appears — even for 45°C. One character is the culprit class: which operator is wrong?

D6 — The flipped boundary

// Rule: scholarship for marks of 60 or more.
int marks;
std::cin >> marks;
if (marks > 60)
{
    std::cout << "Scholarship\n";
}
else
{
    std::cout << "No scholarship\n";
}

Task: the rule says “60 or more”, but marks of exactly 60 get No scholarship. Which test value catches this, and what is the one-character fix? Then state the general flip rule (Lesson 2 §5).

D7 — The missing default

char cmd;
std::cin >> cmd;
switch (cmd)
{
    case 'y':
        std::cout << "Confirmed\n";
        break;
    case 'n':
        std::cout << "Cancelled\n";
        break;
}

Task: a user types x and the program says nothing at all — they can’t tell their keypress was received. Add the missing piece and decide what it should print (echo the input!).

D8 — The leaky switch

// Rule: level 1 prints Intro, level 2 prints Intro + Standard, level 3 prints all three.
switch (level)
{
    case 1:
        std::cout << "Intro\n";
        break;
    case 2:
        std::cout << "Standard\n";
    case 3:
        std::cout << "Advanced\n";
        break;
}

Task: level 2 was supposed to print exactly Standard\nAdvanced — which it does. But level 3 prints Advanced too, and the client now reports: “level 1 must print Intro only” (it does) — however the real bug the tester found: entering level 5 prints nothing. Two fixes: (1) make level 5 (any other level) print Invalid level; (2) the spec changed — level 2 must now print Standard only. Fix both without breaking 1 or 3.

D9 — The chained comparison

// Rule: teenager is age 13..19 inclusive.
int age;
std::cin >> age;
if (13 <= age <= 19)
{
    std::cout << "Teenager\n";
}
else
{
    std::cout << "Not a teenager\n";
}

Task: age 45 prints Teenager. Explain why the compiler accepts this and what it actually computes, then fix it. (Full walk-through: P7.)

D10 — The dangling else

// Rule: adults with ID enter; adults without ID are turned away; minors denied.
int age;
bool hasId;
std::cin >> age >> hasId;
if (age >= 18)
    if (hasId)
        std::cout << "Enter\n";
else
    std::cout << "Denied\n";

Task: a minor (age 15) prints nothing at all. To which if does that else bind, and why? Rewrite with braces so the structure matches the rule. (Binding rule: Lesson 1 §6.)

Fix-list recap

Each class also appears in the mistake galleries — after your hunt, the table entry reads differently: as confirmation, not revelation.