Debugging Hunts

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Ten programs. Each compiles (unless it doesn’t — that’s a clue) but misbehaves. For every one: write the symptom, name the bug, fix it — before opening the fix list. Hints escalate: ① what to notice ② where to look ③ the fix. The rule: Try It Yourself.

D1 — The wrong slash. Compiles. Output looks wrong.

#include <iostream>

int main() {
    std::cout << "Attendance report" << "/n";
    std::cout << "Week 1: perfect" << "/n";
    return 0;
}

Student report: “My newlines print as letters.”

D2 — The glued prompt. Compiles. Screen looks cramped.

#include <iostream>
#include <string>

int main() {
    std::string name;
    int age;

    std::cout << "Name: ";
    std::cin >> name;
    std::cout << "Age: ";
    std::cin >> age;
    std::cout << name << " is " << age << "\n";
    return 0;
}

Student report: “Age: appears at the end of the line where I typed the name, not on its own line.”

D3 — The word eater. Compiles. Loses data.

#include <iostream>
#include <string>

int main() {
    std::string fullName;
    int rollNo;

    std::cout << "Roll no: " << std::endl;
    std::cin >> rollNo;
    std::cout << "Full name: " << std::endl;
    std::cin >> fullName;
    std::cout << rollNo << ": " << fullName << "\n";
    return 0;
}

Student report: “I typed 41 and Ali Raza Khan — the program printed 41: Ali and forgot the rest.”

D4 — The vanishing name. Compiles. Asks the second question and ignores the answer.

#include <iostream>
#include <string>

int main() {
    int age;
    std::string name;

    std::cout << "Age: " << std::endl;
    std::cin >> age;
    std::cout << "Name: " << std::endl;
    std::getline(std::cin, name);

    std::cout << "[" << age << "][" << name << "]\n";
    return 0;
}

Student report: “It never lets me type the name — output is [19][].”

D5 — The wrong order. Compiles. Second read explodes.

#include <iostream>
#include <string>

int main() {
    int marks;
    std::string subject;

    std::cout << "Enter subject and marks: " << std::endl;
    std::cin >> marks >> subject;
    std::cout << subject << " = " << marks << "\n";
    return 0;
}

Student report: “I typed Maths 88 and it printed nothing sensible — something like ` = 0` and the stream seemed dead.”

D6 — The clear that did nothing. Compiles. Infinite prompt loop on bad input.

#include <iostream>

int main() {
    int age = -1;

    while (age < 0) {
        std::cout << "Age: " << std::endl;
        std::cin.ignore(1000, '\n');
        std::cin.clear();
        std::cin >> age;
        if (std::cin.fail()) {
            age = -1;
        }
    }
    std::cout << "Accepted " << age << "\n";
    return 0;
}

Student report: “When I type letters it spams Age: forever. Numbers work.”

D7 — The price that lost its decimals. Compiles. Money wrong.

#include <iostream>

int main() {
    int qty;
    double unitPrice, total;

    std::cout << "Qty and unit price: " << std::endl;
    std::cin >> qty >> unitPrice;
    total = qty * unitPrice;
    std::cout << "Total: " << total << "\n";
    return 0;
}

Student report: “For 3 and 7.5 it prints Total: 22.5 — where did my .00 go? The receipt looks fake.”

D8 — The column that refuses to line up. Compiles. Table wobbles.

#include <iostream>
#include <iomanip>

int main() {
    std::cout << std::setw(10) << "Item" << std::setw(6) << "Qty\n";
    std::cout << std::setw(10) << "Chai" << 3 << "\n";
    std::cout << std::setw(10) << "Samosa" << 12 << "\n";
    return 0;
}

Student report: “The header lines up but the numbers sit right after the names — Chai3, Samosa12 — the qty column vanished.”

D9 — The two Enters. Compiles. Second getline reads empty when the user types both fields on one line.

#include <iostream>
#include <string>

int main() {
    std::string item;
    int qty;

    std::cout << "Item and qty on one line (e.g. 'Chai 2'): " << std::endl;
    std::getline(std::cin, item);
    std::cin >> qty;

    std::cout << qty << " x " << item << "\n";
    return 0;
}

Student report: “Typing Chai 2 on one line prints nothing useful; typing them on two lines works. I thought getline was broken.”

D10 — The menu that ate a letter. Compiles. First menu choice works, second is skipped.

#include <iostream>
#include <string>

int main() {
    char choice;
    std::string note;

    std::cout << "Choice (a/b): " << std::endl;
    std::cin >> choice;                     // user types: a⏎
    std::cout << "Leave a note: " << std::endl;
    std::getline(std::cin >> ws, note);
    std::cout << "Choice2 (y/n): " << std::endl;
    std::cin >> choice;                     // user types: y⏎
    std::cout << "[" << choice << "][" << note << "]\n";
    return 0;
}

Student report: “The note line gets skipped entirely when I answer the menus quickly — [y][].” (The seeded bug is the first cure applied in the wrong place — trace which newline is left where.)

Fix list (after your own fixes)

Reveal after each bug is found and fixed on paper

D1. "/n" is a two-character string; the newline character is '\n' (backslash). Fix both lines. Detection habit: when output “prints as letters”, read the literal character by character.

D2. Prompts cout << "Name: "; never end their line, so the next prompt continues on the same line as the typed text. Fix: end each prompt with '\n' (or print the prompt, then read on the same line — deliberately — and say so). Either is fine; deciding is the lesson.

D3. cin >> fullName reads one word: Ali. ` Raza Khan\nwaits in the buffer. Fix: read the name withgetline(after the mandatory cure —ignore or » ws) whenever spaces are legal. Symptom pattern: "only the first word survived" →»` on a multi-word field.

D4. The mixing trap verbatim: cin >> age leaves '\n'; bare getline reads it as an empty line. Fix: std::cin.ignore(1000, '\n'); after cin >> age, or getline(std::cin >> ws, name).

D5. Extraction is positional: marks (int) is asked to read Maths → fails → stream dead → everything after is skipped; marks holds 0. Fix: match the read order to the prompt order (subject then marks), and — better — validate with fail()/clear()/ignore() so nonsense input recovers instead of dying.

D6. clear() and ignore() are reversed. In a failed state, >> does nothing — but so does ignore() (an ignored stream ignores commands). Fix: clear() first, ignore(1000, '\n') second, then retry. Order is not cosmetic; it is the whole fix.

D7. Nothing is wrong with the math — 22.5 is the true product — the format is default. Fix: std::cout << std::fixed << std::setprecision(2); before printing money (once, sticky). Lesson: money without fixed/setprecision(2) is a formatting bug even when the arithmetic is perfect.

D8. setw applies to the next item only. Header: two setws, both applied. Rows: setw(10) padded the name, but the qty had no setw of its own. Fix: << std::setw(6) << qty on both rows. The “vanished column” is always a forgotten one-shot setw.

D9. The user followed the prompt; the program lied. getline consumed Chai 2\n wholesale into item, then cin >> qty found an empty buffer and blocked for a second line. Fix: change the prompt to promise two lines (item line, then qty line) or parse the one-line form (read a word with >>, then the qty — Lab 5 uses this). Prompt and read-style must make the same promise.

D10. After the first cin >> choice, the buffer holds \n. The getline(cin >> ws, note) would have handled it — but the user answered both menus before the note in this transcript; the seeded bug: the second cin >> choice runs before the note getline, and the y Enter’s newline waits behind… nothing, so the next read (the printed note in a later iteration) would be the one poisoned. Fix for this program: reorder to ask the note before the second menu choice, and keep >> ws on the getline — plus the general law: every >> followed by a getline needs its cure, wherever they touch.

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