Project 1 — The Calculator

← Projects home · Tier: Beginner · Units first: 01–03 — output, variables, arithmetic, input, decisions

Overview

A console calculator the user runs repeatedly: it asks for two numbers and an operation, prints the result (or an honest error), and offers another calculation until the user quits. It is deliberately small — the point is finishing something whole: validation, division-by-zero honesty, clean formatting, and a quit path, all in one program you wrote end to end.

Learning objectives

After completing this project you can:

• build a complete, repeatable console interaction from input to formatted output
• validate numeric input and division-by-zero at the right moments
• structure a program as decisions over arithmetic — and keep the arithmetic in one place
• run a program against a written test plan and record the results

Prerequisites

Requirements

A single-file program calculator.cpp that:

1. greets the user and enters a calculation loop;
2. per round: reads two double operands and one operator (+ - * /);
3. computes and prints the result with 2-decimal money-safe formatting;
4. refuses an unknown operator with a message and no crash;
5. refuses division by zero with a message — the program must not print inf;
6. offers y/n after each calculation; n (or any non-y) quits with a goodbye line.

Functional requirements

Suggested data structures

None beyond variables — deliberately. Three doubles, one char, one accumulator-free loop. The project’s structural lesson is flow control, not data organization.

Milestones

Each milestone ends in a running program.

• M1 — one calculation. Read two numbers and an operator; print a bare result for + only. Exit: compiles, runs once, quits.
• M2 — the operator ladder. All four operations via if/else if. Exit: T1 passes.
• M3 — the refusals. Division-by-zero and unknown-operator branches. Exit: T2 and T3 pass.
• M4 — the loop. Wrap in the repeat-until-quit flow. Exit: T4 passes; the program never needs restarting.

Tasks

1. Write the file header comment (name, date, purpose) and main skeleton.
2. M1: the three reads and the + result.
3. M2: complete the ladder; add fixed << setprecision(2).
4. M3: insert the guard before the division branch; add the final else.
5. M4: wrap in do { ... } while (again == 'y'); reading the choice at the loop’s end.
6. Run the full test plan; fill in the actual-output column.

Test plan

Edge cases

• Negative operands (-3.5 2 *) — formatting and the minus sign.
• Very large values (1e9 1e9 *) — overflow is not your problem to fix at this tier, but note what prints.
• The choice prompt answered with Enter-as-first-char of a word (yes) — document how cin >> char behaves and why.

Extension ideas

1. Add % (integer-only: validate both operands are whole numbers first).
2. Add a running session count of calculations performed, printed at goodbye.
3. A “memory” feature: recall the previous result as either operand (r).

Grading / self-assessment

Score against the shared rubric. Project-specific additions:

• Division by zero produces a message, never inf (+1)
• The unknown operator never produces a result line (+1)
• The loop is honest: y continues, everything else quits, and the behaviour is documented (+1)

Hints

1. fixed << setprecision(2) set once before the first result print stays set for the session.
2. The zero check belongs before the / branch — inside it, the division has already happened.
3. The quit test runs on the choice character, not on the calculation result.

Complete reference solution

// calculator.cpp — Programming Fundamentals Using C++
// Project 1 · The Calculator
// Build: g++ -std=c++17 -Wall -Wextra calculator.cpp -o calculator

#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    cout << "=== Console Calculator ===\n";
    cout << fixed << setprecision(2);

    bool again = true;
    do {
        double a = 0.0, b = 0.0;
        char op = '?';

        cout << "\nFirst number: ";
        cin >> a;
        cout << "Operator (+ - * /): ";
        cin >> op;
        cout << "Second number: ";
        cin >> b;

        if (op != '+' && op != '-' && op != '*' && op != '/') {
            cout << "Unknown operator: " << op << "\n";
        } else if (op == '/' && b == 0.0) {
            cout << "Cannot divide by zero\n";
        } else {
            double result = 0.0;
            if      (op == '+') result = a + b;
            else if (op == '-') result = a - b;
            else if (op == '*') result = a * b;
            else                result = a / b;
            cout << a << " " << op << " " << b << " = " << result << "\n";
        }

        cout << "Another calculation? (y/n): ";
        char choice;
        cin >> choice;
        again = (choice == 'y' || choice == 'Y');
    } while (again);

    cout << "Goodbye — " << "thanks for calculating!\n";
    return 0;
}

Explanation of important design decisions

• The vocabulary check comes first. Testing op against the four legal symbols before any arithmetic means the ladder’s else is genuinely unreachable-bad-input, and the zero-division test only runs for a legal operator. Ordering the guards cheapest-and-broadest first is the validation habit the whole course builds on.
• One result variable, one print. The ladder assigns; the print happens once after it. Every alternative (printing inside each branch) quadruples the formatting surface and is where students first meet “why does this line differ from that one?”
• do-while for the conversation. The user must see the prompts before any exit condition can exist — the repeat-until shape matches the domain (a conversation), and the choice read at the end of the body keeps the exit test next to the data it tests.
• Y is accepted. Strictly y-only is defensible; accepting both cases and documenting it in the test plan is the professional choice — the program’s vocabulary is its contract, and T4 records it.

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— Engr. Dr. Muhammad Nadeem Majeed, Professor, Department of Data Science (PUCIT), University of the Punjab, Lahore · About the instructor