Project: Calculator in Dart

Project Goal

Build a console-based calculator that supports addition, subtraction, multiplication, division, modulo, and power operations. It reads input from the user, validates it, and shows results in a loop until the user quits.

Design

• A Calculator class encapsulates operations
• A display() function handles console I/O
• Input validation with clear error messages
• History of last 5 calculations

Full Code

import 'dart:io';
import 'dart:math';

class Calculator {
  final List<String> _history = [];

  double calculate(double a, String op, double b) {
    double result = switch (op) {
      '+' => a + b,
      '-' => a - b,
      '*' => a * b,
      '/' => b == 0 ? throw ArgumentError('Division by zero') : a / b,
      '%' => b == 0 ? throw ArgumentError('Modulo by zero') : a % b,
      '^' => pow(a, b).toDouble(),
      _ => throw ArgumentError('Unknown operator: $op'),
    };
    _history.add('$a $op $b = $result');
    if (_history.length > 5) _history.removeAt(0);
    return result;
  }

  void showHistory() {
    if (_history.isEmpty) {
      print('No history yet.');
      return;
    }
    print('\n--- Last $${_history.length} calculations ---');
    _history.asMap().forEach((i, h) => print('$${i + 1}. $h'));
  }
}

double? parseNumber(String input) => double.tryParse(input.trim());

void main() {
  var calc = Calculator();
  print('=== Dart Calculator ===');
  print('Operators: + - * / % ^  |  Commands: history, quit');

  while (true) {
    stdout.write('\nEnter expression (e.g. 5 + 3): ');
    var input = stdin.readLineSync()?.trim() ?? '';

    if (input == 'quit' || input == 'q') {
      print('Goodbye!');
      break;
    }
    if (input == 'history') {
      calc.showHistory();
      continue;
    }

    var parts = input.split(RegExp(r'\s+'));
    if (parts.length != 3) {
      print('Invalid format. Use: <number> <operator> <number>');
      continue;
    }

    var a = parseNumber(parts[0]);
    var b = parseNumber(parts[2]);
    if (a == null || b == null) {
      print('Invalid numbers: $${parts[0]}, $${parts[2]}');
      continue;
    }

    try {
      var result = calc.calculate(a, parts[1], b);
      print('Result: $${result % 1 == 0 ? result.toInt() : result}');
    } on ArgumentError catch (e) {
      print('Error: $${e.message}');
    }
  }
}

Running the Project

dart run bin/calculator.dart

# Sample session:
# Enter expression: 10 / 3
# Result: 3.3333333333333335
# Enter expression: 2 ^ 8
# Result: 256
# Enter expression: history
# 1. 10.0 / 3.0 = 3.333...
# 2. 2.0 ^ 8.0 = 256.0

Extension Challenges

• Add trigonometric functions (sin, cos, tan)
• Add parentheses support by writing an expression parser
• Save history to a JSON file and load on startup
• Build a Flutter GUI version of the calculator