Lesson 20 · 20 of 20

Project: shopping cart · Dart

A Dart project that brings the course together: a shopping cart with maps, functions, exceptions, null safety, fold and a switch expression for the discount.

Dart · Project: shopping cart in DMarlon Studio: the example's code and its output in the terminal

This program puts the whole course together. The shop's prices live in a `const` map outside main, because they never change. The cart is a list of product,units lines, just as they might come from a form or a file, and three of them contain mistakes on purpose.

The lineCost function works out the cost of one line. split(',') separates the product from the units; prices[parts[0]] gives a double?, because the product may not exist, and int.tryParse gives an int?, because the text may not be a number. If either is missing, or the quantity is not positive, `throw` raises an ArgumentError with a clear message; otherwise it returns price times units. After those if checks, Dart knows neither is null.

In main, the loop adds each line's cost into the totals map with (totals[product] ?? 0) + ..., so the milk, which appears twice, builds up. The `try` with `on ArgumentError catch (e)` handles the bad lines: they are printed with Skipped: and the cart carries on.

For the receipt, forEach prints each product with the arrow function row, which lines up the columns with padRight and padLeft. `fold` adds up the map's values and a `switch` expression with >= 20 and >= 10 patterns picks the discount: 17.60 euros earns 5%, and the total comes to 16.72.

A good next step is to give it a screen: in DMarlon Studio you can design a Flutter app on the phone canvas with the product list and a pay button, and reuse these same Dart functions.

Code · main.dart

const prices = {'bread': 1.20, 'milk': 0.95, 'cheese': 4.50, 'apples': 2.30};// Gives the cost of one "product,units" line, or throws an error.double lineCost(String line) {  var parts = line.split(',');  double? price = prices[parts[0]];  int? units = int.tryParse(parts[1]);  if (price == null) throw ArgumentError('${parts[0]} is not sold here');  if (units == null || units <= 0) throw ArgumentError('bad quantity in "$line"');  return price * units;}String row(String label, double euros) =>    label.padRight(14) + euros.toStringAsFixed(2).padLeft(7);void main() {  var cart = ['bread,2', 'cheese,1', 'milk,x', 'apples,3', 'ham,1', 'milk,4', 'bread,0'];  var totals = <String, double>{};  for (var line in cart) {    var product = line.split(',')[0];    try {      totals[product] = (totals[product] ?? 0) + lineCost(line);    } on ArgumentError catch (e) {      print('Skipped: ${e.message}');    }  }  totals.forEach((product, cost) => print(row(product, cost)));  double subtotal = totals.values.fold(0, (a, b) => a + b);  double rate = switch (subtotal) { >= 20 => 0.10, >= 10 => 0.05, _ => 0.0 };  print('-' * 21);  print(row('Subtotal', subtotal));  print(row('Discount (${(rate * 100).round()}%)', -subtotal * rate));  print(row('TOTAL', subtotal * (1 - rate)));}

Output · dart run main.dart

Skipped: bad quantity in "milk,x"
Skipped: ham is not sold here
Skipped: bad quantity in "bread,0"
bread            2.40
cheese           4.50
apples           6.90
milk             3.80
---------------------
Subtotal        17.60
Discount (5%)   -0.88
TOTAL           16.72

Key points

Fixed data goes in a const outside main.
double? and int? make you think about what may be missing.
throw in the function and try in the loop keep the error apart from its handling.
fold adds up the values of a map.
A switch expression turns the subtotal into a discount.

Exercise

Add a map with each product's VAT rate (4 for bread and milk, 10 for the rest) and print a VAT line on the receipt with the tax included in the total.