Lesson 14 · 14 of 20

Named and optional parameters · Dart

Make flexible Dart functions with optional parameters in [ ], named parameters in { }, default values and required.

Dart · Named and optional parameters in DMarlon Studio: the example's code and its output in the terminal

A parameter in square brackets is optional and positional: in greet(String name, [String greeting = 'Hello']), the greeting can be left out and then it is 'Hello'. greet('Marta') gives Hello, Marta! and greet('Marta', 'Good morning') changes the greeting.

Parameters in braces are named parameters: when calling the function you write the name before the value, as in nights: 3. That makes it obvious what each number means, and they can go in any order: book('Hotel Mar', guests: 3, nights: 2).

Named parameters are optional unless they are marked `required`: nights is compulsory and, if it is missing, as in the commented-out line, Dart will not compile. The others have a default value: without guests it counts 2 people, and without breakfast, no breakfast.

The button function shows what they are really for: long calls read by themselves. This is how Flutter code is written: Text('Hi', style: ..., textAlign: ...) or Padding(padding: ..., child: ...) are calls with named parameters, and it is what DMarlon Studio generates for the widgets you place on the phone canvas.

Code · main.dart

// Optional positional parameters go in [ ], with a default value.String greet(String name, [String greeting = 'Hello']) => '$greeting, $name!';// Named parameters go in { }; required makes one compulsory.void book(String hotel, {required int nights, int guests = 2, bool breakfast = false}) {  int cost = nights * guests * 45;  if (breakfast) cost += nights * guests * 8;  String extra = breakfast ? ' with breakfast' : '';  print('$hotel: $nights nights, $guests guests$extra = $cost euros');}// Named parameters with defaults make long calls easy to read.String button(String text, {String colour = 'blue', bool rounded = true}) =>    '[$text] $colour${rounded ? ', rounded' : ''}';void main() {  print(greet('Marta'));  print(greet('Marta', 'Good morning'));  // Named arguments carry their name and can go in any order.  book('Hotel Sol', nights: 3);  book('Hotel Mar', guests: 3, nights: 2);  book('Casa Rural', nights: 4, breakfast: true, guests: 4);  // book('Hotel Sol');  // Error: nights is required.  print(button('Buy'));  print(button('Cancel', colour: 'grey', rounded: false));}

Output · dart run main.dart

Hello, Marta!
Good morning, Marta!
Hotel Sol: 3 nights, 2 guests = 270 euros
Hotel Mar: 2 nights, 3 guests = 270 euros
Casa Rural: 4 nights, 4 guests with breakfast = 848 euros
[Buy] blue, rounded
[Cancel] grey

Key points

[type name = value] is optional and goes by position.
{type name = value} goes by name: name: value.
Named arguments can go in any order.
required makes a named parameter compulsory.
Flutter uses named parameters in all its widgets.

Exercise

Write a pizza function with a required named size parameter plus cheese (default true) and toppings (default 0), which prints the order and its price; call it in three different ways.