Objects

This is lesson 3 of the beginner tutorial. Start with lists and functions if you have not yet written a function or used do to process a list.

The previous lesson kept each room's name, readings, and warm limit separately. We will now group them, so we can pass a room's whole report around as one value.

An object can hold several named values. These values are called its fields. A class describes the fields its objects have and the operations they support. Our class will be named TemperatureReport. Class names use UpperCamelCase: each word starts with a capital letter. Variables and functions use kebab-case, as in warm-limit and print-reading. We will introduce methods below; their names also use kebab-case.

The constructor runs when a new object is created. Here it receives three arguments and stores them in the new object's fields. The ? in room := ? means that the constructor must supply the field's initial value. It is not a value stored in the field. We use the same notation for readings and limit:

class TemperatureReport:
  room := ?
  readings := ?
  limit := ?

  constructor room readings limit:
    this.room = room
    this.readings = readings
    this.limit = limit

main:
  report := TemperatureReport "Kitchen" [21, 26] 25
  print report.room
  print report.readings

Expected output:

Kitchen
[21, 26]

TemperatureReport "Kitchen" [21, 26] 25 creates an object and runs its constructor with those three values. this refers to the object being created: in this.room = room, the left side is its field and the right side is the constructor's argument. report.room reads that object's room field.

Change the room name or the readings and run again. Then create a second report and print both room names. Each object keeps its own field values.

Initialize fields with constructor parameters

Toit lets a constructor parameter initialize a field directly by prefixing its name with a dot. This version stores the same three arguments:

class TemperatureReport:
  room := ?
  readings := ?
  limit := ?

  constructor .room .readings .limit:

main:
  report := TemperatureReport "Kitchen" [21, 26] 25
  print report.room

The constructor body is empty because the field assignments are expressed in its parameters. This is shorthand for the explicit assignments used above.

Put an operation on the object

A function inside a class is called a method. It can use the object's fields, so callers do not need to pass the room, readings, and limit each time.

The print-report method below uses what we learned in the previous lessons: printing a string, iterating over a list, and making a decision.

class TemperatureReport:
  room := ?
  readings := ?
  limit := ?

  constructor .room .readings .limit:

  print-report:
    print room
    readings.do: |temperature|
      if temperature >= limit:
        print "$temperature C: Warm"
      else:
        print "$temperature C: Comfortable"

main:
  kitchen := TemperatureReport "Kitchen" [21, 26] 25
  bedroom := TemperatureReport "Bedroom" [22, 24] 23
  kitchen.print-report
  bedroom.print-report

Expected output:

Kitchen
21 C: Comfortable
26 C: Warm
Bedroom
22 C: Comfortable
24 C: Warm

Calling kitchen.print-report uses the kitchen object's fields. Calling bedroom.print-report uses the bedroom object's fields. The method takes no arguments, so its name is all we need to call it.

Handle a room without readings

An empty list currently produces only the room heading. We can make the report clearer by printing No readings when there are none.

The list's size gives the number of values it contains. readings.size == 0 asks whether that number is zero. Use == to compare values; a single = is an assignment, as in the first constructor we wrote.

class TemperatureReport:
  room := ?
  readings := ?
  limit := ?

  constructor .room .readings .limit:

  print-report:
    print room
    if readings.size == 0:
      print "No readings"
      return
    readings.do: |temperature|
      if temperature >= limit:
        print "$temperature C: Warm"
      else:
        print "$temperature C: Comfortable"

main:
  kitchen := TemperatureReport "Kitchen" [21, 26] 25
  bedroom := TemperatureReport "Bedroom" [] 23
  kitchen.print-report
  bedroom.print-report

This prints the kitchen report, then:

Bedroom
No readings

return stops the method immediately and resumes execution after the call. Here there is no reason to continue into the loop once we know the list is empty. The kitchen report still reaches the loop because its list is not empty.

Practice

Add readings to the bedroom report and check that No readings disappears. Then create a third report for another room. You should only need to add its creation and method call under main; the class can stay the same.

Continue when you need more

You can now build a program using variables, decisions, lists, functions, and objects. You do not need to learn every feature before using these tools.