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Remote Control Car
Remote Control Car

Remote Control Car

Learning Exercise
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Introduction

Structs

Structs are an extension built on top of maps which provide compile-time checks and default values. A struct is named after the module it is defined in. To define a struct use the defstruct construct. The construct usually immediately follows after the module definition. defstruct accepts either a list of atoms (for nil default values) or a keyword list (for specified default values). The fields without defaults must precede the fields with default values.

defmodule Plane do
  defstruct [:engine, wings: 2]
end

plane = %Plane{}
# => %Plane{engine: nil, wings: 2}

Accessing fields and updating

Since structs are built on maps, we can use most map functions to get and manipulate values. The Access Behaviour is not implemented for structs. It is recommended to use the static access operator . to access struct fields instead.

  • get/fetch field values:

    plane = %Plane{}
plane.engine
# => nil
Map.fetch(plane, :wings)
# => {:ok, 2}

  • update field values

    plane = %Plane{}
%{plane | wings: 4}
# => %Plane{engine: nil, wings: 4}

Pattern matching

Structs can be used in pattern matching with or without the struct name.

plane = %Plane{}
%Plane{wings: wings} = plane
%{wings: wings} = plane

By including the struct name in the pattern, you can ensure that both the left and right side are structs of the same type.

defmodule Helicopter do
  defstruct [:engine, rotors: 1]
end

%Plane{} = %Helicopter{}
# => (MatchError) no match of right hand side value: %Helicopter{engine: nil, rotors: 1}

Enforcing field value initialization

We can use the @enforce_keys module attribute with a list of the field keys to ensure that the values are initialized when the struct is created. If a key is not listed, its value will be nil as seen in the above example. If an enforced key is not initialized, an error is raised.

defmodule User do
  @enforce_keys [:username]
  defstruct [:username]
end

%User{}
# => (ArgumentError) the following keys must also be given when building struct User: [:username]

Instructions

In this exercise you'll be playing around with a remote controlled car, which you've finally saved enough money for to buy.

Cars start with full (100%) batteries. Each time you drive the car using the remote control, it covers 20 meters and drains one percent of the battery. The car's nickname is not known until it is created.

The remote controlled car has a fancy LED display that shows two bits of information:

  • The total distance it has driven, displayed as: "<METERS> meters".
  • The remaining battery charge, displayed as: "Battery at <PERCENTAGE>%".

If the battery is at 0%, you can't drive the car anymore and the battery display will show "Battery empty".

1. Create a brand-new remote controlled car

Implement the RemoteControlCar.new/0 function to return a brand-new remote controlled car struct:

RemoteControlCar.new()
# => %RemoteControlCar{
#      battery_percentage: 100,
#      distance_driven_in_meters: 0,
#      nickname: "none"
#    }

The nickname is required by the struct, make sure that a value is initialized in the new function, but not in the struct.

2. Create a brand-new remote controlled car with a nickname

Implement the RemoteControlCar.new/1 function to return a brand-new remote controlled car struct with a provided nickname:

RemoteControlCar.new("Blue")
# => %RemoteControlCar{
#      battery_percentage: 100,
#      distance_driven_in_meters: 0,
#      nickname: "Blue"
#    }

3. Display the distance

Implement the RemoteControlCar.display_distance/1 function to return the distance as displayed on the LED display:

car = RemoteControlCar.new()
RemoteControlCar.display_distance(car)
# => "0 meters"

Make sure the function only accepts a RemoteControlCar struct as the argument.

4. Display the battery percentage

Implement the RemoteControlCar.display_battery/1 function to return the battery percentage as displayed on the LED display:

car = RemoteControlCar.new()
RemoteControlCar.display_battery(car)
# => "Battery at 100%"

Make sure the function only accepts a RemoteControlCar struct as the argument. If the battery is at 0%, the battery display will show "Battery empty".

5. Driving changes the battery and distance driven

Implement the RemoteControlCar.drive/1 function that:

  • updates the number of meters driven by 20
  • drains 1% of the battery
RemoteControlCar.new("Red")
|> RemoteControlCar.drive()

# => %RemoteControlCar{
#      battery_percentage: 99,
#      distance_driven_in_meters: 20,
#      nickname: "Red"
#    }

Make sure the function only accepts a RemoteControlCar struct as the argument.

6. Account for driving with a dead battery

Update the RemoteControlCar.drive/1 function to not increase the distance driven nor decrease the battery percentage when the battery is drained (at 0%):

%RemoteControlCar{
  battery_percentage: 0,
  distance_driven_in_meters: 1980,
  nickname: "Red"
}
|> RemoteControlCar.drive()

# => %RemoteControlCar{
#      battery_percentage: 0,
#      distance_driven_in_meters: 1980,
#      nickname: "Red"
#    }
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