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Learning Exercise
This exercise discusses some details of method parameters and their use in C#.
Parameters convey information from a calling method to a called method.
The behavior of parameters can be modified by the use of modifiers such as ref and out.
A parameter with the out modifier conveys a value back from the called method to the caller. The parameter can be passed to the called method without being initialized and in any case it is treated within the called method as if, on entry, it had not been initialized. An understanding of the behavior and rules regarding parameter modifiers can be gained most easily through examples (see below) and compiler messages.
void Foo(out int val)
{
val = 42;
}
void Bar(int val)
{
val = 42;
}
int importantValue = 1729;
Foo(out importantValue);
var result = importantValue == 42;
// => true
importantValue = 1729;
Bar(importantValue);
result = importantValue == 1729;
// => true
out parameters must be assigned to within a called method.
A parameter with the ref modifier passes a value into a called method. When the method returns the caller will find any updates made by the called method in that parameter.
void Foo(ref int val)
{
val *= 7;
}
int importantValue = 6;
Foo(ref importantValue);
return importantValue;
// => 42
ref parameters must be variables as the called method will be operating directly on the parameter as seen by the caller.out and ref modifiers are required both in the called method signature and at the call site.The remote control car project you kicked off in the classes exercise has gone well (congratulations!) and due to a number of recent sponsorship deals there is money in the budget for enhancements.
Part of the budget is being used to provide some telemetry.
To keep the sponsors sweet a panel has been installed on the car to display the sponsors names as it goes round the track.
You will note that the introduction of these fancy new technologies has dramatically reduced the car's battery life.
Implement SetSponsors() to take one or more sponsor names and store them on the car.
Note that the SetSponsors() method's argument is guaranteed to be non-null.
Implement DisplaySponsor() to display the selected sponsor. The first sponsor passed in has a sponsorNum of 0, the second, 1 etc.
var car = RemoteControlCar.Buy();
car.SetSponsors("Exercism", "Walker Industries", "Acme Co.");
var sp2 = car.DisplaySponsor(sponsorNum: 1);
// => "Walker Industries"
Implement the RemoteControlCar.GetTelemetryData() method.
GetTelemetryData() should make the battery percentage and distance driven in meters available via out parameters.
GetTelemetryData() should return false if the serialNum argument is less than the previously received value. (There is some issue of multiple telemetry nodes being involved). In this case serialNum will be set to the highest previous serialNum and battery percentage and meters driven will both be set to -1. Where the call is successful serialNum remains unchanged.
var car = RemoteControlCar.Buy();
car.Drive();
car.Drive();
int serialNum = 4;
car.GetTelemetryData(ref serialNum, out int batteryPercentage, out int distanceDrivenInMeters);
// => true, 4L, 80, 4
serialNum = 1;
car.GetTelemetryData(ref serialNum, out batteryPercentage, out distanceDrivenInMeters);
// => false, 4L, -1, -1
Implement the TelemetryClient.GetBatteryUsagePerMeter() method.
This will call RemoteControlCar.GetTelemetryData(). If GetTelemetryData() returns false or the car has not been driven, then this routine should return "no data". If GetTelemetryData() returns true then a message in the following form should be returned "usage-per-meter=BATTERY-USAGE-PER-METER". Where the calculation is (100 - current battery percentage) divided by the distance driven in meters so far.
var car = RemoteControlCar.Buy();
car.Drive();
car.Drive();
var tc = new TelemetryClient(car);
int serialNum = 4;
tc.GetBatteryUsagePerMeter(serialNum);
// => "usage-per-meter=5"
serialNum = 1;
tc.GetBatteryUsagePerMeter(serialNum);
// => "no data"
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