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5
Rounds & Score Written by Matthijs Hollemans & Fahim Farook

OK, so you have made quite a bit of progress on the game, and the to-do list is getting ever shorter! So what’s next on the list now that you can generate a random number and display it on screen?

A quick look at the task list shows that you now have to “compare the value of the slider to that random number and calculate a score based on how far off the player is.” Let’s get to it!

This chapter covers the following:

  • Get the difference: Calculate the difference between the target value and the value that the user selected.
  • Other ways to calculate the difference: Other approaches to calculating the difference.
  • What’s the score?: Calculate the user’s score based on the difference value.
  • The total score: Calculate the player’s total score over multiple rounds.
  • Display the score: Display the player score on screen.
  • One more round…: Implement updating the round count and displaying the current round on screen.

Getting the difference

Now that you have both the target value (the random number) and a way to read the slider’s position, you can calculate how many points the player scored.

The closer the slider is to the target, the more points for the player.

To calculate the score for each round, you look at how far off the slider’s value is from the target:

Calculating the difference between the slider position and the target value
Calculating the difference between the slider position and the target value

A simple approach to finding the distance between the target and the slider is to subtract currentValue from targetValue.

Unfortunately, that gives a negative value if the slider is to the right of the target because now currentValue is greater than targetValue.

You need some way to turn that negative value into a positive value — or you end up subtracting points from the player’s score (unfair!).

Doing the subtraction the other way around — currentValue minus targetValue — won’t always solve things either because, then, the difference will be negative if the slider is to the left of the target instead of the right.

Hmm, it looks like we’re in trouble here…

Exercise: How would you frame the solution to this problem if I asked you to solve it in natural language? Don’t worry about how to express it in computer language for now, just think it through in plain English.

I came up with something like this:

  • If the slider’s value is greater than the target value, then the difference is: slider value minus the target value.

  • However, if the target value is greater than the slider value, then the difference is: target value minus the slider value.

  • Otherwise, both values must be equal, and the difference is zero.

This will always lead to a difference that is a positive number, because you always subtract the smaller number from the larger one.

Do the math:

If the slider is at position 60 and the target value is 40, then the slider is to the right of the target value, and the difference is 60 - 40 = 20.

However, if the slider is at position 10 and the target is 30, then the slider is to the left of the target and has a smaller value. The difference here is 30 - 10 = also 20.

Algorithms

What you’ve just done is come up with an algorithm, which is a fancy term for a series of steps for solving a computational problem. This is only a very simple algorithm, but it is an algorithm nonetheless.

There are many famous algorithms, such as quicksort for sorting a list of items and binary search for quickly searching through such a sorted list. Other people have already invented many algorithms that you can use in your own programs — that’ll save you a lot of thinking!

However, in the programs that you write, you’ll probably have to come up with a few algorithms of your own at some time or other. Some are simple such as the one above; others can be pretty hard and might cause you to throw up your hands in despair. But that’s part of the fun of programming. :]

The academic field of Computer Science concerns itself largely with studying algorithms and finding better ones.

You can describe any algorithm in plain English. It’s just a series of steps that you perform to calculate something. Often, you can perform that calculation in your head or on paper, the way you did above. But for more complicated algorithms, doing that might take you forever so, at some point, you’ll have to convert the algorithm to computer code.

The point I’m trying to make is this: If you ever get stuck and you don’t know how to make your program calculate something, take a piece of paper and try to write out the steps in English. Set aside the computer for a moment and think the steps through. How would you perform this calculation by hand?

Once you know how to do that, converting the algorithm to code should be a piece of cake.

The difference algorithm

Getting back to the problem at hand, it is possible you came up with a different way to solve it. I’ll show you two alternatives in a minute, but let’s convert the above algorithm to computer code first:

var difference: Int
if currentValue > targetValue {
  difference = currentValue - targetValue
} else if targetValue > currentValue {
  difference = targetValue - currentValue
} else {
  difference = 0
}

The if construct is new. It allows your code to make decisions, and it works much like you would expect:

if something is true {
  then do this
} else if something else is true {
  then do that instead
} else {
  do something when neither of the above are true
}

Basically, you put a logical condition after the if keyword. If that condition turns out to be true, for example currentValue is greater than targetValue, then the code in the block between the { } brackets is executed.

However, if the condition is not true, then the computer looks at the else if condition and evaluates that. There may be more than one else if, and code execution moves one by one from top to bottom until one condition proves to be true.

If none of the conditions are found to be valid, then the code in the final else block is executed.

In the implementation of this little algorithm, you first create a local variable named difference to hold the result. This will either be a positive whole number or zero, so an Int will do:

var difference: Int

Then you compare the currentValue against the targetValue. First, you determine if currentValue is greater than targetValue:

if currentValue > targetValue {

The > is the greater-than operator. The condition currentValue > targetValue is considered true if the value stored in currentValue is at least one higher than the value stored in targetValue. In that case, the following line of code is executed:

  difference = currentValue - targetValue

Here, you subtract targetValue (the smaller one) from currentValue (the larger one) and store the result in the difference variable.

Notice how I chose variable names that clearly describe what kind of data the variables contain. Often, you will see code such as this:

a = b - c

It is not immediately clear what this is supposed to mean, other than that some arithmetic is taking place. The variable names “a”, “b” and “c” don’t give any clues as to their intended purpose or what kind of data they might contain.

Back to the if statement. If currentValue is equal to or less than targetValue, the condition is untrue (or false in computer-speak) and execution will move on to the next condition:

} else if targetValue > currentValue {

The same thing happens here as before, except now the roles of targetValue and currentValue are reversed. The computer will only execute the following line when targetValue is the greater of the two values:

  difference = targetValue - currentValue

This time, you subtract currentValue from targetValue and store the result in the difference variable.

There is only one situation you haven’t handled yet: when currentValue and targetValue are equal. If this happens, the player has put the slider exactly at the position of the target random number, a perfect score.

In that case the difference is 0:

} else {
  difference = 0
}

Since, by now, you’ve already determined that one value is not greater than the other, nor is it smaller, you can only draw one conclusion: The numbers must be equal.

Displaying the difference

➤ Let’s put this code into action. Add it to the top of showAlert():

@IBAction func showAlert() {
  var difference: Int
  
  if currentValue > targetValue {
    difference = currentValue - targetValue
  } else if targetValue > currentValue {
    difference = targetValue - currentValue
  } else {
    difference = 0
  }

  let message = "The value of the slider is: \(currentValue)" +
                "\nThe target value is: \(targetValue)" +
                "\nThe difference is: \(difference)"
  . . .
}

Just so you can see that it works, you add the difference value to the alert message as well.

➤ Run it and see for yourself.

The alert shows the difference between the target and the slider
The alert shows the difference between the target and the slider

Simplifying the algorithm

I mentioned earlier that there are other ways to calculate the difference between currentValue and targetValue as a positive number. The above algorithm works well, but it is eight lines of code. I think we can come up with a simpler approach that takes up fewer lines.

The new algorithm goes like this:

  1. Subtract the target value from the slider’s value.
  2. If the result is a negative number, then multiply it by -1 to make it a positive number.

Here, you no longer avoid the negative number since computers can work just fine with negative numbers. You simply turn it into a positive number.

Exercise: Convert the above algorithm into source code. Hint: The English description of the algorithm contains the words “if” and “then,” which is a pretty good indication that you’ll have to use an if statement.

You should have arrived at something like this:

var difference = currentValue - targetValue
if difference < 0 {
  difference = difference * -1
}

This is a pretty straightforward translation of the new algorithm.

You first do the subtraction and put the result into the difference variable.

Notice that you can create the new variable and assign the result of a calculation to it, all in one line. You don’t need to put it onto two different lines, like so:

var difference: Int
difference = currentValue - targetValue

Also, in the one-liner version, you didn’t have to tell the compiler that difference takes Int values. Because both currentValue and targetValue are Ints, Swift is smart enough to figure out that difference should also be an Int.

This feature, as mentioned before, is called type inference and it’s one of the big selling points of Swift.

Once you have the subtraction result, you use an if statement to determine whether difference is negative, i.e., less than zero. If it is, you multiply by -1 and put the new result — now a positive number — back into the difference variable.

When you write:

difference = difference * -1

the computer first multiplies difference’s value by -1. Then, it puts the result of that calculation back into difference. In effect, this overwrites difference’s old contents (the negative number) with the positive number.

Because this is a common thing to do, there is a handy shortcut:

difference *= -1

The *= operator combines * and = into a single operation. The end result is the same: The variable’s old value is gone and it now contains the result of the multiplication.

You could also have written this algorithm as follows:

var difference = currentValue - targetValue
if difference < 0 {
  difference = -difference
}

Instead of multiplying by -1, you now use the negation operator to ensure difference’s value is always positive. This works because negating a negative number makes it positive again. (Ask a math professor if you don’t believe me.)

Using the new algorithm

➤ Give these new algorithms a try. You should replace the old stuff at the top of showAlert() as follows:

@IBAction func showAlert() {
  var difference = currentValue - targetValue
  if difference < 0 {
    difference *= -1
  }

  let message = . . .
}

When you run this new version of the app (try it!), it should work exactly the same as before. The result of the computation does not change, only the technique used changed.

Another variation

The final alternative algorithm I want to show you uses a function.

You’ve already seen functions a few times before. For example, when you used lroundf() for rounding off the slider’s decimals.

Similarly, to make sure a number is always positive, you can use the abs() function.

If you took math in school, you might remember the term “absolute value,” which is the value of a number without regard to its sign.

That’s exactly what you need here, and the standard library contains a convenient function for it, which allows you to reduce this entire algorithm down to a single line of code:

let difference = abs(targetValue - currentValue)

It really doesn’t matter whether you subtract currentValue from targetValue or the other way around. If the number is negative, abs() turns it positive. It’s a handy function to remember.

➤ Make the change to showAlert() and try it out:

@IBAction func showAlert() {
  let difference = abs(targetValue - currentValue)

  let message = . . .
}

It doesn’t get much simpler than that!

Exercise: Something else has changed… can you spot it?

Answer: You wrote let difference instead of var difference.

Variables and constants

Swift makes a distinction between variables and constants. Unlike a variable, the value of a constant, as the name implies, cannot change.

You can only put something into the box of a constant once and cannot replace it with something else afterwards.

The keyword var creates a variable while let creates a constant. That means difference is now a constant, not a variable.

In the previous algorithms, the value of difference could possibly change. If it was negative, you turned it positive. That required difference to be a variable, because only variables can have their value change.

Now that you can calculate the whole thing in a single line, difference will never have to change once you’ve given it a value. In that case, it’s better to make it a constant with let. Why is that better? It makes your intent clear, which in turn helps the Swift compiler understand your program better.

By the same token, message, alert, and action are also constants (and have been all along!). Now, you know why you declared these objects with let instead of var. Once they’ve been given a value, they never need to change.

Constants are very common in Swift. Often, you only need to hold onto a value for a very short time. If, in that time, the value never has to change, it’s best to make it a constant (let) and not a variable (var).

What’s the score?

Now that you know how far off the slider is from the target, calculating the player’s score for each round is easy.

➤ Change showAlert() to:

@IBAction func showAlert() {
  let difference = abs(targetValue - currentValue)
  let points = 100 - difference

  let message = "You scored \(points) points"
  . . . 
}

The maximum score you can get is 100 points if you put the slider right on the target and the difference is 0. The farther away from the target you are, the fewer points you earn.

➤ Run the app and score some points!

The alert with the player’s score for the current round
The alert with the player’s score for the current round

Exercise: Because the maximum slider position is 100 and the minimum is 1, the biggest difference is 100 - 1 = 99. That means the absolute worst score you can have in a round is 1 point. Explain why this is so. (Eek! It requires math!)

Showing the total score

In this game, you want to show the player’s total score on the screen. After every round, the app should add the newly scored points to the total and then update the score label.

Storing the total score

Because the game needs to keep the total score around for a long time, you will need an instance variable.

➤ Add a new score instance variable to ViewController.swift:

class ViewController: UIViewController {

  var currentValue: Int = 0
  var targetValue = 0
  var score = 0              // add this line

Again, we make use of type inference to not specify a type for score.

Note: If you are not sure about the inferred type of a variable, there is an easy way to find out. Simply hold down the Alt / Option key, and hover your cursor over the variable in question. The variable will be highlighted in blue and your cursor will turn into a question mark. Now, click on the variable and you will get a handy popup, which tells you the type of the variable, as well as the source file in which the variable was declared.

Discover the inferred type for a variable
Discover the inferred type for a variable

➤ Now that you’re using type inference, you can clean up currentValue as well (and make its initial value 0, if you haven’t already):

  var currentValue = 0

Thanks to type inference, you only have to specify the data type when you’re not giving the variable an initial value. But most of the time, you can safely make Swift guess at the type.

I think type inference is pretty sweet! It will definitely save you some, uh, typing (in more ways than one!).

Updating the total score

Now, showAlert() can be amended to update this score variable.

➤ Make the following changes:

@IBAction func showAlert() {
  let difference = abs(targetValue - currentValue)
  let points = 100 - difference

  score += points        // add this line

  let message = "You scored \(points) points"
  . . . 
}

Nothing too shocking, here. You just added the following line:

score += points

This adds the points that the user scored in this round to the total score. You could also have written it like this:

score = score + points

Personally, I prefer the shorthand += version, but either one is OK. Both accomplish exactly the same thing.

Displaying the score

To display your current score, you’re going to do the same thing that you did for the target label: hook up the score label to an outlet and put the score value into the label’s text property.

Exercise: See if you can do the above by yourself. You’ve already done these things before for the target value label, so you should be able to repeat those steps for the score label.

Done? You should have added this line to ViewController.swift:

@IBOutlet var scoreLabel: UILabel!

Then, you connect the relevant label on the storyboard (the one that says 999999) to the new scoreLabel outlet.

Unsure how to connect the outlet? There are several ways to make connections from user interface objects to the view controller’s outlets:

  • Control-click on the object to get a context-sensitive popup menu. Then, drag from New Referencing Outlet to View Controller (you did this with the slider).
  • Go to the Connections Inspector for the label. Drag from New Referencing Outlet to View Controller (you did this with the target label).
  • Control-drag from View Controller to the label (give this one a try now) — doing it the other way, Control-dragging from the label to the View Controller, won’t work.

There is more than one way to skin a cat, or, connect outlets. :]

Great, that gives you a scoreLabel outlet that you can use to display the score. Now, where in the code can you do that? In updateLabels(), of course.

➤ Back in ViewController.swift, change updateLabels() to the following:

func updateLabels() {
  targetLabel.text = String(targetValue)
  scoreLabel.text = String(score)     // add this line
}

Nothing new here. You convert the score — which is an Int — into a String and then pass that string to the label’s text property. In response to that, the label will redraw itself with the new score.

➤ Run the app and verify that the points for this round are added to the total score label whenever you tap the button.

The score label keeps track of the player’s total score
The score label keeps track of the player’s total score

One more round…

Speaking of rounds, you also have to increment the round number each time the player starts a new round.

Exercise: Keep track of the current round number (starting at 1) and increment it when a new round starts. Display the current round number in the corresponding label. I may be throwing you into the deep end here, but if you’ve been able to follow the instructions so far, then you’ve already seen all the pieces you will need to pull this off. Good luck!

If you guessed that you had to add another instance variable, then you are right. You should add the following line (or something similar) to ViewController.swift:

var round = 0

It’s also OK if you included the name of the data type, even though that is not strictly necessary:

var round: Int = 0

Also, add an outlet for the label:

@IBOutlet var roundLabel: UILabel!

As before, you should connect the label to this outlet in Interface Builder.

Don’t forget to make those connections.

Forgetting to make the connections in Interface Builder is an often-made mistake, especially by yours truly.

It happens to me all the time that I make the outlet for a button and write the code to deal with taps on that button but, when I run the app, it doesn’t work. Usually, it takes me a few minutes and some head scratching to realize that I forgot to connect the button to the outlet or the action method.

You can tap on the button all you want but, unless that connection exists, your code will not respond.

Finally, updateLabels() should be modified like this:

func updateLabels() {
  targetLabel.text = String(targetValue)
  scoreLabel.text = String(score)
  roundLabel.text = String(round)    // add this line
}

Did you also figure out where to increment the round variable? I’d say the startNewRound() method is a pretty good place. After all, you call this method whenever you start a new round. It makes sense to increment the round counter there.

➤ Change startNewRound() to:

func startNewRound() {
  round += 1           // add this line
  targetValue = ...
}

Note that when you declared the round instance variable, you gave it a default value of 0. Therefore, when the app starts up, round is initially 0. When you call startNewRound() for the very first time, it adds 1 to this initial value and, as a result, the first round is properly counted as round 1.

➤ Run the app and try it out. The round counter should update whenever you press the Hit Me! button.

The round label counts how many rounds have been played
The round label counts how many rounds have been played

You’re making great progress; well done!

You can find the project files for the app up to this point under 05-Rounds-and-score in the Source Code folder. If you get stuck, compare your version of the app with these source files to see if you missed anything.

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