Exercise: Greater Expressions

The less than (<) and greater than (>) methods on Int work in the usual way. What do you think the type an expression using these methods is? For example, what would you guess the type of

1 < 2

is? How can you verify your answer?

What about

(3 + 2) > 7


(3 / 0) < 6


These are all Boolean expressions. The first evalutes to true, the second to false, and the third does not evaluate to any value. You can verify this in the worksheet, by looking at the output for expressions that succeed, by hovering over the expressions, or by adding a type annotation and seeing if the compiler complains about it.

Exercise: Abyssal Depths

Below is a quote from Wuthering Heights. Write an expression to extract "leave me in this abyss" from this String.

  Be with me always—take any form—drive me mad! 
  only do not leave me in this abyss, where I cannot find you!"

Using the substring method will do the job.

  Be with me always—take any form—drive me mad! 
  only do not leave me in this abyss, where I cannot find you!"
""".substring(76, 98)
// res0: String = "leave me in this abyss"

From this exercise you will probably notice that indices into the String start at 0; a common convention in programming but perhaps a little surprising the first time you encounter it.

Exercise: Floating Point Failings

When we introduced Doubles, I said they are an approximation to the real numbers. Why do you think this is? Think about representing numbers like ⅓ and π. How much space would it take to represent these numbers in decimal?

Double is an approximation because it has the fit within the computer's finite memory. A Double takes up precisely 64-bits, which is enough space to store a lot of digits but not enough to store a number that, like π, has an infinite expansion.

The number ⅓ also has an infinite expansion in decimal. Because Doubles are stored in binary there are some numbers that can be represented in a finite number of decimal digits but have no finite representation in binary. 0.1 turns out to be one such number.

In general, floating point numbers can lead to nasty surprises if you expect them to act like the reals. They are fine for our purposes in Creative Scala, but don't go using them to write accounting software!

Exercise: Appending Strings

Join together two strings (known as appending strings) using the ++ method. Write equivalent expressions using both the normal method call style and the operator style.

Something like the below should do.

"It is a truth ".++("universally acknowledged")
// res1: String = "It is a truth universally acknowledged"
"It is a truth " ++ "universally acknowledged"
// res2: String = "It is a truth universally acknowledged"

Exercise: Stylish Operator

When we discussed operator style, we said that

a b c d e

is equivalent to


and not

a.b(c, d, e)

Can you write an expression that shows this is the case? (Hint: think about using a method with two or more parameters.)

We know the substring method has two parameters.

"The only way to test a hypothesis is to look for ... information that disagrees with it.".substring(16, 33)
// res3: String = "test a hypothesis"

If we try writing this in operator style, as below, the code will not compile.

"The only way to test a hypothesis is to look for ... information that disagrees with it." substring 16 33

However if we try a method with only a single parameter, we can use either style.

1 + 2 + 3
// res4: Int = 6
// res5: Int = 6

So, if we assume there is a consistent translation between the two (one that does not depend on the method being called, for example) then we have shown the conversion between the two is what we claimed.

Exercise: Beyond Expressions

In our current model of computation there are only three components: expressions (program text) with types, that evaluate to values (something within the computer's memory). Is this sufficient? Could we write a stock market or a computer game with just this model? Can you think of ways to extend this model? (We haven't so far addressed this in the book, so there is no reason you should be able to answer this question. It's here to provoke thought.)

This is very open ended question. There are several ways to go beyond the model we have so far.

To be useful our programs must be capable of creating effects—changes in the world that go beyond the computer's memory. For example, displaying things on the screen, making sound, sending messages to other computers, and the like. The console implicitly does some of this for us, by printing values on the screen. We'll need to go a bit beyond that for more useful programs.

We also don't have any way to define our own objects and methods, or reuse values in our programs. If we want to, say, use someone's name across a program we have to repeat that name everywhere. We need more methods of abstraction and that's what we'll turn to soon.