Variables and Types

What Computers Are Good At

What Computers Are Good At

Over the next few lessons we’ll be discussing several core computer capabilties:

• Storing and manipulating data
• Making simple decisions
• Repeating the same steps over and over again

These abilities form the foundation of everything that computers can do. Even if computers sometimes seem complex—they are actually quite simple. (That doesn’t mean that computer programming or computer science is simple or easy, far from it!)

Variables

Variables

Computers are great at storing information or data. Let’s look at how we can store a single number in a Kotlin program:

This code does three things:

1. It tells the computer that we are going to store data in a variable named i
2. It tells the computer that the variable i will store integer data (Int)
3. It tells the computer that the value of i may change (var)
4. It sets the initial value of i to be zero

We have declared our first variable! As its name implies, a variable’s value can and usually does change as the program runs.

Declaration and Initialization

Declaration and Initialization

Technically the statement var i: Int = 0 is combining two things that we can do separately: variable declaration and initialization. Let’s write it out on two separate lines:

While we can do it this way, Kotlin will complain if we don’t set an initial value:

Run the code above and see what happens. As a result, it’s more common to see the variable declaration and initialization combined into a single statement.

Type Inference

Type Inference

When we declare and initialize our variable in the same statement, Kotlin does not require that we explicitly provide a type. For example, this:

is equivalent to

In the second case, Kotlin can infer the type of i from the value that we use to initialize it: 0, an Int, literal. Because this is pretty convenient, we’ll normally omit the unnecessary type notation when we can. There are times when explicit type notation is needed, and we’ll discuss them when we get there.

Literals

Literals

In the code above we also used our first literal—the value 0 that we used to initialize i. A literal is a value that appears directly in your code. Unlike a variable, its value does not change.

Note that in Kotlin, character literals must be enclosed in single (not double) quotes. So this won’t work:

But be careful here, since Kotlin’s type inference might “help” you in a way that you don’t expect!

Types

Types

Here are all 8 Kotlin basic types, broken into four categories:

1. Integer values: Byte, Short, Int, and Long
2. Floating point values: Float and Double
3. Boolean values: Boolean
4. Characters: Char

Integer Values

Integer Values

Kotlin has four different basic types for representing integer values. An integer is a number without a decimal point. So 0, 8, 16, and 333 are all integers, but 8.7, 9.001, and 0.01 are not.

Why does Kotlin have four different types for representing the same kind of data? Because each can hold a different range of values. Byte variables can store values from -128 to 127, while Int variables can store values from -2,147,483,648 to 2,147,483,647: But Int variables also take up more computer memory. Don’t worry too much about these distinctions now. We’ll almost always use Int variables to store integers in this class.

Floating Point Values

Floating Point Values

Kotlin provides two types for storing decimal values: Float and Double. Similar to with integer values, Float variables can store a smaller range of values than Double variables. We’ll commonly use a Double when we need to store a floating point value.

Boolean (or Truth) Values

Boolean (or Truth) Values

In order to make decisions about what to do in our programs, we’ll frequently want to determine whether something is true or false. Variables with Kotlin’s Boolean type can store only two values: true and false.

Character Values

Character Values

Finally, Kotlin provides the Char type for storing a single character value.

Why Types?

Why Types?

Not every programming language requires that you indicate what type of data a variable will hold. For example, this is valid code in the Python programming language:

So why does Kotlin enforce rules about what type of data each variable holds? Because it allows the computer to help you write correct programs and avoid errors. By keeping track of what type of data a variable holds, Kotlin can help us make sure that certain operations on that variable are valid. This will make more sense as we go, but we can already see these checks at work in a simple case:

Representation in the Digital Age

Representation in the Digital Age

Examining the primitive types, you’ll notice that 6—Byte, Short, Int, Long, Float, and Double—are explicitly for storing numbers. What about Boolean? Well, we represent false as 0 and true as 1.

That leaves Char as the outlier that doesn’t seem to store a numeric value. But… it does! Here’s how:

What is shown above is the mapping from numbers to character values. You can see that in action as you use the char type in Kotlin:

The mapping above wasn’t sent down on stone tablets. It was agreed upon at the dawn of the computer age. (And it leaves a lot to be desired, since there are many symbols in many alphabets that are not included!)

But it makes an important point. Internally, computers store everything as a number. Any non-numeric data must be converted to numeric form—or digitized—before it can be manipulated by a computer.

We live in the digital age. You enjoy music delivered in a digital format and take photos with a digital camera. You can increasingly enjoy fine art from a distance due to high-resolution scans. One day soon you’ll have a completely digital medical record, consisting of medical information that was itself digitized so that it could be analyzed by a computer. Learning about computer science and programming will allow you to be a full participant in our digital present and future.

Course Design: Daily Lessons

Course Design: Daily Lessons

CS 124 does not hold lectures.

Study after study has found that lectures are an ineffective way to teach. Learning requires active engagement, not passive watching⁽¹⁾.

In CS 124, you’ll learn through active engagement with our interactive daily lessons. Each lesson mixes text, runnable examples, interactive code walkthroughs (unique to CS 124), and short videos. Along the way you have many chances to test your knowledge through practice problems, debugging challenges, and homework exercises, with course staff supporting you every step of the way.

Daily lessons have many other advantages compared to lecturing. Returning to the content each day helps achieve spaced repetition, helping you learn even more efficiently. Students with different levels of prior experience can move at their own pace, which is impossible with one-speed-fits-all lecturing. This is particularly important in a course like CS 124, which enrolls students with a wide range of prior experience. Eliminating lecture space and time constraints has also allowed us to open up the course to anyone on campus.

We’ve also amassed a large and growing set of explanations from multiple instructors—Colleen and Geoff, and even some new faces from outside Illinois—and from course staff. So if you don’t understand something, there’s probably another explanation that might help.

Homework Problem

Homework Problem

As a reminder, CS 124 now allows students to collaborate freely on the homework problems. Please see the syllabus for more details.

CS People: Ada Lovelace

CS People: Ada Lovelace

As we teach you the basics of computer science and programming, we’re also going introduce you to computer science people—the humans behind technology’s remarkable achievements. Note that we include a few questions on each quiz on these short videos, so don’t skip them!

It may surprise you, but the first computer program was written long before there was a computer to run it on! But everything about this person and their contributions to computer science is quite remarkable.

More Practice

Need more practice? Head over to the practice page.