KotlinCS 124 LogoJava

Conditional Expressions and Statements

var temperature = 53.8
var isHot = temperature > 50
if (isHot) {
println("It's hot... for winter!")
}

In this lesson we’ll talk about how computers make decisions. We’ll start simple, and get a bit more complicated tomorrow. But simple computer decision making is what gives birth to all of the more complex computer behaviors.

An important reminder: you are not alone. Our calendar is packed with opportunities to get help and meet the course staff and fellow students.

Warm Up Debugging Challenge
Warm Up Debugging Challenge

Let’s warm up with your first graded debugging challenge!

Conditional Expressions
Conditional Expressions

Computers can make decisions based on data. They do this by evaluating whether a given expression is true or false. We can do this with only literal values:

var goodGrade = 95 > 90 // 95 > 90 is the conditional expression
println(goodGrade)

Try changing the literal values in the example above and see what happens. You should be able to get goodGrade to be either true or false, depending on the literal values that you choose.

But the example above is a bit silly. We don’t need a computer to tell us whether 95 is greater than 90! So usually our conditional expressions involve at least one variable:

var grade = 95
var goodGrade = grade > 90
println(goodGrade)

In the examples above we’re using a conditional expression to set the value of a boolean variable. What makes the expresion a conditional expression is the use of the > operator. For example:

// The right side of this assignment evaluates to an int because the operator is +
var grade = 10 + 80
// The right side of this assignment evaluates to a boolean because the operator is >
var goodGrade = grade > 90

Kotlin includes a variety of useful conditional operators that you can use on numeric variables and literals: <, >, <=, >=, ==, and !=.

// Let's have fun with Kotlin conditional operators!
var i = 10
var j = 20

Tomorrow we’ll look at how we can chain multiple conditional expressions together to make more complicated decisions. For now, we’ll keep things simple.

Assignment v. Equality Testing
Assignment v. Equality Testing

In the previous lesson we emphasized how = is an assignment operator in Kotlin and does not test equality. The way that we do test for equality is using a slightly-different but similar-looking operator: ==.

When you are getting started it can be very hard to see the difference between = and ==. (You’ll get way, way better at this.)

But it’s worth emphasizing the difference now and starting to be on the lookout for it.

var i = 10
i == 20

Conditional Statements
Conditional Statements

Conditional expressions form the basis for computer decision making because they are the basis for conditional statements. A conditional statement allows you to tell the computer that it should do one thing or another thing depending on the result of a conditional expression. Put more simply, they allow our programs to make decisions based on data.

Let’s dive right in and look at our first example. There are multiple new ideas emerging here all at once—so don’t worry, we’ll go over it slowly and carefully below.

var temperature = 88.8
if (temperature > 80) {
println("It's hot")
}

Parts of this snippet should be familiar to us. The first line is a standard variable declaration and initialization (var temperature = 88.8) of a variable that Kotlin will infer to be a Double since we initialize it with a Double literal. The middle of the second line is a conditional expression (temperature > 80). It evaluates to true if the value in the variable temperature is greater than 80 and false otherwise.

But the rest is new—and represents our first conditional statement. Run the example above and see what happens. Now, change the value of temperature or the threshold (80) and see what happens. See if you get the program to not display any output.

Now let’s go through it carefully together:

var temperature = 88.8
if (temperature > 80) {
println("It's hot")
}

Blocks
Blocks

if statements introduce us to our first block of code. Blocks begin with a { and end with a }. In the case of an if statement, the block contains the code that should be executed if the condition is true:

var i = 10
if (i != 8) { // Block starts here
// What to do if i is not equal to 8
} // Block ends here
println("The rest of the program...")

Blocks of code can contain all of the basic building blocks that we’ve already seen. We can declare variables inside blocks, manipulate them, and even include other conditional statements:

var distanceInKilometers = 9.8
if (distanceInKilometers > 14.8) {
var distanceInMeters = 1000 * distanceInKilometers
if (distanceInMeters > 500) {
println("That's more than half a kilometer!")
}
}

We use blocks to help organize our programs. In the case above, blocks identify the code that should be executed if the condition is true. Over the next few lessons we’ll see how blocks can be used to identify code that should be repeated, and name parts of the code that we want to reuse.

if-else Statements
if-else Statements

To conclude, let’s look at one useful addition to the if statement that we introduced above. We’ve seen how it can be used to decide what to do:

var temperature = 88.8
if (temperature > 80) {
println("It's hot")
}

In the example above, either the block is executed or it is not. But we can also extend this idea to make more complicated decisions. Here’s an example:

var temperature = 88.8
if (temperature > 80) {
println("It's hot")
} else {
println("Today is a nice day")
}

Big Little Decisions
Big Little Decisions

While the decision-making capabilities that we’ve introduced may seem simple, they are the basis for all more complex computer decision making. When you combine lots of tiny decisions, the end result can be astonishing…

CS 124 Honors
CS 124 Honors

Some of you may be interested in CS 199 124, the CS 124 honors section. It’s a fantastic experience! It also has its own website and sources of information. Please use those resources to ask questions about the class. But here are the leads to pitch the course to you:

To sign up to find out more about CS 199 124, use this form. And here are the slides from their presentation show above.

Hack4Impact Product Designer
Hack4Impact Product Designer

Hack4Impact is an organization on campus dedicated to creating software for nonprofits to further their mission for social good. This semester we are looking for more Product Designers to join our team. As a Product Designer, you will work alongside a talented group of peers, using your knowledge and skills to design impactful software for our nonprofit clients.

If you’re interested in applying, head over to h4i.app/ly and submit your application by Saturday, January 21st at 11:59 PM CST. And if you just want to stay in touch, use this form to join our mailing list.

More Practice

Need more practice? Head over to the practice page.