KotlinJava

# Conditional Expressions and Statements

var temperature = 83.8
var isHot = temperature > 80
if (isHot) {
println("It's hot!")
}

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.

## Conditional ExpressionsConditional 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

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:

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 >

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 TestingAssignment 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 StatementsConditional 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")
}

## BlocksBlocks

`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")
}

## Practice: Simple Conditional

Created By: Geoffrey Challen
/ Version: 2020.8.0

Computers can make decisions based on data. In Kotlin we do this using conditional statements. While they may seem simple, conditional statements are the building block of every more complex computer behavior.

Let's get some practice with real world decision making using conditional statements. The University of Illinois is using regular frequent testing to monitor the coronavirus on campus. Maybe the most important piece of data produced by coronavirus testing is the positivity rate: the percent of tests that come back positive. A high positivity rate indicates community spread, and may mean that it is time to shut down the campus again.

Your friend is fairly unconcerned about the coronavirus. They wrote the following snippet of code that uses a 10% positivity rate to decide whether to continue an in-person semester. You think that is too high. (10% is fairly high.) Modify the code below to use 5% as the positivity rate threshold. When you are correct, your code should print "Time to go home" if the value of the variable `positivity` is greater than 5%. Do not modify the value of `positivity`.

if (positivity > 0.10) {
println("Time to go home")
} else {
println("On-campus activities can continue")
}

## Big Little DecisionsBig 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…

## Course Design: Online SupportCourse Design: Online Support

CS 124 is designed to give every student a chance to succeed. Below we explain one unusual aspect of the course. You can find more information on the syllabus.

CS 124 focuses on providing online tutoring. We do also run in-person tutoring sessions, but place more emphasis and staff time on supporting our online tutoring platform.

Our reasoning here is simple: We want to be there whenever you have a question! Due to our daily lesson format, we know that students are engaging with material at all hours throughout the day. This way, whenever you have a question, or a small doubt, or are curious about something, or confused, or frustrated, or need a bit of guidance—we’re always a click away. No need to wait hours or days for the next set of office hours. No need to trudge across campus into some basement or stand in line waiting to cram into some tiny office.

We will provide a variety of opportunities for students to connect with each other and with course staff, in both academic and social settings. But we also want to make it as easy as possible for you to ask questions and access support—wherever, whenever.

For an already initialized `Int` variable `friendsCount`, if a user has more than 500 friends, print the message "Adopt a Dog Today!" and otherwise print "Buy Cat Food At 20% Off".