Values and variables

What is a variable?

A variable is a storage for a value, which can be a string, a number, or something else. Every variable has a name (or an identifier) to distinguish it from other variables. You can access a value by the name of the variable.

Variables are one of the most often used elements in programs; therefore, it is important to understand how to use them.

Declaring variables

Before you can start using a variable, you must declare it. To declare a variable, Kotlin provides two keywords:

• val (for value) declares an immutable variable (just a named value or a constant), which cannot be changed after it has been initialized (this is actually not entirely true, we will discuss this issue in more detail later);
• var (for variable) declares a mutable variable, which can be changed (as many times as needed).

Both val and var keywords provide you a variable!

When you declare a variable, you must add its name after one of these two keywords. Be careful: the name of a variable cannot start with a digit. Usually, it starts with a letter. You should choose meaningful and readable names for variables to make your code easy to understand.

To assign a certain value to a variable, we should use the assignment operator =.

Let’s declare an immutable variable named language and initialize it with the string "Kotlin".

val language = "Kotlin"

Now we can access this string by the variable’s name. Let’s print it!

println(language) // prints "Kotlin" without quotes

This variable cannot be modified after it has been initialized because it was declared as val.

Names are case-sensitive: language is not the same as Language.

Now, let’s declare a mutable variable named dayOfWeek and print its value before and after changing it.

var dayOfWeek = "Monday"
println(dayOfWeek) // prints Monday

dayOfWeek = "Tuesday"
println(dayOfWeek) // prints Tuesday

In the example above, we declared a variable named dayOfWeek and initialized it with the value "Monday". Then we accessed the value by the variable name and printed it. After that, we changed the variable’s value to "Tuesday" and printed this new value.

You do not need to declare a variable again to change its value. Just assign a new value to it using the = operator.

It is also possible to declare and initialize a variable with the value of another variable:

val cost = 3
val costOfCoffee = cost
println(costOfCoffee) // prints 3

Storing different types of values

We’ve already mentioned that variables can store different types of values: strings, numbers, characters, and other data types, which we will encounter later.

Let’s declare three immutable variables to store a number, a string, and a character and then print their values.

val ten = 10
val greeting = "Hello"
val firstLetter = 'A'

println(ten) // prints 10
println(greeting) // prints Hello
println(firstLetter) // prints A

There is one restriction for mutable variables (the ones declared with the keyword var), though. When reassigning their values, you can only use new values of the same type as the initial one. So, the piece of code below is not correct:

var number = 10
number = 11 // ok
number = "twelve" // an error here!

Please remember this restriction!

Conclusion

Now you know, there are two keywords that are used to declare variables. Actually, in many cases, it is better to use immutable variables (those declared with the keyword val). Before using var, you should make sure that val is not suitable in that case. If it really isn’t, use var. Although, keep in mind that the more mutable variables in your code, the harder it is to read. Remember, immutable variables help write more readable code. So, please, use val whenever possible! You can easily replace it with var when you absolutely have to.