Lambda

Learn lambda expressions with { x -> x * 2 }.

Lambda

A lambda is a function without a name. It is written as { x -> x * 2 }; the parameter and body sit inside curly braces.

Example: val double: (Int) -> Int = { x -> x * 2 }. This lambda takes an Int and returns twice its value. Calling double(5) returns 10. Lambdas can be assigned to variables, passed as parameters or used in collection methods. When the lambda is the last parameter, Kotlin lets you place it outside the parentheses.

Lambda call

After a lambda is assigned to a variable, it is called by the variable name. The invoke method also works.

Example: val double: (Int) -> Int = { x -> x * 2 }. Calling double(5) or double.invoke(5) returns 10. The lambda type is (Int) -> Int; parameter and return types are explicit. Lambdas are ideal for short one-line helpers. For long bodies, a fun declaration is preferred.

Exercise The double lambda

Declare val double: (Int) -> Int = { x -> x * 2 }. In main call println(double(5)).

Show example solution

val double: (Int) -> Int = { x -> x * 2 }

fun main() {
    println(double(5))
}

Test yourself

Match the lambda parts

{ x -> ... }

Lambda syntax
x

Parameter
x * 2

Body
double(5)

Call
How is a <strong>lambda</strong> called?
- Only with invoke
- By the variable name, in parentheses
- Only with fun
- With return
Show answer

By the variable name, in parentheses — A lambda is called by the variable name with the argument in parentheses.

Complete the lambda definition

val double: (Int) -> Int = { ____ -> x * 2 }

Show answer

val double: (Int) -> Int = { x -> x * 2 }

Next lesson: it

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