Passing lambdas to functions in Kotlin - Step-by-Step Execution

Concept Flow - Passing lambdas to functions

Define lambda

↓

Define function with lambda parameter

↓

Call function, pass lambda

↓

Inside function: invoke lambda

↓

Use lambda result or side effect

↓

Function ends

This flow shows how a lambda is defined, passed to a function, invoked inside it, and then the function completes.

Execution Sample

Kotlin

fun operateOnNumber(x: Int, op: (Int) -> Int): Int {
    return op(x)
}

val result = operateOnNumber(5) { it * 2 }
println(result)

This code defines a function that takes a number and a lambda, applies the lambda to the number, and prints the result.

Execution Table

Step	Action	Evaluation	Result
1	Call operateOnNumber with x=5 and lambda { it * 2 }	Pass x=5 and lambda	Function starts
2	Inside operateOnNumber, invoke lambda with x=5	lambda(5) = 5 * 2	10
3	Return lambda result 10 from function	Return 10	result = 10
4	Print result	println(10)	Output: 10

💡 Function ends after returning the lambda result and printing it.

Variable Tracker

Variable	Start	After Call	After Lambda Invocation	Final
x	undefined	5	5	5
op (lambda)	undefined	{ it * 2 }	{ it * 2 }	{ it * 2 }
result	undefined	undefined	undefined	10

Key Moments - 3 Insights

Why do we write { it * 2 } outside the parentheses when calling the function?

In Kotlin, if the last parameter of a function is a lambda, you can pass it outside the parentheses. See execution_table step 1 where the lambda is passed this way.

What does 'it' mean inside the lambda?

'it' is the implicit name for the single parameter of the lambda. In step 2, 'it' is 5, the value passed to the lambda.

How does the function get the lambda result?

The function calls op(x) in step 2, which runs the lambda and returns the result 10, then returns it in step 3.

Visual Quiz - 3 Questions

Look at the execution_table step 2, what is the value of 'it' inside the lambda?

A 2

B 10

C 5

D undefined

💡 Hint

Check the 'Evaluation' column in step 2 where lambda(5) = 5 * 2

At which step does the function return the lambda's result?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Action' column in step 3 about returning the lambda result

If we change the lambda to { it + 3 }, what will be printed?

A 15

B 8

C 10

D 5

💡 Hint

Refer to variable_tracker 'result' and how lambda transforms x=5

Concept Snapshot

Passing lambdas to functions in Kotlin:
- Define function with lambda parameter: fun f(x: Int, op: (Int) -> Int)
- Call function passing lambda outside parentheses if last param: f(5) { it * 2 }
- Inside function, invoke lambda: op(x)
- Lambda uses 'it' for single param
- Function returns lambda result

Full Transcript

This example shows how to pass a lambda to a Kotlin function. The function operateOnNumber takes an integer and a lambda that transforms an integer. When called with 5 and the lambda { it * 2 }, the function invokes the lambda with 5, getting 10. It returns 10, which is printed. The lambda parameter 'it' represents the input number. Kotlin allows passing the lambda outside parentheses if it is the last argument. This flow helps beginners see how lambdas are passed, invoked, and results returned.