Let function behavior and use cases in Kotlin - Step-by-Step Execution

Concept Flow - Let function behavior and use cases

Object or Value

↓

Call let function

↓

Pass object as 'it' to lambda

↓

Execute lambda block

↓

Return lambda result

↓

Use returned value or chain calls

The let function takes an object, passes it as 'it' to a lambda, executes the lambda, and returns the lambda's result.

Execution Sample

Kotlin

val name: String? = "Alice"
val length = name?.let {
    println("Name is $it")
    it.length
}
println(length)

This code uses let to safely print a nullable string and get its length if not null.

Execution Table

Step	Variable 'name'	Condition	Action	Output	Variable 'length'
1	"Alice"	name != null	Call let with 'Alice'	Prints: Name is Alice	Not set yet
2	"Alice"	Inside let lambda	Calculate it.length (5)	No output	Not set yet
3	"Alice"	End of let	Return 5 from let	No output	Set length = 5
4	"Alice"	After let	Print length	Prints: 5	5

💡 Execution stops after printing length; let returns lambda result or null if receiver is null.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
name	null or "Alice"	"Alice"	"Alice"	"Alice"	"Alice"
length	undefined	undefined	undefined	5	5

Key Moments - 3 Insights

Why does the lambda inside let use 'it' instead of the variable name?

Inside the let function, the object is passed as 'it' by default to the lambda, so you use 'it' to refer to the object (see execution_table step 1 and 2).

What happens if the variable is null before calling let?

If the variable is null, let is not called and the lambda is skipped, so the result is null (not shown in this trace but implied by the condition in step 1).

Why do we assign the result of let to a variable?

Because let returns the result of the lambda, you can assign it to a variable to use the transformed or computed value later (see execution_table step 3 and 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'length' after step 3?

A undefined

B 5

C null

D "Alice"

💡 Hint

Check the 'Variable length' column at step 3 in the execution_table.

At which step does the program print "Name is Alice"?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Output' column in the execution_table for the print statements.

If 'name' was null, what would happen to the 'length' variable?

A It would be null

B It would be "null" string

C It would be set to 0

D It would cause an error

💡 Hint

Recall that let is only called if the variable is not null; otherwise, the result is null.

Concept Snapshot

let function syntax:
object?.let { it -> /* use it */ }

Behavior:
- Executes lambda only if object is not null
- Passes object as 'it' inside lambda
- Returns lambda result or null

Use cases:
- Safe calls on nullable objects
- Scoping and chaining transformations

Full Transcript

The let function in Kotlin is used to execute a block of code only if an object is not null. It passes the object as 'it' to a lambda, runs the lambda, and returns the lambda's result. In the example, a nullable string 'name' is checked. If not null, let prints the name and returns its length. The length is then printed. If 'name' were null, let would skip the lambda and return null. This helps safely work with nullable values and chain operations.