It keyword for single parameter in Kotlin - Step-by-Step Execution

Concept Flow - It keyword for single parameter

Start Lambda

↓

Single Parameter?

Yes↓

Use 'it' as parameter

↓

Execute Lambda Body

↓

Return Result

↓

End

When a lambda has one parameter, Kotlin lets you use 'it' to refer to it without naming it explicitly.

Execution Sample

Kotlin

val numbers = listOf(1, 2, 3)
val doubled = numbers.map { it * 2 }
println(doubled)

This code doubles each number in the list using 'it' as the single lambda parameter.

Execution Table

Step	Lambda Input (it)	Operation	Result	Output List State
1	1	1 * 2	2	[2]
2	2	2 * 2	4	[2, 4]
3	3	3 * 2	6	[2, 4, 6]
4	-	End of list	-	[2, 4, 6]

💡 All elements processed, lambda applied to each using 'it', resulting list created.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
it	-	1	2	3	-
doubled	[]	[2]	[2, 4]	[2, 4, 6]	[2, 4, 6]

Key Moments - 3 Insights

Why can we use 'it' without declaring a parameter name?

Because the lambda has exactly one parameter, Kotlin automatically provides 'it' as its name (see execution_table steps 1-3).

What if the lambda had more than one parameter?

Then you must name the parameters explicitly; 'it' only works for single-parameter lambdas (not shown in this example).

Is 'it' a keyword or a variable?

'it' is an implicit name Kotlin gives to the single parameter inside the lambda, acting like a variable you can use directly.

Visual Quiz - 3 Questions

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

A 2

B 1

C 3

D 4

💡 Hint

Check the 'Lambda Input (it)' column at step 2 in the execution_table.

At which step does the output list first contain [2, 4]?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Output List State' column in the execution_table.

If the lambda had two parameters, could we still use 'it'?

A Yes, 'it' always works

B No, 'it' only works for one parameter

C Only if we rename 'it'

D Only if we use a different keyword

💡 Hint

Refer to the second key_moment about multiple parameters.

Concept Snapshot

Kotlin lambdas with one parameter can use 'it' implicitly.
No need to name the parameter explicitly.
Use 'it' inside the lambda body to refer to the parameter.
Example: list.map { it * 2 } doubles each element.
If more than one parameter, 'it' cannot be used.

Full Transcript

This example shows how Kotlin lets you use 'it' as a shortcut for the single parameter in a lambda. The code doubles each number in a list by multiplying 'it' by 2. The execution table traces each step: the input value 'it', the operation, the result, and how the output list grows. Beginners often wonder why 'it' works without declaration; it's because Kotlin automatically names the single parameter 'it'. If there were more parameters, you would have to name them explicitly. This visual helps you see how 'it' changes with each element and how the final list is built.