Kotlin - Destructuring Declarations

Using underscores for unused variables

In a destructuring declaration, we can use an underscore in place of a non-required variable. If you don’t need the value of one or more variables, an underscore can be used in the place of that variable. For example,

// Creating rectangleData using the data class
data class rectangleData(val length:Int,val width:Int)

// This function returns two values
fun createRectangle():rectangleData{
    return rectangleData(100,50)
}

fun main(){
    // Calling createRectangle() function to get the parameters of rectangle
    var rectangle = createRectangle()
    // Using the component function to get the values of length and width
    val (_,width) = rectangle
    // printing the value of width
    println("Width of the rectangle: "+ width)
}

Output:

Width of the rectangle: 50

In the above example, we used an underscore in place of the length variable. Here, we only needed the width of the rectangle, so we used an underscore to ignore the value of the length variable.

Using Lambda Parameters with Destructuring Declaration

We can use the lambda functions with destructuring declarations to make some in place changes. Lamba is a function without a name, and it is defined using curly braces {}. Inside the curly braces, we first pass the value of variables, followed by an arrow(->) operator. After the arrow operator, we write the body of the function. 

Syntax of Lambda

{variables -> function_body}

In the example below, we will see how we can change the value of a map using destructuring declarations with a lambda function:

fun main(){
    // Initialising a map from int to int
    val squares = mutableMapOf<Int,Int>()
    squares.put(1,1)
    squares.put(2,4)
    squares.put(3,9)
    squares.put(4,16)
    squares.put(5,25)
    print("Old map is: ")
    println(squares)
    // Changing the key value
    val cubes = squares.mapValues { (key,value) -> value*key }
    print("New Map: ")
    println(cubes)
}

Output:

Old map is: {1=1, 2=4, 3=9, 4=16, 5=25}
New Map: {1=1, 2=8, 3=27, 4=64, 5=125}

In this example, we have used both key and value. In case you don’t need one or both of these, you can use an underscore in place of them. For example,

fun main(){
    // Initialising a map from int to int
    val squares = mutableMapOf<Int,Int>()
    squares.put(1,1)
    squares.put(2,4)
    squares.put(3,9)
    squares.put(4,16)
    squares.put(5,25)
    print("Old map is: ")
    println(squares)
    // Using _ in place of key
    val square_2 = squares.mapValues { (_,value) -> value*value }
    print("New Map: ")
    println(square_2)

    // Using _ in place of both keys and values
    val unit_map = square_2.mapValues { (_,_) -> 1 }
    print("Unit Map: ")
    println(unit_map)
}

Output:

Old map is: {1=1, 2=4, 3=9, 4=16, 5=25}
New Map: {1=1, 2=16, 3=81, 4=256, 5=625}
Unit Map: {1=1, 2=1, 3=1, 4=1, 5=1}

As we can see from the above example, an underscore can be used in place of either key or value or both. 

FAQs

1. How is a data class different from a class in Kotlin?
   Data class is a class that only contains a state. It doesn’t perform any operation. It only holds the data.
    
2. How are lambda expressions different from normal functions?
   A lambda expression is just a function without a name. It is written at the place it needs to be used.
    
3. Can we use an underscore at more than one place in a destructuring declaration?
   Yes, an underscore can be used at any number of places in a destructuring declaration. It will simply ignore the value of all the fields marked as underscores.

Key Takeaways  

Cheers if you reached here!!!

In this blog, we discussed Destructuring Declarations in Kotlin. Additionally, we looked at lambda expressions. We saw how lambdas and underscores could be used with Destructuring Declarations.

After reading this blog, I believe you will work with Destructuring Declarations smoothly. If you wish to learn about Kotlin functions in detail, you can check out our other blog on Kotlin Functions. And to learn in-depth about android development, check out our Android Development course on the Coding Ninjas website.