Kotlin Programming 2022 Spring - Lecture 5
===
###### tags: `Kotlin 2022 Spring` `Kotlin` `2022 Spring`

Roughly cover the materials in Chapter 9.

## Scope Functions

### [`apply`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/apply.html)

import java.io.File

fun main() {
    val readOnlyDir = File("readonly")
    readOnlyDir.mkdir()
    readOnlyDir.setReadable(true)
    readOnlyDir.setWritable(false)
    readOnlyDir.setExecutable(false)
}

is equivalent to

import java.io.File

fun main() {
    val readOnlyDir = File("readonly").apply { 
        // this === File("readonly")
        mkdir() // this.mkdir()
        setReadable(true)
        setWritable(false)
        setExecutable(false)
    }
}

+ `apply` returns the "receiver"
+ `File("readonly")`

### [`let`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/let.html)

fun main() {
    val inputLine = readLine()!!
    val inputIntSquare = inputLine.toInt() * inputLine.toInt()
    println("The square of input is $inputIntSquare.")
}

is can be replaced by

fun main() {
    val inputLine = readLine()!!
    val inputIntSquare = inputLine.toInt().let{
        // it === inputLine.toInt()
        it * it
    }
    println("The square of input is $inputIntSquare.")
}

+ `let` returns the return value of the anonymous function.

### [`run`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/run.html)

fun main() {
    val score = readLine()?.toIntOrNull() ?: 0
    val status = run {
        if (score >= 60) "P" else "F"
    }
    println("Score: $score, $status.")
}

+ `run` returns the return value of the anonymous function.

### [`T.run`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/run.html)

fun main() {
    val score = readLine()?.toIntOrNull() ?: 0
    val status = score.run {
        // this === score
        if (this >= 60) "P" else "F"
    }
    println("Score: $score, $status.")
}

+ Like `apply` but returns the result of the anonymous function.
+ Like `let` but use `this` rather than `it`.

fun main() {
    val inputLine = readLine()!!
    val inputIntSquare = inputLine.toInt().run{
        // this === inputLine.toInt()
        this * this
    }
    println("The square of input is $inputIntSquare.")
}

### `with`

fun main() {
    val score = readLine()?.toIntOrNull() ?: 0
    // val status = score.run {
    val status = with(score) {
        // this === score
        if (this >= 60) "P" else "F"
    }
    println("Score: $score, $status.")
}

+ Similar to `T.run`

### `also`

+ Like `apply` but uses `it` rather than `this`.
+ Like `let` but returns the "receiver"
+ Special usage: swap

fun main() {
    var a = 5
    var b = 10
    println("Before swap: a=$a, b=$b")
    a = b.also {b = a}
    println("After swap: a=$a, b=$b")
}

### Function Selection

Please refer to the [official document](https://kotlinlang.org/docs/reference/scope-functions.html#function-selection).

### `takeIf` and `takeUnless`

fun main() {
    val x = "456".takeIf{it.length % 2 == 1}?.toInt()
    val y = "456".takeUnless{it.length % 2 == 1}?.toInt()
    println("x is $x, and y is $y.")
}