notes.md

Kotlin

Created: Mar 22, 2020 6:19 PM Updated: Apr 06, 2020 8:36 PM

joinToString and IntArray and repeat

import java.util.*

fun main(args: Array<String>) {
    val scanner = Scanner(System.`in`)
    // put your code here
    val grades = IntArray(4)
    val n = scanner.nextInt()
    repeat(n) {
        grades[scanner.nextInt() - 2]++
    }
    println(grades.joinToString(separator = " "))
}

Ranges

4 downTo 1 // reverse
1..4 // inclusive
1 until 4 // excluding upper limit
i in 1..7 step 2 // just odds

Iterating Through Arrays

fun main(args: Array<String>) {
    val daysOfWeek = arrayOf("Sun", "Mon", "Tues", "Wed", "Thur", "Fri", "Sat")

    for (index in daysOfWeek.indices){
        println("$index: ${daysOfWeek[index]}")
    }

        for (i in 0..array.lastIndex) {
        array[i] = scanner.nextInt()
    }
}

MutableMap

// Does not keep natural ordering
val marathon = mutableMapOf(1 to "John Doe", 2 to "Marty McFly", 3 to "Marie Curie")
val marathon = mutableMapOf<Int, String>()
marathon.clear()
print(marathon.containsKey(2))
print(marathon.containsValue("John Doe"))
marathon.size
marathon.isEmpty()

Functional Programming and Lambdas

val originalText = "I don't know... what to say..."
val textWithoutDots = originalText.filter { it != '.' }

Reading into Files

val fileName = "src/reading.txt"
val file = File(fileName)
val lines = file.readText()
print(lines)

val fileName = "src/reading.txt"
val lines = File(fileName).readLines()
for (line in lines){
    println(line)
}

val lines = File(fileName).readBytes()

// Lambda approach to read large files
val fileName = "src/reading.txt"
File(fileName).forEachLine { print(it) }

// Charset
val line = File(fileName).readText(Charsets.US_ASCII)

Data Class

Removes the need for hashCode() , equals(), toString() , and copy() implementations

data class Client(val name: String, val age: Int, val gender: String)

But you can still override all of them except copy()

data class Client(val name: String, val age: Int, val gender: String) {
    var balance: Int = 0

    override fun toString(): String {
        return "Client(name='$name', age=$age, gender='$gender', balance=$balance)"
    }

}
// Copying objects with different parameters
fun main() {
    val bob = Client("Bob", 29, "Male")
    val john = bob.copy(name = "John")
    println(bob)
    println(john)
}