Kotlin cheat sheet for white board problem solving

How to execute Kotlin code

kotlinc KotlinFileName.kt -include-runtime -d output.jar
java -jar output.jar

Loop

for (item in collection) { print(item) }
for (i in 1..5) print(i) = 12345
for (i in 1 until 5) { print(i) } = 1234
for (i in 5..1) print(i) = 
for (i in 5 downTo 1) print(i) = 54321
for (i in 1..4 step 2) print(i) = 135
for (i in 4 downTo 1 step 2) print(i) = 531
for (index in collection.indices) { println("index = $index, element = ${collection[index]}") }
for ((index, element) in collection.withIndex()) { println("index = $index, element = $element") }
collection.forEach { element -> println(element) }
collection.forEachIndexed{index, element -> println("index = $index, element = $element") }

Main Method

fun main(args: Array<String>) {
    // body
}

fun main() {
    // body
}

String

String can be treated as a collection

val text = "kotlin"
for (letter in text) { println(letter) }
for (item in text.indices) { println(text[item]) }

Length

text.length

Substring

fun String.substring(startIndex: Int): String
fun String.substring(startIndex: Int, endIndex: Int): String // exclusive of endIndex

Print collection

collection.joinToString()
collection.joinToString(separator = "DILIMITER")

Basic Types

val a: Int = 10000
val d: Double = 100.00
val f: Float = 100.00f
val l: Long = 1000000004
val l = 1L
val letter: Char = 'A' 
val b: Boolean = true
val s: String = "hello"

IntArray

• Arrays of primitive types without boxing overhead
• no inheritance relation to the Array class but same set of methods and properties

val x: IntArray = intArrayOf(1, 2, 3)
val arr = IntArray(5) // [0, 0, 0, 0, 0]
val arr = IntArray(5) { 42 } // [42, 42, 42, 42, 42]
var arr = IntArray(5) { it * 1 } // [0, 1, 2, 3, 4]

x.size // size property
arr[index] = value
fun IntArray.sortedArray(): IntArray // stable sorting

Notes

• Collection which includes list, set, and map are resizeable.
• IntArray stores primitive and unboxed items. Array<Int> and collection stores boxed types.

To-the-power-of operator

Manual

fun pow(base: Int, exponent: Int): Long {
    var result: Long = 1

    while (exponent != 0) {
        result *= base.toLong()
        --exponent
    }

    return result
}

pow() from Math class

Math.pow(base.toDouble(), exponent.toDouble())

Pre and post increment

++a increments and then returns the variable
a++ returns and then increments the variable
var a = 1
println(a++); //You will see 1
//Now a is 2
println(++a); //You will see 3

Structural Jump Expressions (return, break, and continue)

• return - returns from the nearest enclosing function or anonymous function
• break - terminates the nearest enclosing loop
• continue - skip remaining and continue with next step or proceeds to the next step of the nearest enclosing loop

here@ for (i in 1..5) {
    for (j in 1..4) {
        if (i == 3 || j == 2)
            continue@here
        println("i = $i; j = $j")
    }
}

Map<K, V>

• Doesn’t inherit Collection interface but is Kotlin Collection type.
• Stores key-value pairs. Keys are unique.

Map (immutable)

// creates an immutable map without write operations
val numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)

println(numbersMap.get("1"))
println(numbersMap["key1"])
println(numbersMap.getOrDefault("key5", 0)) // return default if key not found

println("${numbersMap.keys}") // [key1, key2, key3, key4]
println("${numbersMap.values}") // [1, 2, 3, 1]

if ("key2" in numbersMap) // return true if key2 is found
if (1 in numbersMap.values) // returns true if value 1 is in the map
if (numbersMap.containsKey(key)) // same as previous
if (numbersMap.containsValue(1)) // same as previous
for ((k, v) in numbersMap) // returns the pair if found
println("Entries: " + numbersMap.entries) // Entries: [key1=1, key2=2, key3=3, keys4=1]
println("Keys:" + numbersMap.keys) // Keys:[key1, key2, key3, key4]
println("Values:" + numbersMap.values) // Values:[1, 2, 3, 1]

MutableMap (Mutable or with write operation)

// creates an immutable map without write operations
val numbersMap = mutableMapOf("key1" to 1, "key2" to 2, "key3" to 3)
numbersMap.put("key4", 1)
numbersMap["key4"] = 1
numbersMap.remove("key1") // remove a pair
numbersMap.clear() // remove all pair

HashMap

• Default implementation is LinkedHashMap which preserves insertion order
• HashMap - doesn’t say anything about the order

val numbersMap = HashMap<String, Int>()
items["A"] = 90

Size operations

numbersMap.count
items.count { it.value > 10 } // count values greater than 10
numbersMap.isEmpty() // check if it is empty

Array

• Arrays (Array<T>) in Kotlin has a fixed size. It can’t be resized after initialization.

// Initialize
val arr = arrayOf(1, 2, 3) // [1, 2, 3]
val arr = Array(3) {index -> index} // [1, 2, 3]
// Length
val size = arr.size
// Getter
val num = arr[0]
// Setter
arr[0] = 5
// Print
println(arr.joinToString(separator = "; ")) // 1; 2; 3
println(arr.contentToString()) // [1, 2, 3]

Scope functions

Function	Object reference	Return value	is extension function
let	it	lambda result	yes
run	this	lambda result	yes
run	-	lambda result	No: called without the context object
with	this	Lambda result	No: takes the context object as an argument
apply	this	Context object	Yes
also	it	Context object	Yes

Don’t abuse Kotlin’s scope functions