Python to Kotlin

Variable Assignment

This is basic variable assignment. In Kotlin we have mutable, where a variable can be changed. Immutable is where a variable cannot be changed. This is a compiler check, and not enforced in the JVM.

user = "Bob"

//Mutable 
var user = "Bob"
//Immutable
val user = "Bob"

Lists

numbers = [1, 2, 3, 4, 5, 6]

//Immuatble List
val numbers = listOf(1, 2, 3, 4, 5, 6)
//Mutable List
var numbers = mutableListOf(1, 2, 3, 4, 5, 6)

Adding Items to List

numbers.append(7)
numbers += 7

numbers.add(7)

List Comprehension

>>> [n * 2
    for n in [1, 2, 3, 4, 5, 6]]
[2, 4, 6, 8, 10, 12]

numbers.map { it * 2 }

Filter List

>>> filter(lambda x: (x % 2) == 0, numbers)
[2, 4, 6]

numbers.filter { (it % 2) == 0 }

List Iteration

>>> for n in numbers:
...     print(n)

//Extension Loop Method
numbers.forEach { 
    println(it)
}
//Control Syntax
for (item in numbers) print(item)

Loop with Index

for i, num in enumerate(numbers):
    print(i, num)

numbers.forEachIndexed { index, i ->
    println("$index $i")
}

String Formatting

>>> "My Name is {} and I like {}".format("Bob", "Pie")
'My Name is Bob and I like Pie'
>>> "My Name is {name} and I like {food}".format(name="Jane", food="Cake")
'My Name is Jane and I like Cake'

val food = "Pie",
val name = "Bob"
//Standard String
val message = "My Name is $name and I like $food"
//With nested calls
val message = "My Name is $name and I Like ${food.toUpperCase()}"

Function Definition

>>> def add(x, y):
...     return x + y
... 
>>> add(1, 2)
3

fun add(x:Int, y:Int) : Int = x +y 

fun add(x:Int, y:Int) : Int {
  return x +y  
} 

Lambda

multiply_by_two = lambda x: x * 2

// Explicit parameter declaration
multiplyByTwo = { x -> x * 2}
// Implicit parameter declaration
multiplyByTwo = { it * 2 }

Partial Application

from functools import partial
from collections import namedtuple
from time import time

metric = namedtuple("Metric",
    ["value", "timestamp", "metric", "unit"])

loaded_metric = partial(metric, metric="Weight", unit="lb")

current_weight = loaded_metric(value=179, timestamp=time())

import kotlin.js.Date

data class Metric(
    val value : Number,
    val timestamp : Date,
    val metric : String,
    val unit : String
)

val loadedMetric = { value, timestamp -> 
    Metric(value = value, timestamp = timestamp, metric="Weight", unit="lb")
}

loadedMetric(179, Date.now())

Ternary Conditions

1 if True is True else False

// Standard Ternary
val x = if (true is true) 1 else 0 
// We can also support more complex
val number = if (x < 10) {
    1
} else if ( x < 20) {
    10
} else if ( x < 30) {
    20
} else {
    0
}

None/Null Checking

if x is not None:
    return 1

// Standard If Statement
if (x != null) 1
// If x is null default to 1
val rsp = x ?: 1 

Dictionary Checking

map = {
    "one": 1,
    "two": 2
}

if "one" in map:
    return map["one"]
    
map.get("one", 0)

val map = mapOf("one" to 1, "two" to 2)
//If conditional approach
if ("one" in map) map["one"] else 0
//Takes if key present if not return default of 0
map.takeIf { it.contains("one") }?.get("one") ?: 0
//Get with a default
map.getOrElse("one", 0)
//When Statement
when {
    "one" in map -> map["one"]
    else -> 0
}

Pattern Matching Statement

Not supported

...

enum class HTTPVerbs {
    GET, POST, PUT, UPDATE, DELETE
}
//Explicit for each cast
fun test(verb: HTTPVerbs) {
    when(verb) {
        HTTPVerbs.GET -> TODO()
        HTTPVerbs.POST -> TODO()
        HTTPVerbs.PUT -> TODO()
        HTTPVerbs.UPDATE -> TODO()
        HTTPVerbs.DELETE -> TODO()
    }
}
//Chaining conditions together
fun test(verb: HTTPVerbs) {
    when(verb) {
        HTTPVerbs.GET, HTTPVerbs.POST -> TODO()
        HTTPVerbs.UPDATE, HTTPVerbs.DELETE -> TODO()
        HTTPVerbs.PUT -> TODO()
    }
}