val test: Int.(String) -> Int = {
plus((this))
}
When defining this type of extension function, how can I use arguments( Here, the argument of type String) inside the block?
When defining extension functions at the same time as declaration like this, can only this be used?
You can access it using it:
val test: Int.(String) -> Int = {
println("this = $this")
println("it = $it")
42
}
fun main() {
println("result = " + 1.test("a"))
}
This will output
this = 1
it = a
result = 42
An alternative is to introduce a lambda-parameter:
val test: Int.(String) -> Int = { s ->
println("this = $this")
println("s = $s")
42
}
fun main() {
println("result = " + 1.test("a"))
}
This will output
this = 1
s = a
result = 42
Related
I wrote a code in kotlin for evaluating a given string from Divison to subtraction, it works but it gives the wrong answer, for example i got a string x in the first line which have answer -215.96666 whereas the answer from code is -237.366666 idk where the error is. i saw more people using stack for doing it, but i wanted to do this way for evaluating a string
var x : String = "5+4-10x20-40/30x20+34/20"
fun div(x: String): String {
val lis1 = x.split("-").toMutableList()
for (ele1 in lis1) {
if ("/" in ele1) {
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+").toMutableList()
for (ele2 in lis2) {
if ("/" in ele2) {
val pos3 = lis2.indexOf(ele2)
val lis3 = ele2.split("x").toMutableList()
for (ele3 in lis3) {
if ("/" in ele3) {
val pos4 = lis3.indexOf(ele3)
val lis4 = ele3.split("/")
val div = (lis4[0].toDouble() / lis4[1].toDouble()).toString()
lis3[pos4] = div
}
}
lis2[pos3] = lis3.joinToString("x")
}
}
lis1[pos1] = lis2.joinToString("+")
}
}
return (lis1.joinToString("-"))
}
fun mul(x : String) : String{
val lis1 = x.split("-").toMutableList()
for (ele1 in lis1) {
if("x" in ele1){
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+").toMutableList()
for(ele2 in lis2){
if("x" in ele2){
val pos2 = lis2.indexOf(ele2)
val lis3 = ele2.split("x")
val mul = (lis3[0].toDouble() * lis3[1].toDouble()).toString()
lis2[pos2] = mul
}
}
lis1[pos1] = lis2.joinToString("+")
}
}
return (lis1.joinToString("-"))
}
fun add(x : String): String {
val lis1 = x.split("-").toMutableList()
for(ele1 in lis1){
if("+" in ele1){
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+")
val add = (lis2[0].toDouble() + lis2[1].toDouble()).toString()
lis1[pos1] = add
}
}
return (lis1.joinToString("-"))
}
fun sub(x : String) : String{
val lis1 = x.split("-").toMutableList()
var sub = 0.0
for(ele1 in lis1){
sub -= ele1.toDouble()
}
return (sub.toString())
}
fun eval(x: String): String {
val divanswer = div(x)
val mulanswer = mul(divanswer)
val addanswer = add(mulanswer)
return sub(addanswer)
}
fun main(args: Array<String>){
println(eval(x))
}
i wanted the answer as -215.96666 but the answer given by code is -237.366666
In the case of this specific input, there are two bugs involved.
First bug is related to the fact you first process + and then -, but in mathematics we should process them together, from left to right. As a result, your algorithm processes this: 1-1+1 (correct answer: 1) as: 1-(1+1) (answer: -1). Alternatively, I believe handling - first and then + should work properly.
Second bug is that in sub() you start from 0 and subtract even the first number, but the first number should be actually added or used as an initial value.
I believe after fixing these two bugs, it should provide a correct answer for this specific input, but there are probably more bugs like these. For example, you incorrectly handle chains of the same operator, you only care about the first pair, so 1+1+1 becomes 2 (and then it is turned to -2 due to earlier mentioned bug).
As a general advice, learn to use a debugger and analyze how your data changes while your code is processing it. Even if you simply print your intermediate results: divanswer, mulanswer and addanswer and then run them through Google for processing, you will notice there is a problem somewhere at the add() step. Then rinse and repeat until everything works correctly.
Thanks to #broot for spotting this, the problem was with the addition and subtraction part, now that i merged them into one and got the correct answers for strings i input, here is the full code corrected.
var x : String = "5+4-10x20-40/30x20+34/20"
fun div(x: String): String {
val lis1 = x.split("-").toMutableList()
for (ele1 in lis1) {
if ("/" in ele1) {
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+").toMutableList()
for (ele2 in lis2) {
if ("/" in ele2) {
val pos3 = lis2.indexOf(ele2)
val lis3 = ele2.split("x").toMutableList()
for (ele3 in lis3) {
if ("/" in ele3) {
val pos4 = lis3.indexOf(ele3)
val lis4 = ele3.split("/")
val div = (lis4[0].toDouble() / lis4[1].toDouble()).toString()
lis3[pos4] = div
}
}
lis2[pos3] = lis3.joinToString("x")
}
}
lis1[pos1] = lis2.joinToString("+")
}
}
return (lis1.joinToString("-"))
}
fun mul(x : String) : String{
val lis1 = x.split("-").toMutableList()
for (ele1 in lis1) {
if("x" in ele1){
val pos1 = lis1.indexOf(ele1)
val lis2 = ele1.split("+").toMutableList()
for(ele2 in lis2){
if("x" in ele2){
val pos2 = lis2.indexOf(ele2)
val lis3 = ele2.split("x")
val mul = (lis3[0].toDouble() * lis3[1].toDouble()).toString()
lis2[pos2] = mul
}
}
lis1[pos1] = lis2.joinToString("+")
}
}
return (lis1.joinToString("-"))
}
fun final(x: String): Double {
val substr = StringBuffer()
var answer = 0.0
var opr = '+'
for(ele in x.indices){
var current = x[ele]
if(current in '0'..'9' || current == '.'){
substr.append(current)
}
if(current == '-' || current == '+'){
var operand = substr.toString().toDouble()
substr.setLength(0)
when (opr){
'+' -> answer += operand
'-' -> answer -= operand
}
opr = current
}
}
val num = substr.toString().toDouble()
when(opr){
'+' -> answer += num
'-' -> answer -= num
}
return answer
}
fun eval(x: String): Double {
val divanswer = div(x)
return final(mul(divanswer))
}
fun main(args: Array<String>){
println(eval(x))
}
I have the following structure at present:
#Entity
#Table(name = "table_app_settings")
data class AppSetting(
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "app_setting_id")
val id: Long? = null,
#Column(name = "app_setting_name")
val name: String = "",
#Column(name = "app_setting_value")
var value: String = "",
#Column(name = "app_setting_type")
val type: AppSettingType,
)
enum class AppSettingType {
CHAR,
STRING,
BYTE,
SHORT,
INT,
LONG,
DOUBLE,
FLOAT,
BOOLEAN,
}
This is then saved to the database with the following:
override fun saveAppSetting(setting: AppSetting): DatabaseResult<AppSetting> {
log.info("Saving App Setting ${setting.name} to database.")
return try {
// Attempt to save the entity to the database. If we do not throw an exception, return success.
val savedSetting = appSettingsRepository.save(setting)
DatabaseResult(
code = ResultCode.CREATION_SUCCESS,
entity = savedSetting
)
} catch(exception: DataAccessException) {
log.error("Unable to save App Setting ${setting.name} to database. Reason: ${exception.message}")
DatabaseResult(
code = ResultCode.CREATION_FAILURE
)
}
}
Now, let's say that I wish to save a Char type to database, I figure I would use the following:
override fun saveAppSetting(name: String, value: Char): DatabaseResult<Char> {
val appSettingResult = saveAppSetting(AppSetting(
name = name,
value = value.toString(),
type = AppSettingType.CHAR,
))
return if(appSettingResult.code != ResultCode.CREATION_FAILURE) {
val entity = getAppSetting<Char>(appSettingResult.entity?.name!!).entity.toString().first()
DatabaseResult(
code = appSettingResult.code,
entity = entity
)
} else {
DatabaseResult(
code = ResultCode.CREATION_FAILURE,
)
}
}
I also figured that I would need to do the following in order to retrieve the correct object type:
override fun getAppSetting(name: String): DatabaseResult<Any?> {
log.info("Getting App Setting $name from database.")
val appSetting = appSettingsRepository.findAppSettingByName(name)
return if(appSetting != null) {
log.info("App Setting $name has ID of ${appSetting.id} within the database")
when(appSetting.type) {
AppSettingType.CHAR -> {
DatabaseResult<Char>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.first(),
)
}
AppSettingType.STRING -> {
DatabaseResult<String>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value,
)
}
AppSettingType.BYTE -> {
DatabaseResult<Byte>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toByte(),
)
}
AppSettingType.SHORT -> {
DatabaseResult<Short>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toShort(),
)
}
AppSettingType.INT -> {
DatabaseResult<Int>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toInt(),
)
}
AppSettingType.LONG -> {
DatabaseResult<Long>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toLong(),
)
}
AppSettingType.DOUBLE -> {
DatabaseResult<Double>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toDouble(),
)
}
AppSettingType.FLOAT -> {
DatabaseResult<Float>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toFloat()
)
}
AppSettingType.BOOLEAN -> {
DatabaseResult<Boolean>(
code = ResultCode.FETCH_SUCCESS,
entity = appSetting.value.toBoolean()
)
}
}
} else {
log.error("App Setting $name does not seem to exist within the database.")
DatabaseResult(
code = ResultCode.FETCH_FAILURE
)
}
However, when I then wish to use said object, I still have to write something like the following:
val newBarcode = getAppSetting("barcode_value").entity.toString().toInt()
Assuming I've "initialised" barcode_value with a value of 177 (for example).
How can I get the function to return what I need without having to do .toString.to...()?
Yes this all possible, here is a simplified demo, firstly
import kotlin.reflect.KClass
data class AppSetting(
val id: Long? = null,
val name: String = "",
var value: String = "",
val type: AppSettingType,
)
enum class AppSettingType(val clazz: KClass<out Any>) {
CHAR(Char::class),
STRING(String::class),
INT(Int::class),
}
So I added a clazz so from the enum we know the Kotlin type
and now a function to simulate your repository fetch
fun findAppSettingByName(name: String): AppSetting? {
return when(name) {
"Char thing" -> AppSetting(value= "C", type = AppSettingType.CHAR)
"String thing" -> AppSetting(value= "Str", type = AppSettingType.STRING)
"Int thing" -> AppSetting(value= "42", type = AppSettingType.INT)
else -> throw IllegalArgumentException()
}
}
Next in the function declaration I have made it generic with T and for the purposes of the demo removed the DatabaseResult container. Then I added a clazz parameter which is the typical Java way of carrying the required class information into the function:
fun <T : Any> getAppSetting(name: String, clazz: KClass<T>): T? {
val appSetting: AppSetting? = findAppSettingByName(name)
return appSetting?.let {
require(clazz == appSetting.type.clazz) {
"appSetting.type=${appSetting.type.clazz} mismatched with requested class=${clazz}"
}
when (appSetting.type) {
AppSettingType.CHAR -> appSetting.value.first()
AppSettingType.STRING -> appSetting.value
AppSettingType.INT -> appSetting.value.toInt()
} as T
}
}
the as T is important to cast the values into the required return type - this is unchecked but the when() clause should be creating the correct types.
Now let's test it:
val c1: Char? = getAppSetting("Char thing", Char::class)
val s1: String? = getAppSetting("String thing", String::class)
val i1: Int? = getAppSetting("Int thing", Int::class)
println("c1=$c1 s1=$s1 i1=$i1")
val c2: Char? = getAppSetting("Char thing")
val s2: String? = getAppSetting("String thing")
val i2: Int? = getAppSetting("Int thing")
println("c2=$c2 s2=$s2 i2=$i2")
}
The output is
c1=C s1=Str i1=42
c2=C s2=Str i2=42
But how do c2/s2/i2 work, the final part is this function
inline fun <reified T : Any> getAppSetting(name: String) = getAppSetting(name, T::class)
This is reified generic parameters... there is no need to pass the clazz because this can be found from the data type of the receiving variable.
There are many articles about this advanced topic, e.g.
https://typealias.com/guides/getting-real-with-reified-type-parameters/
https://medium.com/kotlin-thursdays/introduction-to-kotlin-generics-reified-generic-parameters-7643f53ba513
Now, I didn't completely answer what you wanted because you wanted to receive a DatabaseResult<T> wrapper. What might be possible, is to have a function that returns DatabaseResult<T> and you can obtain the T from it as the "clazz" parameter, but I'll leave that for someone else to improve on :-) but I think that gets you pretty close.
I am learning Kotlin and writing code to check my understanding. I'm trying to use a toString override to print the values of a hashMap that is a property of a class. I can't get it to work. Instead I get output like "kotlin.Unit() -> kotlin.Unit". Also, I don't understand why the values of the hashMap ARE printing out before the toString output. I don't know where that output is coming from. Please help me. Thanks. Below is my code and the output I'm getting.
Code:
package ch07.ExpandoObject
import java.beans.PropertyChangeListener
import java.beans.PropertyChangeSupport
import kotlin.properties.Delegates
import kotlin.reflect.KProperty
class Person(
val name: String = "",
age: Int? = null,
var isMarried: Boolean? = null ,_attributes: kotlin.collections.HashMap<String,String>? = hashMapOf<String, String>()
)
:PropertyChangeAware()
{
var _attributes : kotlin.collections.HashMap<String,String>? = hashMapOf<String, String>()
fun setAttribute(attrName: String, value: String) {
_attributes!!.set(attrName, value)
_attributes!!.set("name", this.name)
}
override fun toString() = "Person(name=\"${name?:""}\", age=${age?:99999}, isMarried=$isMarried) " +
"${_attributes?.get("name")} " + "$name " +
this._attributes!!.forEach { (attrName, value) -> println("$attrName = $value") } +
{
for ((attrName, value) in this._attributes!!) {
println("attribute $attrName = ${this._attributes!![attrName]}")
}
}
val _age = ObservableProperty(propName = "age", propValue = age, changeSupport = changeSupport)
private val observer = {
prop: KProperty<*>, oldValue: Int, newValue: Int ->
changeSupport.firePropertyChange(prop.name, oldValue, newValue)
}
var age: Int by Delegates.observable(initialValue = age?:99999,onChange = observer)
}
class ObservableProperty(val propName: String,
var propValue: Int?, val changeSupport: PropertyChangeSupport
) {
fun getValue(): Int? = propValue
fun setValue( newValue: Int) {
val oldValue = propValue
propValue = newValue
changeSupport.firePropertyChange(propName, oldValue, newValue)
}
}
open class PropertyChangeAware {
val changeSupport = PropertyChangeSupport(this)
fun addPropertyChangeListener(listener: PropertyChangeListener) {
changeSupport.addPropertyChangeListener(listener)
}
fun removePropertyChangeListener(listener: PropertyChangeListener) {
changeSupport.removePropertyChangeListener(listener)
}
}
fun main(args: Array<String>) {
val p = Person("Bob", 89, isMarried = false)
val data = mapOf("lastname" to "Jones", "company" to "JetBrains")
for ((attrName, value) in data)
p.setAttribute(attrName, value)
println(p)
}
Here is the current output:
name = Bob
company = JetBrains
lastname = Jones
Person(name="Bob", age=89, isMarried=false) Bob Bob kotlin.Unit() -> kotlin.Unit
Thanks, again, for any help.
You should not use print() or println() functions inside toString() because they output their arguments to the standard output immediately instead of appending them to the string returned to the caller.
Let's examine the output kotlin.Unit() -> kotlin.Unit you're getting. It consists of two parts:
kotlin.Unit is the string representation of attributes!!.forEach { ... } expression. forEach function returns without value, and in Kotlin it's expressed by returning the Unit object value. Its string representation is appended to the string you're returning.
the second part, () -> kotlin.Unit, is also the string representation of the lambda function expression { for((attrName, value) in ...) }. This function takes no parameters, and returns without value, which means that its type is () -> Unit. Note that in Kotlin the block { ... } declares a local lambda function. If you instead want to run the code inside of that block, use the run function: run { ... }
The goal of toString function is to build the string representation of an object. And for that you can use buildString function:
override fun toString() = buildString {
append("Person(name=\"${name?:""}\", age=${age?:99999}, isMarried=$isMarried) ")
append("${_attributes?.get("name")} ").append("$name ")
this._attributes!!.forEach { (attrName, value) -> append("$attrName = $value") }
for ((attrName, value) in this._attributes!!) {
append("attribute $attrName = ${this._attributes!![attrName]}")
}
}
This function creates a StringBuilder and passes it as a receiver to its functional argument, where you call append or appendln on that receiver. Then buildString converts that string builder to a string and returns it.
I am trying to learn functional Kotlin and have written this test code:
import java.util.*
data class BorrowerX(val name: String, val maxBooks: Int) {
companion object {
fun getName(br: BorrowerX): String = br.name
fun findBorrowerX(n: String, brs: ArrayList<BorrowerX>): BorrowerX? {
val coll: List<BorrowerX> = brs.filter { BorrowerX.getName(it) == n }
if (coll.isEmpty()) {
return null
} else return coll.first()
}
fun findBorrowerX2(n: String, brs: ArrayList<BorrowerX>, f: (BorrowerX) -> String): BorrowerX? {
val coll: List<BorrowerX> = brs.filter { f(it) == n }
if (coll.isEmpty()) {
return null
} else return coll.first()
}
}
}
In the REPL I can successfully call "findBorrowerX":
import BorrowerX
val br1 = BorrowerX(name = "Borrower1", maxBooks = 1)
val br2 = BorrowerX(name = "Borrower2", maxBooks = 2)
val br3 = BorrowerX(name = "Borrower3", maxBooks = 3)
val brs1 = arrayListOf(br1, br2, br3)
BorrowerX.findBorrowerX("Borrower1", brs1)
BorrowerX(name=Borrower1, maxBooks=1)
BorrowerX.findBorrowerX("Borrower-Bad", brs1)
null
But how do I make the call to "findBorrowerX2":
BorrowerX.findBorrowerX2("Borrower1", brs1, BorrowerX.getName(???))
And pass the iterated BorrowerX to getName??
This looks related, but I'm not sure:
Kotlin: how to pass a function as parameter to another?
Thank you in advance for your help with this!
EDIT:
Here is the equivalent Scala code for what I want to do:
def findBorrowerX2(n: String, brs: List[BorrowerX], f: BorrowerX => String): BorrowerX = {
val coll: List[BorrowerX] = brs.filter(f(_) == n)
if (coll.isEmpty) {
null
} else {
coll.head
}
}
scala> BorrowerX.findBorrowerX2("Borrower3", brs1, BorrowerX.getName(_))
res1: BorrowerX = BorrowerX(Borrower3,3)
scala> BorrowerX.findBorrowerX2("Borrower33", brs1, BorrowerX.getName(_))
res2: BorrowerX = null
Perhaps this is not possible in Kotlin?
You can use :: operator to get a function reference:
BorrowerX.findBorrowerX2("Borrower1", brs1, BorrowerX.Companion::getName)
Here BorrowerX.Companion::getName is a reference to the function getName declared in the companion object (named Companion) of the class BorrowerX. It has the type KFunction1<BorrowerX, String> which is a subtype of the required functional parameter type (BorrowerX) -> String.
It's worth noting that you can use :: operator to get a property reference too:
BorrowerX.findBorrowerX2("Borrower1", brs1, BorrowerX::name)
BorrowerX::name has the type KProperty1<BorrowerX, String> which also is a subtype of (BorrowerX) -> String. When invoked with the specified BorrowerX instance it returns the value of its name property.
As stated in the documentation on lambdas:
BorrowerX.findBorrowerX2("Borrower-Bad", brs1, { it.name })
or when the lambda is the last parameter of the method:
BorrowerX.findBorrowerX2("Borrower-Bad", brs1) { it.name }
Stating types and parameter names explicitly often improves readability:
BorrowerX.findBorrowerX2("Borrower-Bad", brs1) { borrower:BorrowerX -> borrower.name }
Let's say I have:
fun addInvoker(adder: () -> Int = ::add): Int{
return adder()
}
fun add(num1:Int = 1, num2:Int = 1): Int{
return num1 + num2
}
I get an error since ::add has two parameters, but the signature of addInvoker requires it to have zero parameters. However, if I change it to:
fun addInvoker(adder: (Int, Int) -> Int = ::add): Int{
return adder()
}
fun add(num1:Int = 1, num2:Int = 1): Int{
return num1 + num2
}
Then I can't invoke adder(), i.e. invoking add with its default arguments.
So, is there some way I can make ::add the default argument to invokeAdder but still invoke add with adder(), thus invoking it with the default args?
You can make a lambda of your add which will be no-argument function and will call add with its default arguments: { add() }.
Complete code:
fun addInvoker(adder: () -> Int = { add() }): Int {
return adder()
}
fun add(num1: Int = 1, num2: Int = 1): Int {
return num1 + num2
}
In Kotlin, functions with default arguments have no special representation in the type system, so the only option is to make wrappers passing only part of arguments to them:
val add0: () -> Int = { add() }
val add1: (Int) -> Int = { add(num1 = it) }
val add2: (Int) -> Int = { add(num2 = it) }