I have an object media that holds descriptions which is a list. I'd love to see some elegant logic in Kotlin to add an element to that descriptions if the field is not null or add a fresh new list (with an initial element) to that field if it is null.
Pseudo:
if (media.descriptions == null) { media.descriptions = listOf("myValue")}
else { media.descriptions.add("myValue") }
I would probably do it the other way around, except you need to alter media itself (see below), i.e. creating your list first and add all the other entries to that list if media.descriptions isn't null:
val myDescriptions = mutableListOf("myValue") // and maybe others
media.descriptions?.forEach(myDescriptions::add)
If you need to manipulate descriptions of media, there is not so much you can do to make it more readable...:
if (media.descriptions == null) {
media.descriptions = mutableListOf("myValue") // just using mutable to make it clear
} else {
media.descriptions += "myValue"
}
or maybe:
if (media.descriptions == null) {
media.descriptions = mutableListOf<String>()
}
media.descriptions?.add("myValue")
You can use the elvis ?: operator to assign the list.
The simplest way I can think of is
media.descriptions = media.descriptions ?: listOf("myValue")
media.descriptions.add("myValue")
Related
I have some code which looks like this, where param is of a data class type:
val options = if (param.language == null) {
param.copy(language = default())
} else {
param
}
Now, however, the language object has been moved into a hierarchy of nullable objects, so the check must look like this:
if (param.subObj?.nextObj?.language == null) { ... }
How do I use the copy idiom in this case?
One way to do this is:
val newParam = when {
param.subObj == null -> param.copy(subObj = SubObj(nextObj = NextObj(language = Language())))
param.subObj.nextObj == null -> param.copy(subObj = param.subObj.copy(nextObj = NextObj(language = Language())))
param.subObj.nextObj.language == null -> param.copy(subObj = param.subObj.copy(nextObj = param.subObj.nextObj.copy(language = Language())))
else -> param
}
I agree that this doesn't look very clean but this seems to be the only way to me, because at each step you need to check if the current property is null or not. If it is null, you need to use the default instance otherwise you need to make a copy.
Could you do something like this?
// you could create a DefaultCopyable interface if you like
data class SubObj(val prop1: Double? = null, val nextObj: NextObj? = null) {
fun copyWithDefaults() =
copy(prop1 = prop1 ?: 1.0, nextObj = nextObj?.copyWithDefaults() ?: NextObj())
}
data class NextObj(val name: String? = null) {
fun copyWithDefaults() = copy(name = name ?: "Hi")
}
I think you need a special function because you're not using the standard copy functionality exactly, you need some custom logic to define defaults for each class. But by putting that function in each of your classes, they all know how to copy themselves, and each copy function that works with other types can just call their default-copy functions.
The problem there though is:
fun main() {
val thing = SubObj(3.0)
val newThing = thing.copyWithDefaults()
println("$thing\n$newThing")
}
> SubObj(prop1=3.0, nextObj=null)
> SubObj(prop1=3.0, nextObj=NextObj(name=null))
Because nextObj was null in SubObj, it has to create one instead of copying it. But the real default value for name is null - it doesn't know how to instantiate one with the other defaults, that's an internal detail of NextObj. You could always call NextObj().copyWithDefaults() but that starts to look like a code smell to me - why isn't the default value for the parameter the actual default value you want? (There are probably good reasons, but it might mean there's a better way to architect what you're up to)
Good afternoon, I want to make a universal sort for all tables. The idea is that the method will receive the name of the column as input and, through reflection, I will receive a link to the field of the same name.
val id = "id"
var a = JobSeekerTable::class
a.memberProperties.forEach { e ->
if (e.name == id) {
transaction {
JobSeeker.all().sortedBy { e.getter }
}
}
}
Unfortunately, this does not work. There was an option, through the fields field that the table has
JobSeekerTable.fields.forEach {v->
transaction {
JobSeeker.all().sortedBy { v }
}
}
but also unsuccessfully :(
If there is any way to refer to the required field through the name. Not using if and stuff like that?
First, you are probably looking for orderBy, not sortedBy. The former is to order SQL query results, the later is to sort a collection.
Second, you want to pass an instance of a column:
val id = "id"
JobSeekerTable.selectAll().orderBy(JobSeekerTable.columns.find {
it.name == id // Here I used the name you provided, although probably it should be named something like columnName
} !! to SortOrder.ASC)
Using "screaming" operator (!!) in Kotlin is a bad practice. So if all of your tables have ID column, for example, you can use "elvis" operator instead.
JobSeekerTable.selectAll().orderBy((JobSeekerTable.columns.find {
it.name == id
} ?: JobSeekerTable.id) to SortOrder.ASC)
I'm making my first app in Kotlin and there is a lot of syntax I don't know, and I was wondering if there is a better way to check if a list contains at least one non null entry.
For now my solution is:
var atLeastOneValue: Boolean
var i = 0
for (x in list) {
if (x != null) atLeastOneValue = true
else i++
}
if (list.size == i) atLeastOneValue = false
return atLeastOneValue
I'm working with MutableList<String>.
You can use contains function for that:
val hasNull = list.contains(null)
contains can also be called in the operator form, it corresponds to the operator in:
val hasNull = null in list
val hasNoNull = null !in list
I'm trying to convert my nested for loop to asSequence in Kotlin. Here, my goal is to get and update the value of all my object array from another object array with the same key.
nested for loop:
val myFields = getMyFields()
val otherFields = getOtherFields()
for (myField in myFields) { // loop tru the my fields
for (otherField in otherFields) { // find the same fields
if (myField.key == otherField.key) { // if the same, update the value
val updatedMyField = myField.copy(value = otherValue.value)
myFields[myFields.indexOf(myField)] = updatedMyField // update my field value
break
}
}
}
What I've tried:
val updatedMyFields = getMyFields().asSequence()
.map { myField ->
getOtherFields().asSequence()
.map { otherField ->
if (myField.key == otherField.key) {
return#map otherField.value
} else {
return#map ""
}
}
.filter { it?.isNotEmpty() == true }
.first()?.map { myField.copy(value = it.toString()) }
}
.toList()
but this does not compile as it will return List<List<MyField>>.
I'm just looking for something much cleaner for this.
As comments suggest, this would probably be much more efficient with a Map.
(More precisely, a map solution would take time proportional to the sum of the list lengths, while the nested for loop takes time proportional to their product — which gets bigger much faster.)
Here's one way of doing that:
val otherFields = getOtherFields().associate{ it.key to it.value }
val myFields = getMyFields().map {
val otherValue = otherFields[it.key]
if (otherValue != null) it.copy(value = otherValue) else it
}
The first line creates a Map from the ‘other fields’ keys to their values. The rest then uses it to create a new list from ‘my fields’, substituting the values from the ‘other fields’ where present.
I've had to make assumptions about the types &c, since the code in the question is incomplete, but this should do the same. Obviously, you can change how it merges the values by amending the it.copy().
There are likely to be even simpler and more efficient ways, depending on the surrounding code. If you expanded it into a Minimal, Complete, and Verifiable Example — in particular, one that illustrates how you already use a Map, as per your comment — we might be able to suggest something better.
Why do you want to use asSequence() ? You can go for something like that:
val myFields = getMyFields()
val otherFields = getOtherFields()
myFields.forEach{firstField ->
otherFields.forEach{secondField ->
if (firstField.key == secondField.key) {
myFields[myFields.indexOf(firstField)] = secondField.value
}
}
}
This will do the same job than your nested for loop and it's easier to read, to understand and so to maintain than your nested asSequence().
Kotlin has some pretty cool functions for collections. However, I have come across a problem in which the solution is not apparent to me.
I have a List of Objects. Those Objects have an ID field which coincides with a SQLite database. SQL operations are performed on the database, and a new list is generated. How can the index of an item from the new list be found based on the "ID" field (or any other field for that matter)?
the Collection.find{} function return the object, but not the index.
indexOfFirst can find the index of the first element of a collection that satisfies a specified predicate.
We have a DB SQlite that a call is made to to retrieve parentList We can obtain the items in the ArrayList with this type of code
fun onDoIt(view: View){
initDB()
for (t in 0..X-1) {
var N:String = parentList[t].dept
// NOTE two syntax here [t] and get(t)
if(t == 1){
var B:String = parentList[0].idD.toString()
println("$$$$$$$$$$$$$$$$$$$$$ ====== "+B)
}
var I:String = parentList.get(t).idD.toString()
println("################### id "+I+" for "+N)
}
}
private fun initDB() {
parentList = db.querySPDept()
if (parentList.isEmpty()) {
title = "No Records in DB"
} else {
X = parentList.size
println("**************************************** SIZE " + X)
title = "SP View Activity"
}
}