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
Related
Is there an easier approach to convert an Intellij IDEA environment variable into a list of Tuples?
My environment variable for Intellij is
GROCERY_LIST=[("egg", "dairy"),("chicken", "meat"),("apple", "fruit")]
The environment variable gets accessed into Kotlin file as String.
val g_list = System.getenv("GROCERY_LIST")
Ideally I'd like to iterate over g_list, first element being ("egg", "dairy") and so on.
And then ("egg", "dairy") is a tuple/pair
I have tried to split g_list by comma that's NOT inside quotes i.e
val splitted_list = g_list.split(",(?=(?:[^\\\"]*\\\"[^\\\"]*\\\")*[^\\\"]*\$)".toRegex()).toTypedArray()
this gives me first element as [("egg", second element as "dairy")] and so on.
Also created a data class and tried to map the string into data class using jacksonObjectMapper following this link:
val mapper = jacksonObjectMapper()
val g_list = System.getenv("GROCERY_LIST")
val myList: List<Shopping> = mapper.readValue(g_list)
data class Shopping(val a: String, val b: String)
You can create a regular expression to match all strings in your environmental variable.
Regex::findAll()
Then loop through the strings while creating a list of Shopping objects.
// Raw data set.
val groceryList: String = "[(\"egg\", \"dairy\"),(\"chicken\", \"meat\"),(\"apple\", \"fruit\")]"
// Build regular expression.
val regex = Regex("\"([\\s\\S]+?)\"")
val matchResult = regex.findAll(groceryList)
val iterator = matchResult.iterator()
// Create a List of `Shopping` objects.
var first: String = "";
var second: String = "";
val shoppingList = mutableListOf<Shopping>()
var i = 0;
while (iterator.hasNext()) {
val value = iterator.next().value;
if (i % 2 == 0) {
first = value;
} else {
second = value;
shoppingList.add(Shopping(first, second))
first = ""
second = ""
}
i++
}
// Print Shopping List.
for (s in shoppingList) {
println(s)
}
// Output.
/*
Shopping(a="egg", b="dairy")
Shopping(a="chicken", b="meat")
Shopping(a="apple", b="fruit")
*/
data class Shopping(val a: String, val b: String)
Never a good idea to use regex to match parenthesis.
I would suggest a step-by-step approach:
You could first match the name and the value by
(\w+)=(.*)
There you get the name in group 1 and the value in group 2 without caring about any subsequent = characters that might appear in the value.
If you then want to split the value, I would get rid of start and end parenthesis first by matching by
(?<=\[\().*(?=\)\])
(or simply cut off the first and last two characters of the string, if it is always given it starts with [( and ends in )])
Then get the single list entries from splitting by
\),\(
(take care that the split operation also takes a regex, so you have to escape it)
And for each list entry you could split that simply by
,\s*
or, if you want the quote character to be removed, use a match with
\"(.*)\",\s*\"(.*)\"
where group 1 contains the key (left of equals sign) and group 2 the value (right of equals sign)
I have a class called Person
data class Person(
val id: Int,
val name: String
)
data class IDs(
val id : Int,
val active : Boolean )
and an array list that has numbers of ids and another list of Persons
val myStu = listOf<Person>(Person(1, "Name_1"), Person(2, "Name_2"), Person(3, "Name_3"))
var ids = listOf<IDs>(IDs(1,false),IDs(2,true),IDs(3,true))
var newIds = listOf<Int>(2,3,4,6)
First I want to apply two actions to the myStu, first is to have a list that include all the items from myStu that his id matches the id in the IDS and only if the active is true
myStu or the new list will have the values
Person(2, "Name_2"), Person(3, "Name_3"))
Then do action two , I need to add a new item to the new list that their id does not exist in the newIds , in another word we will add a new person Person(4,"None") and (6,"None) , 4 and 6 values come from newIds list
the final output will be :
id= 2 name = "Name_2", id= 3 name = "Name_3", id= 4 name = "None" , id =6 name="None"
I want to write the code with filter , I failed with first step because I don't know how to use contains() with the list inside the filter
val newArr = myStu.filter {
ids.contains(it.id)
}
The "easiest" way of doing that would be to use filter directly, there's no need for contains. If we were to use contains, then we would need to also search for which element contained the id, in order to get the status. We can just do a .any() to do both at the same time.
V1
val activeStu = myStu.filter { person -> ids.any { it.id == person.id && it.active } }
val result = newIds.map { newId ->
activeStu.find { it.id == newId } ?: Person(id = newId, name = "None")
}
Another method, that might work a bit better if we have big lists, would be to first transform the IDs list into a map. That way the second part of our code is a bit more efficient, since there is no search involved.
V2
val idsMap = ids.associate { it.id to it.active }
val activeStu = myStu.filter { idsMap[it.id] ?: false }
//the creation of the result list is the same
Version without creating 2 new lists. This works, but it might be quite ineficient processing wise, and also harder to understand what is going on IMO.
V3
val result = newIds.map { newId ->
//try to find an IDs with the current newId and status as true
when (ids.find { it.id == newId }?.active) {
//if found, then find the corresponding Person
true -> myStu.find { it.id == newId } ?: Person(newId, "None") // if this happens, it means that an IDs with status true existed and no Person had that id. Not sure what you want in this scenario, this version creates one of the "none" persons.
//if not found, then create a new one
else -> Person(newId, "None")
}
}
Note: depending on what version of kotlin you have, you might have to change the when statement to this:
when (ids.find { it.id == newId }?.active == true)
Since I think I remember that null didn't used to be treated as false in old versions (I've run this with version 1.4.20).
Btw, you can also use this version with the idsMap from V2, just replace the when(...) with when(idsMap[newId] or when(idsMap[newId] == true) depending on the kotlin version.
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)
I have the following code snippet
val cachedNews = listOf(News(9, "https://009"), News(8, "https://234"), News(7, "https://345"))
val freshNews = listOf(News(1, "https://123"), News(2, "https://234"), News(3, "https://345"))
val result = freshNews.filter {fresh -> filter(cachedNews, fresh)}
private fun filter(cached: List<News>, fresh: News): Boolean {
cached.forEach { cachedItem ->
if (cachedItem.url == fresh.url) return true
}
return false }
When the code runs if cachedItem.url == fresh.url the list is filtered and the result is a list where the urls of the two lists are identical. However when i reverse equality like so cachedItem.url != fresh.url the list is not filtered at all. The sequence of execution changes.
When using the == sign, the first item of freshNews is compared with the first Item of cachedNews after that the secondItem of freshNews is compared with secondItem of cachedNews and so on.
When I use the != sign the all items of freshNews are compared against only the firstItem of cachedNews ??
Am I missing something or is my code just wrong?
I'm not sure what the specific problem is because your approach is quite confusing. Your custom filter function is actually more like a contains function.
What might be useful is to:
Extract the cached URLs to a set
Filter the new results by URLs that are not in the set.
fun main() {
val cachedNews = listOf(News(9, "https://009"), News(8, "https://234"), News(7, "https://345"))
val freshNews = listOf(News(1, "https://123"), News(2, "https://234"), News(3, "https://345"))
val cachedUrls = cachedNews.map { it.url }.toSet()
val result = freshNews.filterNot { cachedUrls.contains(it.url) }
println(result)
}
Result:
[News(id=1, url=https://123)]
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")