This is what I have and what I want to achieve:
I have a class which has a mutableList as a field.
I want to find a specific element inside that list and change it.
This is what I've tried so far:
This is the functional statement I was hoping would have worked, after I've also put it in an Extension function:
fun Classroom.setNewParameters(id: String, modifiedName: String) {
this.students.filter { l -> l.id == id }
.map { l -> l.name = modifiedName }
.toList()
}
But the list students appears to be unchanged after this function is called.
I found an "ugly" solution with the code below:
fun Classroom.setNewParameters(id: String, modifiedName: String) {
for (l : Student in this.students) {
if (l.id == id) {
l.name = modifiedName
break
}
}
}
Still tho, I'd really like to know why the first block of code behaves like it does and doesn't actually have any effect on the list.
You can think of map as a way to transform input to get new output. Normally it should not mutate state inside, in other words it should be a function without side effects to pursue maintainability
In your case you clearly want to mutate elements, for that you might want to use that code snippet:
fun Classroom.setNewParameters(id: String, modifiedName: String) {
this.students.filter { l -> l.id == id }
.onEach { l -> l.name = modifiedName }
}
Also, even you used map incorrectly it should must modify field l.name (unless you have implemented you own delegates or getter/setter). Try to debug set breakpoint on this.students.filter { l -> l.id == id } and see if there are any items left after filtering
Noob here but I did just see something related to this the other day.
Is there a reason you wouldn't just check to see if your array contains the old value, return the element id and then assign your new value to that id?
I guess I'm just pointing out that this could be accomplished with a "value in array" type search... but I'm still too new to know the pros and cons of using it vs map.
Kind of like this below, which I got from Kotlin - Idiomatic way to check array contains value
"value" in array
Which translates into the Java API:
array.contains("value")
So I'd check for the old value in the array, return it's index and then reassign that index with the new value.
Related
Why I'm getting "java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0" while running next code??? :
val totalList = mutableListOf<MutableList<Int>>()
fun main() {
for (i in 0..15) {
for (j in 0..10) {
*some operations and calculations with **var element of type Int***
totalList[i].add(element)
}
}
}
I was thinking that in such case while iterating through 'j' it should add elements to mutableList[i], after this it should start adding elements to mutableList[i + 1] etc.... But instead I am recieving IndexOutOfBoundsException....
val totalList = mutableListOf<MutableList<Int>>()
All this does is create one list which is going to contain MutableList<Int> items. Right now, there's nothing in it (you've supplied no initial elements in the parentheses).
Skip forward a bit, and you do this:
totalList[0].add(element)
You're trying to get the first element of that empty list and add to it. But there is no first element (index 0) because the list is empty (length 0). That's what the error is telling you.
There's lots of ways to handle this - one thing you could do is create your lists up-front:
// create the 16 list items you want to access in the loop
// (the number is the item count, the lambda generates each item)
val totalList = MutableList(16) { mutableListOf<Int>() }
// then refer to that list's properties in your loop (no hardcoded 0..15)
for (i in totalList.indices) {
...
// guaranteed to exist since i is generated from the list's indices
totalList[i].add(element)
}
Or you could do it the way you are now, only using getOrElse to generate the empty list on-demand, when you try to get it but it doesn't exist:
for (i in 0..15) {
for (j in 0..10) {
// if the element at i doesn't exist, create a list instead, but also
// add it to the main list (see below)
totalList.getOrElse(i) {
mutableListOf<Int>().also { totalList.add(it) }
}.add(element)
}
}
Personally I don't really like this, you're using explicit indices but you're adding new list items to the end of the main list. That implicity requires that you're iterating over the list items in order - which you are here, but there's nothing enforcing that. If the order ever changed, it would break.
I'd prefer the first approach - create your structure of lists in advance, then iterate over those and fill them as necessary. Or you might want to consider arrays instead, since you have a fixed collection size you're "completing" by adding items to specific indices
Another approach (that I mentioned in the comments) is to create each list as a whole, complete thing, and then add that to your main list. This is generally how you do things in Kotlin - the standard library contains a lot of functional tools to allow you to chain operations together, transform things, and create immutable collections (which are safer and more explicit about whether they're meant to be changed or they're a fixed set of data).
for (i in 0..15) {
// map transforms each element of the range (each number) to an item,
// resulting in a list of items
val items = (0..10).map { j ->
// do whatever you're doing
// the last expression in the lambda is its resulting value,
// i.e. the item that ends up in the list
element
}
// now you have a complete list of items, add them to totalList
totalList.add(items)
}
(Or you could create the list directly with List(11) { j -> ... } but this is a more general example of transforming a bunch of things to a bunch of other things)
That example there is kinda half and half - you still have the imperative for loop going on as well. Writing it all using the same approach, you can get:
val totalList = (0..15).map { i ->
(0..10).map { j ->
// do stuff
element
}
}
I'd probably prefer the List(count) { i -> ... } approach for this, it's a better fit (this is a general example). That would also be better since you could use MutableList instead of List, if you really need them to be mutable (with the maps you could just chain .toMutableList() after the mapping function, as another step in the chain). Generally in Kotlin, collections are immutable by default, and this kind of approach is how you build them up without having to create a mutable list etc. and add items to it yourself
I was trying to generate all permutations of a list in Kotlin. There are a zillion examples out there which return a List<List<T>>, but my input list breaks those as they try to fit all the results in the output list. So I thought I would try to make a version returning Sequence<List<T>>...
fun <T> List<T>.allPermutations(): Sequence<List<T>> {
println("Permutations of $this")
if (isEmpty()) return emptySequence()
val list = this
return indices
.asSequence()
.flatMap { i ->
val elem = list[i]
(list - elem).allPermutations().map { perm -> perm + elem }
}
}
// Then try to print the first permutation
println((0..15).toList().allPermutations().first())
Problem is, Kotlin just seems to give up and asks for the complete contents of one of the nested sequences - so it never (or at least not for a very long time) ends up getting to the first element. (It will probably run out of memory before it gets there.)
I tried the same using Flow<T>, with the same outcome.
As far as I can tell, at no point does my code ask it to convert the sequence into a list, but it seems like something internal is doing it to me anyway, so how do I stop that?
As mentioned in the comments, you have handled the empty base case incorrectly. You should return a sequence of one empty list.
// an empty list has a single permutation - "itself"
if (isEmpty()) return sequenceOf(emptyList())
If you return an empty sequence, first will never find anything - your sequence is always empty - so it will keep evaluating the sequence until it ends, and throw an exception. (Try this with a smaller input like 0..2!)
Let me elaborate:
I need to be able to iterate over a list of objects. Each of the objects has a property which is a list, and I have to check if that list contains any elements that are not in another list.
When I tried to do it by using nested for loops, it kept giving me concurrent modification exceptions, so I tried to use an iterator, but now I'm stuck, since if I make an iterator based on the list of objects, I can't access the individual object's properties to then iterate over.
Here's some example code of what I was trying to accomplish:
for (preference in preferencesWithRestaurant) {
for (restaurantID in preference.restaurantIDs) {
// One method I tried using
preferencesWithRestaurant.removeIf{ !listOfIds.contains(restaurantID) }
/* alternate method I tried using
if (!listOfIds.contains(restaurantID)) {
preferencesWithRestaurant.remove(preference)
}
*/
}
}
If you can replace the value of preferencesWithRestaurant or store the result in another variable then you can filter it:
preferencesWithRestaurant = preferencesWithRestaurant.filter { preference ->
preference.restaurantIDs.all { it in listOfIds }
}
Depending on the exact type of preferencesWithRestaurant you may need to convert it to the proper type, e.g. invoke toMutableList() at the end.
If you prefer to modify preferencesWithRestaurant in-place, then you can use retainAll() (thanks #Tenfour04):
preferencesWithRestaurant.retainAll { preference ->
preference.restaurantIDs.all { it in listOfIds }
}
Alternatively, you can keep your original approach, but use a mutable iterator to remove an item while iterating:
val iter = preferencesWithRestaurant.listIterator()
for (preference in iter) {
for (restaurantID in preference.restaurantIDs) {
if (!listOfIds.contains(restaurantID)) {
iter.remove()
break
}
}
}
I have an ArrayList<String>. I want to add n copies of a new String to it.
I've Googled generally and searched on StackOverflow. I've looked at the documentation.
Surely there's a better way than doing a loop?
I was hoping for something like:
myArray.addAll (ArrayList<String>(count: 10, value: "123"))
You can initialize a List with a given size n and an initializer function like this:
fun main() {
val n = 10
val defaultList = List(n) { it -> "default" } // you can leave "it ->" here
println(defaultList)
}
This piece of code then outputs
[default, default, default, default, default, default, default, default, default, default]
If you want to intialize an Array<String> directly without using a List as intermediate, you can do
val defaultArray: Array<String> = Array(n) { "default" }
println(defaultArray.contentToString())
in the main and get the same output (even without the it ->, which, indeed, isn't necessary in this case).
I have a object list with the same id, then, i want to keep the one with the most recent date, and the delete the other with the kotlin collection operators. For Example I have :
{"id":111,
"date":"02/12/2017"
}
and the other
{"id":111,
"date":"02/8/2018"}
In this case, i would like to delete the first object.
You can achieve it like this
list.groupBy { it.id }.entries.map { it.value.maxBy { it.date } }
it will create a map of id, List<object> while keeping original order and then choose newest object from the list.
Here I am assuming date is long value timestamp
If you really must remove the entries from the current collection the following may help you (I assume your objects (Obj) are contained in a list called list):
list.removeAll { anObj -> list.any { anObj.id == it.id && it.date > anObj.date } }
// or same with removeIf
It's probably easier just collecting what you are actually interested in and then have an immutable list in the first place.
Collecting the elements you are interested in could be done as follows (there are of course lots of other ways to do that... just one of many):
val result = list.groupBy { it.id }.values.mapNotNull { it.maxBy { it.date } } // mapNotNull is only used due to maxBy returning a nullable type... it isn't really needed... or well... depends on what your date type is ;-)
result will then be a List<Obj>. Instead of mapNotNull you could also use it.maxBy { it.date }!! if you know there is at least 1 element.
If you then still need to remove the elements from the list, you could do the following:
list.removeIf { it !in result }
// or
list.removeAll { it !in result }
However I can't really recommend that you mutate your current list just for that... The result in the above example already contains all the elements in the form you want (i.e. List<Obj>). Instead rather use the benefit of having an immutable list :-)