What I want to achieve is to get min/max attribute value of object from ArrayList<Object>. For example if Object has attribute weight(float), I want heaviest object from the list.
I've tried to implement Comparable to get max/min value but this returns same value for min and same for max for some reason. (I don't know if it works with floats)
val maxVal: Float = arrayList.max().floatVal1
val minVal: Float = arrayList.min().floatVal1
data class CustomObject(var val1: String, var floatVal1: Float , var floatVal2: Float?, var floatVal3: Float?, var floatVal4: Float?): Comparable<CustomObject>{
override fun compareTo(other: CustomObject) = (floatVal1 - other.floatVal1).toInt()
}
That specific question from duplicate post does not show me how to get max/min value based on Float. That's the problem. If I want to modify Comparator it accepts only Int. And i cant use that stream feature because my app is for API 23+ not 24+
I think you're looking for minBy and maxBy:
val minObject: CustomObject? = arrayList.minBy { it.floatVal1 }
val maxObject: CustomObject? = arrayList.maxBy { it.floatVal1 }
val maxObj: Object? = arrayList.maxByOrNull { it.floatVal1 }
val minObj: Object? = arrayList.minByOrNull { it.floatVal2 }
maxBy, minBy are deprecated since Kotlin 1.4
This return non-null types:
val max = list.maxOf { it.value }
val min = list.minOf { it.value }
Below Code returns Max value of your parameter in list of objects.
Ex)100 is max marks obtained all students
list.maxOf { it.yourParameter } // Returns Max value of yourParameter value in list of objects
Below Code returns Object of Max value of your parameter in list of objects.
Ex)Student object who got 100 marks
list.maxBy { it.yourParameter } // Returns Object of Max of yourParameter value in list of objects
Related
I am trying to figure out the best way to map my sorted map to the arguments of another function. This is an example of what I have.
data class ValueDescription (val length: Int, val count: Int)
// Now I am trying to map to a variable that looks like this
// This variable cannot be changed, I have to return this variable in this format
// The output will be a list of ValueDescriptions with a length and count for each entry of the map I have
// I will add the results of myValues to a mutable list later
val myValues = ValueDescription(_length here__, __count here__)
I have a sorted map that I want to map to my values
// The map will look like this
// Where both Ints hold the lengths and counts
// For example I would have the length of 7 to count of 8
val out = Map<Int, Int>
How can I take the values in my sorted map and place them into the variable myValues?
I tried to map by looping through my map with the forEach method and doing something like
out.map{it.key to myValues.ValueDescription.length}
but this doesn't seem to work.
I'm not sure I completely understood the question. If I got it correctly, your input is the Map<Int, Int> and you want to transform it to a List<ValueDescription>.
You can definitely use the map function for this:
val inputMap: Map<Int, Int> = TODO("provide the initial map here")
val myValues = inputMap.map { (l, c) -> ValueDescription(l, c) }
The map function here iterates over the entries of the map, and transforms each of them into a value of type ValueDescription by calling our lambda (the part between braces { ... }).
Each entry of the map here contains a key (the length) and a value (the count). Instead of using it.key and it.value, you can also use parentheses like I did here with (l, c) to destructure the entry into its 2 parts and give them names like l and c. The above is equivalent to:
val myValues = inputMap.map { ValueDescription(it.key, it.value) }
Hey I have list in kotlin. I am iterating over it. I want to store true/false as flag index which one is minimum value in whole list. I tried some code, but it's not working.
fun main() {
val list = mutableListOf(4.83, 4.39, 3.58, 3.50, 3.46)
val minValue = mutableListOf<BestMinimumValue>()
var previousValue = 0.0
list.forEach {
minValue.add(BestMinimumValue(compareValue = previousValue > it))
previousValue = it
}
minValue.forEach {
println(it)
}
}
data class BestMinimumValue(
val compareValue: Boolean
)
Actual Output
BestMinimumValue(compareValue=false)
BestMinimumValue(compareValue=true)
BestMinimumValue(compareValue=true)
BestMinimumValue(compareValue=true)
BestMinimumValue(compareValue=true)
I'll explain what I need. In my list 3.46 is minimum value so on that place I need the flag as true and other one will be false.
Expected Output
BestMinimumValue(compareValue=false)
BestMinimumValue(compareValue=false)
BestMinimumValue(compareValue=false)
BestMinimumValue(compareValue=false)
BestMinimumValue(compareValue=true)
It is not possible to create the resulting list while iterating, because we can't know if the current item is the smallest one or not. We have to iterate at least twice: first searching for the minimum value and then creating a resulting list.
One solution is to find the minimum and then compare items to it:
val min = list.minOrNull()!!
val minValue = list.map { BestMinimumValue(it == min) }
Note if there are multiple same minimum values, then all of them will be marked as true.
If this is not expected behavior or if we like to avoid comparing items twice, then we can find the index of the minimum value and just construct the resulting list with it:
val minIndex = list.withIndex().minByOrNull { it.value }!!.index
val minValue = List(list.size) { BestMinimumValue(it == minIndex) }
I'm trying my hands on Kotlin. Being from a Python background is really giving me a tough time to get the knack of the Kotlin syntax. I'm trying to do a simple dictionary (Mutable Map) operation. However, its giving me exceptions.
This is what I tried. Kotlin compiler
Adding the code snippet for reference.
fun main() {
val openActivityMap = mutableMapOf<String, MutableMap<String, Any>>()
val packageName = "amazon"
val currentTime = 23454321234
if(openActivityMap.containsKey(packageName)){
if(openActivityMap[packageName]?.get("isAlreadyApplied")){
if((openActivityMap[packageName]?.get("lastAppliedAt") - currentTime) > 3600){
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
else{
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
}
I'm a bit late to the party, but I'd like to point out another solution here.
As I commented on the OP, heterogeneous maps with fixed string keys like this are usually better expressed with classes in Kotlin. For instance, in your case, the class for your main map's values could be the following:
data class PackageInfo(
var isAlreadyApplied: Boolean,
var lastAppliedAt: Long,
)
(you could obviously add more properties if need be)
This would save you all the casts on the final values.
Another point I'd like to make is that if you access the value for a key anyway, you don't need to check up front the existence of the key with containsKey. Maps return null for keys that are not associated with any value (this is why you need to check for null after getting the value).
The compiler cannot see the correlation between containsKey and the subsequent get or [] access. However, it's smart enough to understand a null check if you simply get the value first and then check for null.
This always applies unless you want to tell the difference between keys that aren't in the map and keys that are in the map but associated null values (which is quite rare).
All in all, I would write something like that:
fun main() {
val openActivityMap = mutableMapOf<String, PackageInfo>()
val packageName = "amazon"
val currentTime = 23454321234
val packageInfo = openActivityMap[packageName]
if (packageInfo != null) { // the key was found and the value is smart cast to non-null in the next block
if (packageInfo.isAlreadyApplied) {
if ((packageInfo.lastAppliedAt - currentTime) > 3600) {
packageInfo.isAlreadyApplied = false
}
} else {
packageInfo.isAlreadyApplied = false
}
}
}
data class PackageInfo(
var isAlreadyApplied: Boolean,
var lastAppliedAt: Long,
)
I would recommend writing tests first and working in small increments, but this should fix your compilation issues:
fun main() {
val openActivityMap = mutableMapOf<String, MutableMap<String, Any>>()
val packageName = "amazon"
val currentTime = 23454321234
if(openActivityMap.containsKey(packageName)){
if(openActivityMap[packageName]?.get("isAlreadyApplied") as Boolean){
if((openActivityMap[packageName]?.get("lastAppliedAt") as Long - currentTime) > 3600){
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
else {
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
}
EDIT: Also I prefer to avoid nullable variables and mutable objects in general, but I suppose there's an exception to every rule.
Couldn't you just declare your Map<String, Any> to return a Boolean instead of Any? So,
val openActivityMap = mutableMapOf<String, MutableMap<String, Boolean>>()
It looks like you're trying to use your second Map to store both Booleans and Ints, which is complicating the logic. You'll need to typecast if you decide to approach it without Typing.
There's a problem with the 2 statement below
if(openActivityMap[packageName]?.get("isAlreadyApplied"))
if((openActivityMap[packageName]?.get("lastAppliedAt") - currentTime) > 3600)
As we all know, an IF statement requires a boolean value for it's param. The types of both statement are unknown at compilation time as they are of a Generic type, Any. As such,
openActivityMap[packageName]?.get("isAlreadyApplied") could be a null or of type Any (Not Boolean).
openActivityMap[packageName]?.get("lastAppliedAt") could be a null or of type Any (an Int was expected here for computation).
This would throw compilation errors as the compiler does not know the types to go with. What could be done is to cast to it's proper types.
Solution
openActivityMap[packageName]?.get("isAlreadyApplied") as Boolean ?: false
((openActivityMap[packageName]?.get("lastAppliedAt") as Int ?: 0) - currentTime)
Giving a default value if it's null.
maybe you can try something like this
if (openActivityMap.containsKey(packageName)) {
val packageMap = openActivityMap[packageName]!!
val applyRequired = (packageMap["lastAppliedAt"] as Long - currentTime) > 3600
packageMap["isAlreadyApplied"] = packageMap.containsKey("isAlreadyApplied") && !applyRequired
}
btw. do you really want to have lastAppliedAt to be in te future? otherewise it will never be > 3600
Given a maximum list size in parameter size and total amount of elements in parameter elements, I need to create a list of lists. What is the syntax for creating variables in for loops in Kotlin?
The way I'm thinking of going about this is to declare and create lists before elements are added to a list. Then, when a list has reached full capacity, it is switched out for the next list that is empty.
Here is the half-baked code:
fun listOfLists(size: Int, vararg elements: String): List<List<String>> {
var amountOfElements = elements.size
var currentSubList: List<String> = mutableListOf<String>()
val numberOfLists: Int = amountOfElements / size + 1
for (n in 0..numberOfLists) {
// Code for creating the total number of lists needed
}
for (e in elements) {
if (amountOfElements % size == 0) {
// Code for switching lists
}
amountOfElements--
}
As #dyukha correctly mentioned, what you need is chunked() function.
fun listOfLists(size: Int, vararg elements: String) =
elements.asList().chunked(size)
Or, if you want to be really efficient, you can also use asSequence():
fun listOfLists(size: Int, vararg elements: String) =
elements.asSequence().chunked(size)
chunked() doesn't work on Array, because it's defined on Iterable and Sequence, and Array doesn't implement any of them.
If I create an array, then fill it, Kotlin believes that there may be nulls in the array, and forces me to account for this
val strings = arrayOfNulls<String>(10000)
strings.fill("hello")
val upper = strings.map { it!!.toUpperCase() } // requires it!!
val lower = upper.map { it.toLowerCase() } // doesn't require !!
Creating a filled array doesn't have this problem
val strings = Array(10000, {"string"})
val upper = strings.map { it.toUpperCase() } // doesn't require !!
How can I tell the compiler that the result of strings.fill("hello") is an array of NonNull?
A rule of thumb: if in doubts, specify the types explicitly (there is a special refactoring for that):
val strings1: Array<String?> = arrayOfNulls<String>(10000)
val strings2: Array<String> = Array(10000, {"string"})
So you see that strings1 contains nullable items, while strings2 does not. That and only that determines how to work with these arrays:
// You can simply use nullability in you code:
strings2[0] = strings1[0]?.toUpperCase ?: "KOTLIN"
//Or you can ALWAYS cast the type, if you are confident:
val casted = strings1 as Array<String>
//But to be sure I'd transform the items of the array:
val asserted = strings1.map{it!!}
val defaults = strings1.map{it ?: "DEFAULT"}
Why the filled array works fine
The filled array infers the type of the array during the call from the lambda used as the second argument:
val strings = Array(10000, {"string"})
produces Array<String>
val strings = Array(10000, { it -> if (it % 2 == 0) "string" else null })
produces Array<String?>
Therefore changing the declaration to the left of the = that doesn't match the lambda does not do anything to help. If there is a conflict, there is an error.
How to make the arrayOfNulls work
For the arrayOfNulls problem, they type you specify to the call arrayOfNulls<String> is used in the function signature as generic type T and the function arrayOfNulls returns Array<T?> which means nullable. Nothing in your code changes that type. The fill method only sets values into the existing array.
To convert this nullable-element array to non-nullable-element list, use:
val nullableStrings = arrayOfNulls<String>(10000).apply { fill("hello") }
val strings = nullableStrings.filterNotNull()
val upper = strings.map { it.toUpperCase() } // no !! needed
Which is fine because your map call converts to a list anyway, so why not convert beforehand. Now depending on the size of the array this could be performant or not, the copy might be fast if in CPU cache. If it is large and no performant, you can make this lazy:
val nullableStrings = arrayOfNulls<String>(10000).apply { fill("hello") }
val strings = nullableStrings.asSequence().filterNotNull()
val upper = strings.map { it.toUpperCase() } // no !! needed
Or you can stay with arrays by doing a copy, but really this makes no sense because you undo it with the map:
val nullableStrings = arrayOfNulls<String>(10000).apply { fill("hello") }
val strings: Array<String> = Array(nullableStrings.size, { idx -> nullableStrings[idx]!! })
Arrays really are not that common in Java or Kotlin code (JetBrains studied the statistics) unless the code is doing really low level optimization. It could be better to use lists.
Given that you might end up with lists anyway, maybe start there too and give up the array.
val nullableStrings = listOf("a","b",null,"c",null,"d")
val strings = nullableStrings.filterNotNull()
But, if you can't stop the quest to use arrays, and really must cast one without a copy...
You can always write a function that does two things: First, check that all values are not null, and if so then return the array that is cast as not null. This is a bit hacky, but is safe only because the difference is nullability.
First, create an extension function on Array<T?>:
fun <T: Any> Array<T?>.asNotNull(): Array<T> {
if (this.any { it == null }) {
throw IllegalStateException("Cannot cast an array that contains null")
}
#Suppress("CAST_NEVER_SUCCEEDS")
return this as Array<T>
}
Then use this function new function to do the conversion (element checked as not null cast):
val nullableStrings = arrayOfNulls<String>(10000).apply { fill("hello") }
val strings = nullableStrings.asNotNull() // magic!
val upperStrings = strings.map { it.toUpperCase() } // no error
But I feel dirty even talking about this last option.
There is no way to tell this to the compiler. The type of the variable is determined when it is declared. In this case, the variable is declared as an array that can contain nulls.
The fill() method does not declare a new variable, it only modifies the contents of an existing one, so it cannot cause the variable type to change.