How do I get the index of a value from a Kotlin array?
My best solution right now is using:
val max = nums.max()
val maxIdx = nums.indices.find({ (i) -> nums[i] == max }) ?: -1
is there a better way?
If you want to get the index of the maximum element you can use 'maxBy' function:
val maxIdx = nums.indices.maxBy { nums[it] } ?: -1
It is more efficient since it will traverse the array only once.
With current Kotlin (1.0) you can use indexOf() extension function on arrays:
val x = arrayOf("happy","dancer","jumper").indexOf("dancer")
All extension functions for arrays are found in the api reference.
In your example:
val maxIdx = nums.indexOf(nums.max())
If you want to find the item based on some predicate, then you can use indexOfFirst and indexOfLast extension functions.
val strings = arrayOf("hello","world","hello")
val firstHelloIndex = strings.indexOfFirst { it == "hello" }
val lastHelloIndex = strings.indexOfLast { it == "hello" }
Related
Problem Statement
I will try to elaborate the case by means of a scenario. Lets take this question for instance.
Link to question: https://leetcode.com/problems/remove-element/
Given an array nums and a value target, remove all instances of
that value in-place and return the new length.
Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.
Example: Given nums = [0,1,2,2,3,0,4,2], target = 2; the output =
5 (number of elements not equal to target) and modify the array to
[0,1,3,0,4]
The order of elements can be changed. It doesn't matter what you leave
beyond the new length.
My Approach
Step-1: Identify all the elements which are equal to the given target and move them to right hand side of the array while maintaining a counter.
Step-2: Drop all the elements from right.
Step-3: return (n - counter), where n is the array length and counter is the number of elements equal to target.
Below is the implementation of the same:
object RemoveElement {
// Link to question: https://leetcode.com/problems/remove-element/
def main(args: Array[String]): Unit = {
var nums = Array(3,2,2,3)
val target = 3
val result = removeElement(nums, target)
// nums = nums.dropRight(_.equals(target)) // POINT 1
println(s"Result: ${result}, Modified Array: [${nums.mkString(", ")}]")
}
def removeElement(nums: Array[Int], target: Int): Int = {
val n = nums.length
var left, counter = 0
var right = n - 1
while(left < right){
if(nums(left) != target){
left += 1
}
else {
// Find position of the elements which is not equal to target
if(nums(right) == target){
counter += 1
right -= 1
}
else{
// Swap the elements
counter += 1
val temp = nums(left)
nums(left) = nums(right)
nums(right) = temp
left += 1
right -= 1
}
}
}
// nums.dropWhile(_.equals(target)) // POINT 2
// nums = nums.dropRight(_.equals(target)) // POINT 3
return (n - counter)
}
}
POINT - 1: Makes absolute sense as the array nums is in the scope of main method, therefore, the statement would work as charm.
POINT - 2: These lines has no impact to the array nums.
POINT - 3: Gives error. I understand that the input argument (array nums) is of type val (i.e. passed by reference, and hence immutable within the scope of the method removeElement).
If I had an option of creating a new array, there wouldn't be any issue. But if I am required to return the modified array by adding/removing the elements (like in this question) to the calling method, how do I achieve that in Scala?
To make the case more generic, what is the way by which we can modify the input collections (passed as arguments) in Scala methods?
P.S.: If I do not remove elements from the input array itself, LeetCode fails my submission with below message:
How do I modify an input array passed to a method by reference?
Scala does not support pass-by-reference. The default is pass-by-value (or more precisely, a special case of pass-by-value which is sometimes known as call-by-object, call-by-sharing, or call-by-object-sharing). Scala also supports call-by-name.
So, you simply cannot pass an array to a method by reference in Scala. You will have to use another language which supports pass-by-reference such as C# (with the ref keyword) or C++. (Note that Java also doesn't support pass-by-reference.)
Something like this
object Solution {
def removeElement(nums: Array[Int], `val`: Int): Int = {
var p: Int = 0
nums.foreach(v => {
if (v != `val`) {
nums(p) = v
p += 1
}
})
p
}
}
I have this problem: I want to iterate on the elements of an array that satisfy a certain condition.
The first thing I though is to use the filter method on the original array and then iterates over the resulting elements. But I had some memory usage problem with that, i.e. java heap space.
When a filter is applied on an array, it will instantiate a new array? So it will copy each element?
Is it better to use this approach:
array.filter(<condition>).foreach{ element =>
do something
}
Or the following one?
for(i <- array.indices if <condition>){
do something
}
Moreover, I wrote these two tests:
With for
val size = 10000000
val elements = Array.ofDim[Double](size)
for (i <- elements.indices) {
elements.update(i, math.random)
}
var cont = 0
val n = 0.5
while(true){
cont = 0
for (j <- elements.indices if elements(j) < n) {
cont += 1
}
println(cont)
}
with filter
val size = 10000000
val elements = Array.ofDim[Double](size)
for (i <- elements.indices) {
elements.update(i, math.random)
}
val n = 0.5
while(true){
val valid = elements.filter(x => x < n)
println(valid.size)
}
and I checked the memory usage with VisualVM, it seem that the first solution uses less memory than the second one.
This is the memory used by the first solution
This is the memory used by the second solution
The for expression use the withFilter method rather than filter, which avoids creating the intermediate Array. So either use the for version or use withFilter rather than filter.
Kotlin has many shorthands and interesting features. So, I wonder if there is some fast and short way of converting array of string to array of integers. Similar to this code in Python:
results = [int(i) for i in results]
You can use .map { ... } with .toInt() or .toIntOrNull():
val result = strings.map { it.toInt() }
Only the result is not an array but a list. It is preferable to use lists over arrays in non-performance-critical code, see the differences.
If you need an array, add .toTypedArray() or .toIntArray().
I'd use something simple like
val strings = arrayOf("1", "2", "3")
val ints = ints.map { it.toInt() }.toTypedArray()
Alternatively, if you're into extensions:
fun Array<String>.asInts() = this.map { it.toInt() }.toTypedArray()
strings.asInts()
If you are trying to convert a List structure that implements RandomAccess (like ArrayList, or Array), you can use this version for better performance:
IntArray(strings.size) { strings[it].toInt() }
This version is compiled to a basic for loop and int[]:
int size = strings.size();
int[] result = new int[size];
int index = 0;
for(int newLength = result.length; index < newLength; ++index) {
String numberRaw = strings.get(index);
int parsedNumber = Integer.parseInt(numberRaw);
result[index] = parsedNumber;
}
If you use Array.map as other answers suggest, you get back a List, not an Array. If you want to map an array strings to another array results, you can do it directly like this:
val results = Array(strings.size) { strings[it].toInt() }
This is more efficient than first mapping to a List and then copying the elements over to an Array by calling .toTypedArray().
Consider the input like this "" (empty string)
It would be better to do the filtering first. And it is true the return value is list but not array.
If you need an array, add .toTypedArray() or .toIntArray().
fun stringToIntList(data: String): List<Int> =
data.split(",").filter { it.toIntOrNull() != null }
.map { it.toInt() }
val result = "[1, 2, 3, 4, 5]".removeSurrounding("[","]").replace(" ","").split(",").map { it.toInt() }
Found following simplest
strings.chars().toArray()
So say I have an array:
var stringArray = ["a","b","c","d","e","f","g","h","i","j"]
Now, how do I delete "a", "c", "e", "g", and "i" (all the even number indexes from the array)?
Thanks!
Instead of using C-style for-loops (which are set to be deprecated in an upcoming version of Swift), you could accomplish this using strides:
var result = [String]()
for i in stride(from: 1, through: stringArray.count - 1, by: 2) {
result.append(stringArray[i])
}
Or for an even more functional solution,
let result = stride(from: 1, to: stringArray.count - 1, by: 2).map { stringArray[$0] }
Traditional
var filteredArray = []
for var i = 1; i < stringArray.count; i = i + 2 {
filteredArray.append(stringArray[i])
}
Functional alternative
var result = stringArray.enumerate().filter({ index, _ in
index % 2 != 0
}).map { $0.1 }
enumerate takes a array of elements and returns an array of tuples where each tuple is an index-array pair (e.g. (.0 3, .1 "d")). We then remove the elements that are odd using the modulus operator. Finally, we convert the tuple array back to a normal array using map. HTH
There are a bunch of different ways to accomplish this, but here are a couple that I found interesting:
Using flatMap() on indices:
let result: [String] = stringArray.indices.flatMap {
if $0 % 2 != 0 { return stringArray[$0] }
else { return nil }
}
Note: result needs to be defined as a [String] otherwise the compiler doesn't know which version of flatMap() to use.
Or, if you want to modify the original array in place:
stringArray.indices.reverse().forEach {
if $0 % 2 == 0 { stringArray.removeAtIndex($0) }
}
In this case you have to call reverse() on indices first so that they're enumerated in reverse order. Otherwise by the time you get to the end of the array you'll be attempting to remove an index that doesn't exist anymore.
Swift 4.2
A function accepting generics and producing reduced result
func stripElements<T>(in array:[T]) -> [T] {
return array.enumerated().filter { (arg0) -> Bool in
let (offset, _) = arg0
return offset % 2 != 0
}.map { $0.element }
}
Given some arrays in Kotlin
let a = arrayOf("first", "second")
val b = arrayOf("first", "second")
val c = arrayOf("1st", "2nd")
Are there built-in functions to the Kotlin std-lib that tests two arrays for (value) equality for each element?
Thus resulting in:
a.equals(b) // true
a.equals(c) // false
equals() is actually returning false in both cases, but maybe there are built-in functions to Kotlin that one could use?
There is the static function java.utils.Arrays.deepEquals(a.toTypedArray(), b.toTypedArray()) but I would rather prefer an instance method as it would work better with optionals.
In Kotlin 1.1 you can use contentEquals and contentDeepEquals to compare two arrays for structural equality. e.g.:
a contentEquals b // true
b contentEquals c // false
In Kotlin 1.0 there are no "built-in functions to the Kotlin std-lib that tests two arrays for (value) equality for each element."
"Arrays are always compared using equals(), as all other objects" (Feedback Request: Limitations on Data Classes | Kotlin Blog).
So a.equals(b) will only return true if a and b reference the same array.
You can, however, create your own "optionals"-friendly methods using extension functions. e.g.:
fun Array<*>.equalsArray(other: Array<*>) = Arrays.equals(this, other)
fun Array<*>.deepEqualsArray(other: Array<*>) = Arrays.deepEquals(this, other)
P.S. The comments on Feedback Request: Limitations on Data Classes | Kotlin Blog are worth a read as well, specifically comment 39364.
Kotlin 1.1 introduced extensions for comparing arrays by content:
contentEquals and contentDeepEquals.
These extensions are infix, so you can use them the following way:
val areEqual = arr1 contentEquals arr2
And if you want to compare contents of two Collections ignoring the order you can add this simple extension:
infix fun <T> Collection<T>.sameContentWith(collection: Collection<T>?)
= collection?.let { this.size == it.size && this.containsAll(it) }
...and use it like this:
a = mutableListOf<String>()
b = mutableListOf<String>()
isListsHasSameContent = a sameContentWith b
For a simple equals (not deep equals!):
otherArray.size == array.size && otherArray.filter { !array.contains(it) }.isEmpty()
This code will compare the size and the items. The items are compared with .equals().
In koltin if your array or ArrayList is type of a data Class you can simply compare array :a and array :b
like this
if(a == b)
it will return simple boolean if it matched all the value of both arrays, but if you are matching other-than data data Class then you can use this extension to compare it with single value
fun <T> Array<T>.isEqual(comparable: Array<T>): Boolean {
var isChanged = true
if (this.size == comparable.size) {
for (index in 0 until comparable.size) {
if (this[index] != comparable[index]) {
isChanged = false
break
}
}
} else {
isChanged = false
}
return isChanged
}
then you can use your array
val a = arrayOf("first", "second")
val b = arrayOf("first", "second")
println(a.isEqual(b)) //true
a[0] = "third"
println(a.isEqual(b)) //false