How to get columns of two dimesional array using functional style in kotlin - kotlin

For example I have such a simple task: count all raws and columns which have all zeros in 2D array.
So for
0 0 0
0 0 0
1 0 1
answer is 2 raws and 1 column.
I can make it just like that for raws: var cntRaws = a.count { it.all { el -> el == 0 } }, but how to solve it for columns in same way?


val x = Array<IntArray>(3) { IntArray(3) { 0 } }
x[2][0] = 1
x[2][2] = 1
val raws = x.count { it.sum() == 0 }
val columns = (x.indices)
.map { columnIndex -> x.map { it[columnIndex] } }
.count { it.sum() == 0 }
println("total raws:$raws")
println("total col:$columns")


I couldn't think of any idomatic / functional style to do this, but I came up with an idea just create a lazily evaluated swapping function that swaps the columns with rows when it wants to pull without creating a new list.
fun <T> List<List<T>>.swapped() = sequence {
var index = 0
while (index < size) {
yield(map { it[index] })
index++
}
}
fun main() {
val list = listOf(
listOf(0, 0, 0),
listOf(0, 0, 0),
listOf(1, 0, 1)
)
val cntRows = list.count { it.all { el -> el == 0 } }
val cntCols = list.swapped().count { it.all { el -> el == 0 } }
println("cntRows: $cntRows")
println("cntCols: $cntCols")
}
I tried my best to optimize it and do it in same O(n*m) steps as done with the regular row counting, since Sequences are lazily evaluated.


This is a functional way of doing it that works if all rows are the same size:
fun <T>List<List<T>>.rowToColumn() = (0 until first().size).map{row -> (0 until size).map {col-> this[col][row] }}

Related

Can someone explain why the below Kotlin code produces incorrect expected result?

Can someone explain why the below code produces [2,3,5,6,7,8,9,10,11,12]?
I know it has something to do with filter function is deferred to the last element but I don't see the picture. It would even be better if you can visualise it. Thank you so much.
val primes: Sequence<Int> = sequence {
var numbers = generateSequence(2) { it + 1 }
var prime: Int
while (true) {
prime = numbers.first()
yield(prime)
numbers = numbers.drop(1).filter { it % prime != 0 }
}
}
print(primes.take(10).toList())

it's because you change prime variable in filter closure. For example, on the second step you have numbers as .filter { it % prime != 0 }.filter { it % prime != 0 } but the prime is one variable, and it is equal to 3
Correct version:
val primes: Sequence<Int> = sequence {
var numbers = generateSequence(2) { it + 1 }
while (true) {
val prime = numbers.first()
yield(prime)
numbers = numbers.drop(1).filter { it % prime != 0 }
}
}
print(primes.take(10).toList())

problems with index of array

I'm writing a function that allows you to remove certain numbers from an int arraylist.
My code
for (i in 1 until 4) {
divider = setDivider(i)
for(index in 0 until numbers.size){
if(index <= numbers.size){
if (numbers[index] % divider == 0 && !isDone) {
numbers.removeAt(index)
}
}else{
isDone = true
}
}
if(isDone)
break
}
the function to set the divider
fun setDivider(divider: Int): Int {
when (divider) {
1 -> return 2
2 -> return 3
3 -> return 5
4 -> return 7
}
return 8
}
I do not know why the ide is giving me the error Index 9 out of bounds for length 9.

Author explained in the comments that the goal is to remove all numbers that are divisible by 2, 3, 5 and 7.
It can be achieved much easier by utilizing ready to use functions from stdlib:
val dividers = listOf(2, 3, 5, 7)
numbers.removeAll { num ->
dividers.any { num % it == 0 }
}
It removes elements that satisfy the provided condition (is divisible) for any of provided dividers.
Also, it is often cleaner to not modify a collection in-place, but to create an entirely new collection:
val numbers2 = numbers.filterNot { num ->
dividers.any { num % it == 0 }
}

A very basic exercise help-----Kotlin

Im trying to do this exercise
https://www.hackerrank.com/challenges/compare-the-triplets/problem?h_r=next-challenge&h_v=zen
I already wrote the code but the result is not right and for my eyes its all good
Could somebody pls tell me whats wrong??
thx
import java.util.Scanner
fun main(){
var loop = 0
var score = Array<Int>(2){0}
val reader = Scanner(System.`in`)
var alice:String = readLine().toString()
var bob:String = readLine().toString()
val numerosa: List<String> = alice.split(" ")
val numerosb:List<String> = bob.split(" ")
for(a in 3..3) {
when (numerosa[loop].toInt()) {
in numerosb[loop].toInt() + 1..100 -> score[0] += 1
in numerosb[loop].toInt() - 1..0 -> score[1] += 1
}
loop += 1
}
println("${score[0]} ${score[1]}")
}

You could do it something like this, you have multiple variables which were not required so I cleaned up the code.
val score = Array(2) { 0 }
val aliceNumbers = readLine()!!.split(" ").map(String::toInt)
val bobNumbers = readLine()!!.split(" ").map(String::toInt)
require(aliceNumbers.size == 3 && bobNumbers.size == 3) { "There must be 3 numbers for each" }
require(!aliceNumbers.any { it !in 1..100 } || !bobNumbers.any { it !in 1..100 }) { "Numbers must be in range 1 to 100" }
for (a in 0..2) {
if(aliceNumbers[a] > bobNumbers[a]) score[0] += 1
if(aliceNumbers[a] < bobNumbers[a]) score[1] += 1
}
println("${score[0]} ${score[1]}")

Kotlin decomposing numbers into powers of 2

Hi I am writing an app in kotlin and need to decompose a number into powers of 2.
I have already done this in c#, PHP and swift but kotlin works differently somehow.
having researched this I believe it is something to do with the numbers in my code going negative somewhere and that the solution lies in declaring one or more of the variable as "Long" to prevent this from happening but i have not been able to figure out how to do this.
here is my code:
var salads = StringBuilder()
var value = 127
var j=0
while (j < 256) {
var mask = 1 shl j
if(value != 0 && mask != 0) {
salads.append(mask)
salads.append(",")
}
j += 1
}
// salads = (salads.dropLast()) // removes the final ","
println("Salads = $salads")
This shoud output the following:
1,2,4,8,16,32,64
What I actually get is:
1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,
Any ideas?

This works for the one input that you specified, at the very least:
fun powersOfTwo(value :Long): String {
val result = ArrayList<String>()
var i = 0
var lastMask = 0
while (lastMask < value) {
val mask = 1 shl i
if (value != 0.toLong() && mask < value) {
result.add(mask.toString())
}
lastMask = mask
i += 1
}
return result.joinToString(",")
}
Ran it in a unit test:
#Test
fun addition_isCorrect() {
val result = powersOfTwo(127)
assertEquals("1,2,4,8,16,32,64", result)
}
Test passed.

You can get a list of all powers of two that fit in Int and test each of them for whether the value contains it with the infix function and:
val value = 126
val powersOfTwo = (0 until Int.SIZE_BITS).map { n -> 1 shl n }
println(powersOfTwo.filter { p -> value and p != 0}.joinToString(","))
// prints: 2,4,8,16,32,64
See the entire code in Kotlin playground: https://pl.kotl.in/f4CZtmCyI

Hi I finally managed to get this working properly:
fun decomposeByTwo(value :Int): String {
val result = ArrayList<String>()
var value = value
var j = 0
while (j < 256) {
var mask = 1 shl j
if ((value and mask) != 0) {
value -= mask
result.add(mask.toString())
}
j += 1
}
return result.toString()
}
I hope this helps someone trying to get a handle on bitwise options!

Somehow you want to do the "bitwise AND" of "value" and "mask" to determine if the j-th bit of "value" is set. I think you just forgot that test in your kotlin implementation.

Pass by value. Array

I have two arrays. But when I change second - first change too.
I tried
.clone()
.copyOf()
but it didn't work for me.
object MatrixObject {
var table: Array<Array<Int>>? = null
fun randOf(n: Int) {
table= Array(n, { Array(n, { Random().nextInt(100 - 0) + 0 }) })
}
var tableF: Array<Array<Int>>? = null
get() {
if (field==null)
factorization()
return field
}
fun factorization() {
tableF = table!!
... //here I change elements of tableF
}
}
I tried
for(row in 0 until table!!.size)
tableF!![row] = Arrays.copyOf(table!![row], table!![row].size)
and
for(row in 0 until table!!.size)
tableF!![row] = table!![row].clone() // and copyOf()
but it still doesn't work.

I found the solution.I initialized the array.
tableF= Array(table!!.size, { Array(table!!.size, {0}) })
for(row in 0 until table!!.size)
tableF!![row] = table!![row].clone()