Related
Suppose I have a list filled with zeroes
val a = List(0,0,0,0,2,4,0,6,0,7)
I want to slice away the zeroes preceding the first non-zero element and also return the index where the 1st non-zero element is present.
Foe the above case I want an output:
output = List(2,4,0,6,0,7)
idx = 4
How do I do it?
First, you can use zipWithIndex to conveniently pair each element with its index. Then use dropWhile to return all of the preceding zero elements. From there, you'll have all of the remaining elements paired with their indices from the original List. You can unzip them. Since this may result in an empty list, the index you're looking for should be optional.
scala> val (remaining, indices) = a.zipWithIndex.dropWhile { case (a, i) => a == 0 }.unzip
remaining: List[Int] = List(2, 4, 0, 6, 0, 7) // <--- The list you want
indices: List[Int] = List(4, 5, 6, 7, 8, 9)
scala> val index = indices.headOption
index: Option[Int] = Some(4) // <--- the index of the first non-zero element
This is a use-case for span:
val a = List(0,0,0,0,2,4,0,6,0,7)
val (zeros, output) = a.span(_ == 0)
val idx = zeros.length
use dropWhile:
val output = a.dropWhile{ _ == 0 }
val idx = output.headOption
.map(_ => a.length - output.length)
.getOrElse(-1) // index starting from 0, -1 if not found
Sightly modified from #bottaio answer, but returning an Option[Int] instead of a plain Int for the index.
def firstNonZero(l: List[Int]): (Option[Int], List[Int]) = {
#annotation.tailrec
def go(remaining: List[Int], idx: Int): (Int, List[Int]) =
remaining match {
case Nil => idx -> Nil
case 0 :: xs => go(remaining = xs, idx + 1)
case xs => idx -> xs
}
l match {
case 0 :: xs =>
val (idx, list) = go(remaining = xs, idx = 1)
Some(idx) -> list
case list =>
None -> list
}
}
Just another solution using foldLeft:
val (i, l) =
a.foldLeft((None: Option[Int], List.empty: List[Int]))((b, n) => {
if (n == 0 && b._2.isEmpty) (b._1.orElse(Some(0)).map(_ + 1), List.empty)
else (b._1.orElse(Some(0)), b._2 :+ n)
})
i: Option[Int] = Some(4)
l: List[Int] = List(2, 4, 0, 6, 0, 7)
You can do it pretty clean with indexWhere:
val idx = a.indexWhere(_!=0)
val output = a.drop(idx)
Others have provided answers that requires multiple list traversals. You can write a recursive function to calculate that in a single pass:
def firstNonZero(l: List[Int]): (Int, List[Int]) = {
#tailrec
def go(l: List[Int], idx: Int): (Int, List[Int]) = l match {
case Nil => (idx, Nil)
case 0 :: xs => go(xs, idx + 1)
case xs => (idx, xs)
}
go(l, 0)
}
what is also equivalent to
val (leadingZeros, rest) = a.span(_ == 0)
val (index, output) = (leadingZeros.length, rest)
I'm trying to roll a list, for example like so:
val notRolled = (1 to 5).toList // List(1,2,3,4,5)
val rolledBy1 = rollList(notRolled,1) // List(2,3,4,5,1)
val rolledBy2 = rollList(notRolled,2) // List(3,4,5,1,2)
val rolledBy3 = rollList(notRolled,3) // List(4,5,1,2,3)
val rolledBy4 = rollList(notRolled,4) // List(5,1,2,3,4)
val rolledBy5 = rollList(notRolled,5) // List(1,2,3,4,5)
val rolledByM1 = rollList(notRolled,-1) // List(5,1,2,3,4)
val rolledByM2 = rollList(notRolled,-2) // List(4,5,1,2,3)
val rolledByM3 = rollList(notRolled,-3) // List(3,4,5,1,2)
val rolledByM4 = rollList(notRolled,-4) // List(2,3,4,5,1)
val rolledByM5 = rollList(notRolled,-5) // List(1,2,3,4,5)
I had a look at scala-lang.org and can't seem to find anything that matches my requirement.
Is there a built-in method for rolling a list?
If not, is there a more efficient way to do it than this:
def rollList[T](theList: List[T], itemsToRoll: Int): List[T] = {
val split = {
if (itemsToRoll < 0) (theList.size + itemsToRoll % theList.size)
else itemsToRoll % theList.size
}
val (beginning, end) = theList.splitAt(split)
end ::: beginning
}
Using the enrich-my-library pattern will allow you to add the roll method directly to all the collections so that you can call myList.roll(2), as opposed to roll(myList, 1).
Using the generic CanBuildFrom pattern allows you to make roll return the best possible type on these collections (so that roll on a List will return a List, but roll on an Iterable will return an Iterable).
All together, here is one option that abides by these patterns:
import scala.collection.generic.CanBuildFrom
import scala.collection.TraversableLike
implicit class TraversableWithRoll[A, Repr <: Traversable[A]](val xs: TraversableLike[A, Repr]) extends AnyVal {
def roll[That](by: Int)(implicit bf: CanBuildFrom[Repr, A, That]): That = {
val builder = bf()
val size = xs.size
builder.sizeHint(xs)
val leftBy = if (size == 0) 0 else ((by % size) + size) % size
builder ++= xs.drop(leftBy)
builder ++= xs.take(leftBy)
builder.result
}
}
This allows you to do some of the following:
List(1, 2, 3, 4, 5).roll(2) //List(4,5,1,2,3)
Seq(3, 4, 5).roll(-1) //Seq(5, 3, 4)
Notice the benefits of the fluent syntax enabled by the enrich-my-library pattern, and the stronger types enabled by the use of the CanBuildFrom implicit .
def rotate[A] (list: List[A], by: Int) = {
if (list.isEmpty) list
else {
val shift = (by % list.size + list.size) % list.size
val (l, r) = list.splitAt(shift)
r ++ l
}
}
val list = List(1, 2, 3, 4, 5)
rotate(list, 1) // List(2, 3, 4, 5, 1)
rotate(list, 2) // List(3, 4, 5, 1, 2)
rotate(list, -1) // List(5, 1, 2, 3, 4)
rotate(list, -2) // List(4, 5, 1, 2, 3)
rotate(list, -9) // List(2, 3, 4, 5, 1)
Here is one way of simulating a circular list in a functional way. I made it so that a positive i will do a right shift and a negative a left shift. This is the reverse of what the question has, but I felt it's more natural.
def shift[T](c: List[T], i: Int) = {
if(c.isEmpty) c
else {
val (h,t) = if (i >= 0) c.splitAt(c.size - i%c.size)
else c.splitAt(- i%c.size)
t ++ h
}
}
Here are some examples in the REPL:
cala> shift(List(1,2,4), 1)
res: List[Int] = List(4, 1, 2)
scala> shift(List(1,2,4), 2)
res: List[Int] = List(2, 4, 1)
scala> shift(List(1,2,4), 3)
res: List[Int] = List(1, 2, 4)
scala> shift(List(1,2,4), 4)
res: List[Int] = List(4, 1, 2)
scala> shift(List(1,2,4), 5)
res: List[Int] = List(2, 4, 1)
scala> shift(List(1,2,4), 6)
res: List[Int] = List(1, 2, 4)
scala> shift(List(1,2,4), -1)
res60: List[Int] = List(2, 4, 1)
scala> shift(List(1,2,4), -2)
res: List[Int] = List(4, 1, 2)
scala> shift(List(1,2,4), -3)
res: List[Int] = List(1, 2, 4)
scala> shift(List(1,2,4), -4)
res: List[Int] = List(2, 4, 1)
scala> shift(List(1,2,4), -5)
res: List[Int] = List(4, 1, 2)
scala> shift(List(1,2,4), -6)
res: List[Int] = List(1, 2, 4)
This is my pure functional very simple proposition:
def circ[A]( L: List[A], times: Int ): List[A] = {
if ( times == 0 || L.size < 2 ) L
else circ(L.drop(1) :+ L.head , times-1)
}
val G = List("1a","2b","3c")
println( circ(G,1) ) //List(2b, 3c, 1a)
println( circ(G,2) ) //List(3c, 1a, 2b)
println( circ(G,3) ) //List(1a, 2b, 3c)
println( circ(G,4) ) //List(2b, 3c, 1a)
Added for times < 0 (rolling backwards):
def circ[A]( L: List[A], times: Int ): List[A] = {
if ( times == 0 || L.size < 2 ) L
else if ( times < 0 ) circ(L.reverse,-times).reverse
else circ(L.drop(1) :+ L.head , times-1)
}
Given:
val notRolled = (1 to 5).toList
Let's say we want rotate with shift = 3
val shift = 3
notRolled.zipWithIndex.groupBy(_._2 < shift).values.flatten.map(_._1)
For negative values we should add 5(lists size), so for shift = -3 we will calculate
val shift = -3
notRolled.zipWithIndex.groupBy(_._2 < 2).values.flatten.map(_._1)
I'm going to incorporate the ideas above, about CanBuildFrom, just as soon as I understand Scala better (!), but in the meantime, I had to do precisely this for the sake of writing a Burrows-Wheeler Transform algorithm. Here, with no take() or drop(), and no errors on empty collections (and, yes, side-effecting code):
def roll[A](xs: Traversable[A]): Iterator[Traversable[A]] = {
var front = xs
var back = collection.mutable.Buffer[A]()
Iterator.continually(front ++ back).takeWhile { _ =>
val done = front.isEmpty
if (!done) {
back :+= front.head
front = front.tail
}
!done
}
}
I have a List
val family=List("1","2","11","12","21","22","31","33","41","44","51","55")
i want to take its first n elements but the problem is that parents size is not fixed.
val familliar=List("1","2","11") //n=3
You can use take
scala> val list = List(1,2,3,4,5,6,7,8,9)
list: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9)
scala> list.take(3)
res0: List[Int] = List(1, 2, 3)
List(1,2,3).take(100) //List(1,2,3)
The signature of take will compare the argument with index, so the incremental index will never more than argument
The signature of take
override def take(n: Int): List[A] = {
val b = new ListBuffer[A]
var i = 0
var these = this
while (!these.isEmpty && i < n) {
i += 1
b += these.head
these = these.tail
}
if (these.isEmpty) this
else b.toList
}
Use take:
val familliar = family.take(3)
How to split a List of elements into lists with at most N items?
ex: Given a list with 7 elements, create groups of 4, leaving the last group possibly with less elements.
split(List(1,2,3,4,5,6,"seven"),4)
=> List(List(1,2,3,4), List(5,6,"seven"))
I think you're looking for grouped. It returns an iterator, but you can convert the result to a list,
scala> List(1,2,3,4,5,6,"seven").grouped(4).toList
res0: List[List[Any]] = List(List(1, 2, 3, 4), List(5, 6, seven))
There is much easier way to do the task using sliding method.
It works this way:
val numbers = List(1, 2, 3, 4, 5, 6 ,7)
Lets say you want to break the list into smaller lists of size 3.
numbers.sliding(3, 3).toList
will give you
List(List(1, 2, 3), List(4, 5, 6), List(7))
Or if you want to make your own:
def split[A](xs: List[A], n: Int): List[List[A]] = {
if (xs.size <= n) xs :: Nil
else (xs take n) :: split(xs drop n, n)
}
Use:
scala> split(List(1,2,3,4,5,6,"seven"), 4)
res15: List[List[Any]] = List(List(1, 2, 3, 4), List(5, 6, seven))
edit: upon reviewing this 2 years later, I wouldn't recommend this implementation since size is O(n), and hence this method is O(n^2), which would explain why the built-in method becomes faster for large lists, as noted in comments below. You could implement efficiently as follows:
def split[A](xs: List[A], n: Int): List[List[A]] =
if (xs.isEmpty) Nil
else (xs take n) :: split(xs drop n, n)
or even (slightly) more efficiently using splitAt:
def split[A](xs: List[A], n: Int): List[List[A]] =
if (xs.isEmpty) Nil
else {
val (ys, zs) = xs.splitAt(n)
ys :: split(zs, n)
}
I am adding a tail recursive version of the split method since there was some discussion of tail-recursion versus recursion. I have used the tailrec annotation to force the compiler to complain in case the implementation is not indeed tail-recusive. Tail-recursion I believe turns into a loop under the hood and thus will not cause problems even for a large list as the stack will not grow indefinitely.
import scala.annotation.tailrec
object ListSplitter {
def split[A](xs: List[A], n: Int): List[List[A]] = {
#tailrec
def splitInner[A](res: List[List[A]], lst: List[A], n: Int) : List[List[A]] = {
if(lst.isEmpty) res
else {
val headList: List[A] = lst.take(n)
val tailList : List[A]= lst.drop(n)
splitInner(headList :: res, tailList, n)
}
}
splitInner(Nil, xs, n).reverse
}
}
object ListSplitterTest extends App {
val res = ListSplitter.split(List(1,2,3,4,5,6,7), 2)
println(res)
}
I think this is the implementation using splitAt instead of take/drop
def split [X] (n:Int, xs:List[X]) : List[List[X]] =
if (xs.size <= n) xs :: Nil
else (xs.splitAt(n)._1) :: split(n,xs.splitAt(n)._2)
I have a List, which may contain elements that will compare as equal. I would like a similar List, but with one element removed. So from (A, B, C, B, D) I would like to be able to "remove" just one B to get e.g. (A, C, B, D). The order of the elements in the result does not matter.
I have working code, written in a Lisp-inspired way in Scala. Is there a more idiomatic way
to do this?
The context is a card game where two decks of standard cards are in play, so there may
be duplicate cards but still played one at a time.
def removeOne(c: Card, left: List[Card], right: List[Card]): List[Card] = {
if (Nil == right) {
return left
}
if (c == right.head) {
return left ::: right.tail
}
return removeOne(c, right.head :: left, right.tail)
}
def removeCard(c: Card, cards: List[Card]): List[Card] = {
return removeOne(c, Nil, cards)
}
I haven't seen this possibility in the answers above, so:
scala> def remove(num: Int, list: List[Int]) = list diff List(num)
remove: (num: Int,list: List[Int])List[Int]
scala> remove(2,List(1,2,3,4,5))
res2: List[Int] = List(1, 3, 4, 5)
Edit:
scala> remove(2,List(2,2,2))
res0: List[Int] = List(2, 2)
Like a charm :-).
You could use the filterNot method.
val data = "test"
list = List("this", "is", "a", "test")
list.filterNot(elm => elm == data)
You could try this:
scala> val (left,right) = List(1,2,3,2,4).span(_ != 2)
left: List[Int] = List(1)
right: List[Int] = List(2, 3, 2, 4)
scala> left ::: right.tail
res7: List[Int] = List(1, 3, 2, 4)
And as method:
def removeInt(i: Int, li: List[Int]) = {
val (left, right) = li.span(_ != i)
left ::: right.drop(1)
}
Unfortunately, the collections hierarchy got itself into a bit of a mess with - on List. For ArrayBuffer it works just like you might hope:
scala> collection.mutable.ArrayBuffer(1,2,3,2,4) - 2
res0: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 3, 2, 4)
but, sadly, List ended up with a filterNot-style implementation and thus does the "wrong thing" and throws a deprecation warning at you (sensible enough, since it is actually filterNoting):
scala> List(1,2,3,2,4) - 2
warning: there were deprecation warnings; re-run with -deprecation for details
res1: List[Int] = List(1, 3, 4)
So arguably the easiest thing to do is convert List into a collection that does this right, and then convert back again:
import collection.mutable.ArrayBuffer._
scala> ((ArrayBuffer() ++ List(1,2,3,2,4)) - 2).toList
res2: List[Int] = List(1, 3, 2, 4)
Alternatively, you could keep the logic of the code you've got but make the style more idiomatic:
def removeInt(i: Int, li: List[Int]) = {
def removeOne(i: Int, left: List[Int], right: List[Int]): List[Int] = right match {
case r :: rest =>
if (r == i) left.reverse ::: rest
else removeOne(i, r :: left, rest)
case Nil => left.reverse
}
removeOne(i, Nil, li)
}
scala> removeInt(2, List(1,2,3,2,4))
res3: List[Int] = List(1, 3, 2, 4)
def removeAtIdx[T](idx: Int, listToRemoveFrom: List[T]): List[T] = {
assert(listToRemoveFrom.length > idx && idx >= 0)
val (left, _ :: right) = listToRemoveFrom.splitAt(idx)
left ++ right
}
How about
def removeCard(c: Card, cards: List[Card]) = {
val (head, tail) = cards span {c!=}
head :::
(tail match {
case x :: xs => xs
case Nil => Nil
})
}
If you see return, there's something wrong.
// throws a MatchError exception if i isn't found in li
def remove[A](i:A, li:List[A]) = {
val (head,_::tail) = li.span(i != _)
head ::: tail
}
As one possible solutions you can find index of the first suitable element and then remove element at this index:
def removeOne(l: List[Card], c: Card) = l indexOf c match {
case -1 => l
case n => (l take n) ++ (l drop (n + 1))
}
Just another thought on how to do this using a fold:
def remove[A](item : A, lst : List[A]) : List[A] = {
lst.:\[List[A]](Nil)((lst, lstItem) =>
if (lstItem == item) lst else lstItem::lst )
}
Generic Tail Recursion Solution:
def removeElement[T](list: List[T], ele: T): List[T] = {
#tailrec
def removeElementHelper(list: List[T],
accumList: List[T] = List[T]()): List[T] = {
if (list.length == 1) {
if (list.head == ele) accumList.reverse
else accumList.reverse ::: list
} else {
list match {
case head :: tail if (head != ele) =>
removeElementHelper(tail, head :: accumList)
case head :: tail if (head == ele) => (accumList.reverse ::: tail)
case _ => accumList
}
}
}
removeElementHelper(list)
}
val list : Array[Int] = Array(6, 5, 3, 1, 8, 7, 2)
val test2 = list.splitAt(list.length / 2)._2
val res = test2.patch(1, Nil, 1)
object HelloWorld {
def main(args: Array[String]) {
var months: List[String] = List("December","November","October","September","August", "July","June","May","April","March","February","January")
println("Deleting the reverse list one by one")
var i = 0
while (i < (months.length)){
println("Deleting "+months.apply(i))
months = (months.drop(1))
}
println(months)
}
}