I want to sum adjacent elements in scala and I'm not sure how to deal with the last element.
So I have a list:
val x = List(1,2,3,4)
And I want to sum adjacent elements using indices and map:
val size = x.indices.size
val y = x.indices.map(i =>
if (i < size - 1)
x(i) + x(i+1))
The problem is that this approach creates an AnyVal elemnt at the end:
res1: scala.collection.immutable.IndexedSeq[AnyVal] = Vector(3, 5, 7, ())
and if I try to sum the elements or another numeric method of the collection, it doesn't work:
error: could not find implicit value for parameter num: Numeric[AnyVal]
I tried to filter out the element using:
y diff List(Unit) or y diff List(AnyVal)
but it doesn't work.
Is there a better approach in scala to do this type of adjacent sum without using a foor loop?
For a more functional solution, you can use sliding to group the elements together in twos (or any number of them), then map to their sum.
scala> List(1, 2, 3, 4).sliding(2).map(_.sum).toList
res80: List[Int] = List(3, 5, 7)
What sliding(2) will do is create an intermediate iterator of lists like this:
Iterator(
List(1, 2),
List(2, 3),
List(3, 4)
)
So when we chain map(_.sum), we will map each inner List to it's own sum. toList will convert the Iterator back into a List.
You can try pattern matching and tail recursion also.
import scala.annotation.tailrec
#tailrec
def f(l:List[Int],r :List[Int]=Nil):List[Int] = {
l match {
case x :: xs :: xss =>
f(l.tail, r :+ (x + xs))
case _ => r
}
}
scala> f(List(1,2,3,4))
res4: List[Int] = List(3, 5, 7)
With a for comprehension by zipping two lists, the second with the first item dropped,
for ( (a,b) <- x zip x.drop(1) ) yield a+b
which results in
List(3, 5, 7)
I have a Scala List that contains some repeated numbers. I want to count the number of times a specific number will repeat itself. For example:
val list = List(1,2,3,3,4,2,8,4,3,3,5)
val repeats = list.takeWhile(_ == List(3,3)).size
And the val repeats would equal 2.
Obviously the above is pseudo-code and takeWhile will not find two repeated 3s since _ represents an integer. I tried mixing both takeWhile and take(2) but with little success. I also referred code from How to find count of repeatable elements in scala list but it appears the author is looking to achieve something different.
Thanks for your help.
This will work in this case:
val repeats = list.sliding(2).count(_.forall(_ == 3))
The sliding(2) method gives you an iterator of lists of elements and successors and then we just count where these two are equal to 3.
Question is if it creates the correct result to List(3, 3, 3)? Do you want that to be 2 or just 1 repeat.
val repeats = list.sliding(2).toList.count(_==List(3,3))
and more generally the following code returns tuples of element and repeats value for all elements:
scala> list.distinct.map(x=>(x,list.sliding(2).toList.count(_.forall(_==x))))
res27: List[(Int, Int)] = List((1,0), (2,0), (3,2), (4,0), (8,0), (5,0))
which means that the element '3' repeats 2 times consecutively at 2 places and all others 0 times.
and also if we want element repeats 3 times consecutively we just need to modify the code as follows:
list.distinct.map(x=>(x,list.sliding(3).toList.count(_.forall(_==x))))
in SCALA REPL:
scala> val list = List(1,2,3,3,3,4,2,8,4,3,3,3,5)
list: List[Int] = List(1, 2, 3, 3, 3, 4, 2, 8, 4, 3, 3, 3, 5)
scala> list.distinct.map(x=>(x,list.sliding(3).toList.count(_==List(x,x,x))))
res29: List[(Int, Int)] = List((1,0), (2,0), (3,2), (4,0), (8,0), (5,0))
Even sliding value can be varied by defining a function as:
def repeatsByTimes(list:List[Int],n:Int) =
list.distinct.map(x=>(x,list.sliding(n).toList.count(_.forall(_==x))))
Now in REPL:
scala> val list = List(1,2,3,3,4,2,8,4,3,3,5)
list: List[Int] = List(1, 2, 3, 3, 4, 2, 8, 4, 3, 3, 5)
scala> repeatsByTimes(list,2)
res33: List[(Int, Int)] = List((1,0), (2,0), (3,2), (4,0), (8,0), (5,0))
scala> val list = List(1,2,3,3,3,4,2,8,4,3,3,3,2,4,3,3,3,5)
list: List[Int] = List(1, 2, 3, 3, 3, 4, 2, 8, 4, 3, 3, 3, 2, 4, 3, 3, 3, 5)
scala> repeatsByTimes(list,3)
res34: List[(Int, Int)] = List((1,0), (2,0), (3,3), (4,0), (8,0), (5,0))
scala>
We can go still further like given a list of integers and given a maximum number
of consecutive repetitions that any of the element can occur in the list, we may need a list of 3-tuples representing (the element, number of repetitions of this element, at how many places this repetition occurred). this is more exhaustive information than the above. Can be achieved by writing a function like this:
def repeats(list:List[Int],maxRep:Int) =
{ var v:List[(Int,Int,Int)] = List();
for(i<- 1 to maxRep)
v = v ++ list.distinct.map(x=>
(x,i,list.sliding(i).toList.count(_.forall(_==x))))
v.sortBy(_._1) }
in SCALA REPL:
scala> val list = List(1,2,3,3,3,4,2,8,4,3,3,3,2,4,3,3,3,5)
list: List[Int] = List(1, 2, 3, 3, 3, 4, 2, 8, 4, 3, 3, 3, 2, 4, 3, 3, 3, 5)
scala> repeats(list,3)
res38: List[(Int, Int, Int)] = List((1,1,1), (1,2,0), (1,3,0), (2,1,3),
(2,2,0), (2,3,0), (3,1,9), (3,2,6), (3,3,3), (4,1,3), (4,2,0), (4,3,0),
(5,1,1), (5,2,0), (5,3,0), (8,1,1), (8,2,0), (8,3,0))
scala>
These results can be understood as follows:
1 times the element '1' occurred at 1 places.
2 times the element '1' occurred at 0 places.
............................................
............................................
.............................................
2 times the element '3' occurred at 6 places..
.............................................
3 times the element '3' occurred at 3 places...
............................................and so on.
Thanks to Luigi Plinge I was able to use methods in run-length encoding to group together items in a list that repeat. I used some snippets from this page here: http://aperiodic.net/phil/scala/s-99/
var n = 0
runLengthEncode(totalFrequencies).foreach{ o =>
if(o._1 > 1 && o._2==subjectNumber) n+=1
}
n
The method runLengthEncode is as follows:
private def pack[A](ls: List[A]): List[List[A]] = {
if (ls.isEmpty) List(List())
else {
val (packed, next) = ls span { _ == ls.head }
if (next == Nil) List(packed)
else packed :: pack(next)
}
}
private def runLengthEncode[A](ls: List[A]): List[(Int, A)] =
pack(ls) map { e => (e.length, e.head) }
I'm not entirely satisfied that I needed to use the mutable var n to count the number of occurrences but it did the trick. This will count the number of times a number repeats itself no matter how many times it is repeated.
If you knew your list was not very long you could do it with Strings.
val list = List(1,2,3,3,4,2,8,4,3,3,5)
val matchList = List(3,3)
(matchList.mkString(",")).r.findAllMatchIn(list.mkString(",")).length
From you pseudocode I got this working:
val pairs = list.sliding(2).toList //create pairs of consecutive elements
val result = pairs.groupBy(x => x).map{ case(x,y) => (x,y.size); //group pairs and retain the size, which is the number of occurrences.
result will be a Map[List[Int], Int] so you can the count number like:
result(List(3,3)) // will return 2
I couldn't understand if you also want to check lists of several sizes, then you would need to change the parameter to sliding to the desired size.
def pack[A](ls: List[A]): List[List[A]] = {
if (ls.isEmpty) List(List())
else {
val (packed, next) = ls span { _ == ls.head }
if (next == Nil) List(packed)
else packed :: pack(next)
}
}
def encode[A](ls: List[A]): List[(Int, A)] = pack(ls) map { e => (e.length, e.head) }
val numberOfNs = list.distinct.map{ n =>
(n -> list.count(_ == n))
}.toMap
val runLengthPerN = runLengthEncode(list).map{ t => t._2 -> t._1}.toMap
val nRepeatedMostInSuccession = runLengthPerN.toList.sortWith(_._2 <= _._2).head._1
Where runLength is defined as below from scala's 99 problems problem 9 and scala's 99 problems problem 10.
Since numberOfNs and runLengthPerN are Maps, you can get the population count of any number in the list with numberOfNs(number) and the length of the longest repitition in succession with runLengthPerN(number). To get the runLength, just compute as above with runLength(list).map{ t => t._2 -> t._1 }.
For a Scala List[Int] I can call the method max to find the maximum element value.
How can I find the index of the maximum element?
This is what I am doing now:
val max = list.max
val index = list.indexOf(max)
One way to do this is to zip the list with its indices, find the resulting pair with the largest first element, and return the second element of that pair:
scala> List(0, 43, 1, 34, 10).zipWithIndex.maxBy(_._1)._2
res0: Int = 1
This isn't the most efficient way to solve the problem, but it's idiomatic and clear.
Since Seq is a function in Scala, the following code works:
list.indices.maxBy(list)
even easier to read would be:
val g = List(0, 43, 1, 34, 10)
val g_index=g.indexOf(g.max)
def maxIndex[ T <% Ordered[T] ] (list : List[T]) : Option[Int] = list match {
case Nil => None
case head::tail => Some(
tail.foldLeft((0, head, 1)){
case ((indexOfMaximum, maximum, index), elem) =>
if(elem > maximum) (index, elem, index + 1)
else (indexOfMaximum, maximum, index + 1)
}._1
)
} //> maxIndex: [T](list: List[T])(implicit evidence$2: T => Ordered[T])Option[Int]
maxIndex(Nil) //> res0: Option[Int] = None
maxIndex(List(1,2,3,4,3)) //> res1: Option[Int] = Some(3)
maxIndex(List("a","x","c","d","e")) //> res2: Option[Int] = Some(1)
maxIndex(Nil).getOrElse(-1) //> res3: Int = -1
maxIndex(List(1,2,3,4,3)).getOrElse(-1) //> res4: Int = 3
maxIndex(List(1,2,2,1)).getOrElse(-1) //> res5: Int = 1
In case there are multiple maximums, it returns the first one's index.
Pros:You can use this with multiple types, it goes through the list only once, you can supply a default index instead of getting exception for empty lists.
Cons:Maybe you prefer exceptions :) Not a one-liner.
I think most of the solutions presented here go thru the list twice (or average 1.5 times) -- Once for max and the other for the max position. Perhaps a lot of focus is on what looks pretty?
In order to go thru a non empty list just once, the following can be tried:
list.foldLeft((0, Int.MinValue, -1)) {
case ((i, max, maxloc), v) =>
if (v > max) (i + 1, v, i)
else (i + 1, max, maxloc)}._3
Pimp my library! :)
class AwesomeList(list: List[Int]) {
def getMaxIndex: Int = {
val max = list.max
list.indexOf(max)
}
}
implicit def makeAwesomeList(xs: List[Int]) = new AwesomeList(xs)
//> makeAwesomeList: (xs: List[Int])scalaconsole.scratchie1.AwesomeList
//Now we can do this:
List(4,2,7,1,5,6) getMaxIndex //> res0: Int = 2
//And also this:
val myList = List(4,2,7,1,5,6) //> myList : List[Int] = List(4, 2, 7, 1, 5, 6)
myList getMaxIndex //> res1: Int = 2
//Regular list methods also work
myList filter (_%2==0) //> res2: List[Int] = List(4, 2, 6)
More details about this pattern here: http://www.artima.com/weblogs/viewpost.jsp?thread=179766
How do you replace an element by index with an immutable List.
E.g.
val list = 1 :: 2 ::3 :: 4 :: List()
list.replace(2, 5)
If you want to replace index 2, then
list.updated(2,5) // Gives 1 :: 2 :: 5 :: 4 :: Nil
If you want to find every place where there's a 2 and put a 5 in instead,
list.map { case 2 => 5; case x => x } // 1 :: 5 :: 3 :: 4 :: Nil
In both cases, you're not really "replacing", you're returning a new list that has a different element(s) at that (those) position(s).
In addition to what has been said before, you can use patch function that replaces sub-sequences of a sequence:
scala> val list = List(1, 2, 3, 4)
list: List[Int] = List(1, 2, 3, 4)
scala> list.patch(2, Seq(5), 1) // replaces one element of the initial sequence
res0: List[Int] = List(1, 2, 5, 4)
scala> list.patch(2, Seq(5), 2) // replaces two elements of the initial sequence
res1: List[Int] = List(1, 2, 5)
scala> list.patch(2, Seq(5), 0) // adds a new element
res2: List[Int] = List(1, 2, 5, 3, 4)
You can use list.updated(2,5) (which is a method on Seq).
It's probably better to use a scala.collection.immutable.Vector for this purpose, becuase updates on Vector take (I think) constant time.
You can use map to generate a new list , like this :
# list
res20: List[Int] = List(1, 2, 3, 4, 4, 5, 4)
# list.map(e => if(e==4) 0 else e)
res21: List[Int] = List(1, 2, 3, 0, 0, 5, 0)
It can also be achieved using patch function as
scala> var l = List(11,20,24,31,35)
l: List[Int] = List(11, 20, 24, 31, 35)
scala> l.patch(2,List(27),1)
res35: List[Int] = List(11, 20, 27, 31, 35)
where 2 is the position where we are looking to add the value, List(27) is the value we are adding to the list and 1 is the number of elements to be replaced from the original list.
If you do a lot of such replacements, it is better to use a muttable class or Array.
following is a simple example of String replacement in scala List, you can do similar for other types of data
scala> val original: List[String] = List("a","b")
original: List[String] = List(a, b)
scala> val replace = original.map(x => if(x.equals("a")) "c" else x)
replace: List[String] = List(c, b)