As I noted in the title, how to compare the element of index N with element of index N+1, if elements compared are exactly the same, yield element only once.
I know I can use toSet, to get a set of unique elements, but this does not help me because, my list can contain duplicated elements but duplicated element can't be the next element in my list.
val ll = List(1, 2, 3, 6, 3, 7, 5, 5, 6, 3)
// Desired output: List(1, 2, 3, 6, 3, 7, 5, 6, 3)
I got a "near working solution" using zipWithIndex.collect, but when I compare inside it, index runs OutOfBounds. I can make this to work if I can use two conditions inside, first check maximum index to be index = (list.size-1) then I can compare list(index) != list(index+1) then yield list(index)
What I have tried without success (because of OutOfBounds), is:
times.zipWithIndex.collect
{
case (element, index)
// index+1 will be incremented out of my list
if (times(index) != times(index+1)) => times(index)
}
This can work if I can use one more condition to limit index, but does not work with two conditions:
times.zipWithIndex.collect
{
case (element, index)
if (index < times.size)
if (times(index) != times(index+1)) => times(index)
}
I appreciate any kind of alternative.
how about
ll.foldLeft(List[Int]())((acc, x) => acc match {case Nil => List(x) case y => if (y.last == x) y else y :+ x})
Here's my alternative using the sliding function:
val ll = List(1, 2, 3, 6, 3, 7, 5, 5, 6, 3)
ll.sliding(2)
.filter( t => t.length > 1 && t(0) != t(1) )
.map( t => t(0) )
.toList :+ ll.last
You can use zip the list with itself, dropping the first element so that you compare elements at index N with N + 1. You only need to append the last element (you may want to use a ListBuffer as appending the last element requires to copy the list).
val r = times.zip(times.drop(1)).withFilter(t => t._1 != t._2).map(_._1) :+ times.last
scala> val times = List(1, 2, 3, 6, 3, 7, 5, 5, 6, 3)
times: List[Int] = List(1, 2, 3, 6, 3, 7, 5, 5, 6, 3)
scala> val r = times.zip(times.drop(1)).withFilter(t => t._1 != t._2).map(_._1) :+ times.last
r: List[Int] = List(1, 2, 3, 6, 3, 7, 5, 6, 3)
Related
If you have one Integer list in Scala, and you want to iterate through it and sum every two neighbours with the same value and return this as a list, how would one do that ?
So for example:
List(4, 4, 2, 6) => List(8, 2, 6)
I'm completely new to Scala, but I can imagine that pattern match or map could be useful.
def sumSameNeighbours: List[Int] => List[Int] = {
ls match {
case l1::l2:ls => l1 == l2
}
}
This is what I can think of.
EDIT: How would I have to change the code in order to iterate from right to left instead from left to right?
So that f.e. it would be:
List(2, 2, 2, 6, 4) => List(2, 4, 6, 4)
instead of
List(2, 2, 2, 6, 4) => List(4, 2, 6, 4)
This is pretty close to your suggestion and seems basically to work:
import scala.annotation.tailrec
def sumSameNeighbors( ls : List[Int] ) : List[Int] = {
#tailrec
def walk( unsummed : List[Int], reverseAccum : List[Int] ) : List[Int] = {
unsummed match {
case a :: b :: rest if a == b => walk( rest, a + b :: reverseAccum )
case a :: rest => walk( rest, a :: reverseAccum )
case Nil => reverseAccum.reverse
}
}
walk( ls, Nil )
}
Note: Based on final OP's specifications clarification, this answer doesn't exactly fit the question requirements.
Here is a solution using List.grouped(2):
list.grouped(2).toList
.flatMap {
case List(a, b) if a == b => List(a + b)
case l => l
}
The idea is to group consecutive elements by pair. If the pair has the same elements, we return their sum to be flatMaped and otherwise both elements untouched.
List(4, 4, 2, 6) => List(8, 2, 6)
List(2, 4, 4, 2, 6) => List(2, 4, 4, 2, 6)
List(2) => List(2)
List(9, 4, 4, 4, 2, 6) => List(9, 4, 8, 2, 6)
Another way using foldRight, which I find a good default for this sort of traversing a collection while creating a new one:
list.foldRight(List.empty[Int]) {
case (x, y :: tail) if x == y => (x + y) :: tail
case (x, list) => x :: list
}
Output of List(2, 2, 2, 6, 4) is List(2, 4, 6, 4) as requested.
The main thing I wasn't clear on from your examples is what the output should be if summing creates new neighbours: should List(8, 4, 2, 2) turn into List(8, 4, 4) or List(16)? This produces the second.
I understand how to use if in map. For example, val result = list.map(x => if (x % 2 == 0) x * 2 else x / 2).
However, I want to use if for only part of the arguments.
val inputColumns = List(
List(1, 2, 3, 4, 5, 6), // first "column"
List(4, 6, 5, 7, 12, 15) // second "column"
)
inputColumns.zipWithIndex.map{ case (col, idx) => if (idx == 0) col * 2 else col / 10}
<console>:1: error: ';' expected but integer literal found.
inputColumns.zipWithIndex
res4: List[(List[Int], Int)] = List((List(1, 2, 3, 4, 5, 6),0), (List(4, 6, 5, 7, 12, 15),1))
I have searched the error info but have not found a solution.
Why my code is not 'legal' in Scala? Is there a better way to write it? Basically, I want to do a pattern matching and then do something on other arguments.
To explain your problem another way, inputColumns has type List[List[Int]]. You can verify this in the Scala REPL:
$ scala
Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_161).
Type in expressions for evaluation. Or try :help.
scala> val inputColumns = List(
| List(1, 2, 3, 4, 5, 6), // first "column"
| List(4, 6, 5, 7, 12, 15) // second "column"
| )
inputColumns: List[List[Int]] = List(List(1, 2, 3, 4, 5, 6), List(4, 6, 5, 7, 12, 15))
Now, when you call .zipWithIndex on that list, you end up with a List[(List[Int], Int)] - that is, a list of a tuple, in which the first tuple type is a List[Int] (the column) and the second is an Int (the index):
scala> inputColumns.zipWithIndex
res0: List[(List[Int], Int)] = List((List(1, 2, 3, 4, 5, 6),0), (List(4, 6, 5, 7, 12, 15),1))
Consequently, when you try to apply a map function to this list, col is a List[Int] and not an Int, and so col * 2 makes no sense - you're multiplying a List[Int] by 2. You then also try to divide the list by 10, obviously.
scala> inputColumns.zipWithIndex.map{ case(col, idx) => if(idx == 0) col * 2 else col / 10 }
<console>:13: error: value * is not a member of List[Int]
inputColumns.zipWithIndex.map{ case(col, idx) => if(idx == 0) col * 2 else col / 10 }
^
<console>:13: error: value / is not a member of List[Int]
inputColumns.zipWithIndex.map{ case(col, idx) => if(idx == 0) col * 2 else col / 10 }
^
In order to resolve this, it depends what you're trying to achieve. If you want a single list of integers, and then zip those so that each value has an associated index, you should call flatten on inputColumns before calling zipWithIndex. This will result in List[(Int, Int)], where the first value in the tuple is the column value, and the second is the index. Your map function will then work correctly without modification:
scala> inputColumns.flatten.zipWithIndex.map{ case(col, idx) => if(idx == 0) col * 2 else col / 10 }
res3: List[Int] = List(2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1)
Of course, you no longer have separate columns.
If you wish each value in each list to have an associated index, you need to firstly map inputColumns into two zipped lists, using inputColumns.map(_.zipWithIndex) to create a List[List[(Int, Int)]] - a list of a list of (Int, Int) tuples:
scala> inputColumns.map(_.zipWithIndex)
res4: List[List[(Int, Int)]] = List(List((1,0), (2,1), (3,2), (4,3), (5,4), (6,5)), List((4,0), (6,1), (5,2), (7,3), (12,4), (15,5)))
We can now apply your original map function to the result of the zipWithIndex operation:
scala> inputColumns.map(_.zipWithIndex.map { case (col, idx) => if(idx == 0) col * 2 else col / 10 })
res5: List[List[Int]] = List(List(2, 0, 0, 0, 0, 0), List(8, 0, 0, 0, 1, 1))
The result is another List[List[Int]] with each internal list being the results of your map operation on the original two input columns.
On the other hand, if idx is meant to be the index of the column, and not of each value, and you want to multiply all of the values in the first column by 2 and divide all of the values in the other columns by 10, then you need to change your original map function to map across each column, as follows:
scala> inputColumns.zipWithIndex.map {
| case (col, idx) => {
| if(idx == 0) col.map(_ * 2) // Multiply values in first column by 1
| else col.map(_ / 10) // Divide values in all other columns by 10
| }
| }
res5: List[List[Int]] = List(List(2, 4, 6, 8, 10, 12), List(0, 0, 0, 0, 1, 1))
Let me know if you require any further clarification...
UPDATE:
The use of case in map is a common Scala shorthand. If a higher-order function takes a single argument, something such as this:
def someHOF[A, B](x: A => B) = //...
and you call that function like this (with what Scala terms a partial function - a function consisting solely of a list of case statements):
someHOF {
case expr1 => //...
case expr2 => //...
...
}
then Scala treats it as a kind-of shorthand for:
someHOF {a =>
a match {
case expr1 => //...
case expr2 => //...
...
}
}
or, being slightly more terse,
someHOF {
_ match {
case expr1 => //...
case expr2 => //...
...
}
}
For a List, for example, you can use it with functions such as map, flatMap, filter, etc.
In the case of your map function, the sole argument is a tuple, and the sole case statement acts to break open the tuple and expose its contents. That is:
val l = List((1, 2), (3, 4), (5, 6))
l.map { case(a, b) => println(s"First is $a, second is $b") }
is equivalent to:
l.map {x =>
x match {
case (a, b) => println(s"First is $a, second is $b")
}
}
and both will output:
First is 1, second is 2
First is 3, second is 4
First is 5, second is 6
Note: This latter is a bit of a dumb example, since map is supposed to map (i.e. change) the values in the list into new values in a new list. If all you were doing was printing the values, this would be better:
val l = List((1, 2), (3, 4), (5, 6))
l.foreach { case(a, b) => println(s"First is $a, second is $b") }
You are trying to multiply a list by 2 when you do col * 2 as col is List(1, 2, 3, 4, 5, 6) when idx is 0, which is not possible and similar is the case with else part col / 10
If you are trying to multiply the elements of first list by 2 and devide the elements of rest of the list by 10 then you should be doing the following
inputColumns.zipWithIndex.map{ case (col, idx) => if (idx == 0) col.map(_*2) else col.map(_/10)}
Even better approach would be to use match case
inputColumns.zipWithIndex.map(x => x._2 match {
case 0 => x._1.map(_*2)
case _ => x._1.map(_/10)
})
This is my first day using scala. I am trying to make a string of the number of times each digit is represented in a string. For instance, the number 4310227 would return "1121100100" because 0 appears once, 1 appears once, 2 appears twice and so on...
def pow(n:Int) : String = {
val cubed = (n * n * n).toString
val digits = 0 to 9
val str = ""
for (a <- digits) {
println(a)
val b = cubed.count(_==a.toString)
println(b)
}
return cubed
}
and it doesn't seem to work. would like some scalay reasons why and to know whether I should even be going about it in this manner. Thanks!
When you iterate over strings, which is what you are doing when you call String#count(), you are working with Chars, not Strings. You don't want to compare these two with ==, since they aren't the same type of object.
One way to solve this problem is to call Char#toString() before performing the comparison, e.g., amend your code to read cubed.count(_.toString==a.toString).
As Rado and cheeken said, you're comparing a Char with a String, which will never be be equal. An alternative to cheekin's answer of converting each character to a string is to create a range from chars, ie '0' to '9':
val digits = '0' to '9'
...
val b = cubed.count(_ == a)
Note that if you want the Int that a Char represents, you can call char.asDigit.
Aleksey's, Ren's and Randall's answers are something you will want to strive towards as they separate out the pure solution to the problem. However, given that it's your first day with Scala, depending on what background you have, you might need a bit more context before understanding them.
Fairly simple:
scala> ("122333abc456xyz" filter (_.isDigit)).foldLeft(Map.empty[Char, Int]) ((histo, c) => histo + (c -> (histo.getOrElse(c, 0) + 1)))
res1: scala.collection.immutable.Map[Char,Int] = Map(4 -> 1, 5 -> 1, 6 -> 1, 1 -> 1, 2 -> 2, 3 -> 3)
This is perhaps not the fastest approach because intermediate datatype like String and Char are used but one of the most simplest:
def countDigits(n: Int): Map[Int, Int] =
n.toString.groupBy(x => x) map { case (n, c) => (n.asDigit, c.size) }
Example:
scala> def countDigits(n: Int): Map[Int, Int] = n.toString.groupBy(x => x) map { case (n, c) => (n.asDigit, c.size) }
countDigits: (n: Int)Map[Int,Int]
scala> countDigits(12345135)
res0: Map[Int,Int] = Map(5 -> 2, 1 -> 2, 2 -> 1, 3 -> 2, 4 -> 1)
Where myNumAsString is a String, eg "15625"
myNumAsString.groupBy(x => x).map(x => (x._1, x._2.length))
Result = Map(2 -> 1, 5 -> 2, 1 -> 1, 6 -> 1)
ie. A map containing the digit with its corresponding count.
What this is doing is taking your list, grouping the values by value (So for the initial string of "15625", it produces a map of 1 -> 1, 2 -> 2, 6 -> 6, and 5 -> 55.). The second bit just creates a map of the value to the count of how many times it occurs.
The counts for these hundred digits happen to fit into a hex digit.
scala> val is = for (_ <- (1 to 100).toList) yield r.nextInt(10)
is: List[Int] = List(8, 3, 9, 8, 0, 2, 0, 7, 8, 1, 6, 9, 9, 0, 3, 6, 8, 6, 3, 1, 8, 7, 0, 4, 4, 8, 4, 6, 9, 7, 4, 6, 6, 0, 3, 0, 4, 1, 5, 8, 9, 1, 2, 0, 8, 8, 2, 3, 8, 6, 4, 7, 1, 0, 2, 2, 6, 9, 3, 8, 6, 7, 9, 5, 0, 7, 6, 8, 7, 5, 8, 2, 2, 2, 4, 1, 2, 2, 6, 8, 1, 7, 0, 7, 6, 9, 5, 5, 5, 3, 5, 8, 2, 5, 1, 9, 5, 7, 2, 3)
scala> (new Array[Int](10) /: is) { case (a, i) => a(i) += 1 ; a } map ("%x" format _) mkString
warning: there were 1 feature warning(s); re-run with -feature for details
res7: String = a8c879caf9
scala> (new Array[Int](10) /: is) { case (a, i) => a(i) += 1 ; a } sum
warning: there were 1 feature warning(s); re-run with -feature for details
res8: Int = 100
I was going to point out that no one used a char range, but now I see Kristian did.
def pow(n:Int) : String = {
val cubed = (n * n * n).toString
val cnts = for (a <- '0' to '9') yield cubed.count(_ == a)
(cnts map (c => ('0' + c).toChar)).mkString
}
I am sure there is an elegant/funny way of doing it,
but I can only think of a more or less complicated recursive solution.
Rephrasing:
Is there any standard lib (collections) method nor simple combination of them to take the first distinct elements of a list?
scala> val s = Seq(3, 5, 4, 1, 5, 7, 1, 2)
s: Seq[Int] = List(3, 5, 4, 1, 5, 7, 1, 2)
scala> s.takeWhileDistinct //Would return Seq(3,5,4,1), it should preserve the original order and ignore posterior occurrences of distinct values like 7 and 2.
If you want it to be fast-ish, then
{ val hs = scala.collection.mutable.HashSet[Int]()
s.takeWhile{ hs.add } }
will do the trick. (Extra braces prevent leaking the temp value hs.)
This is a short approach in a maximum of O(2logN).
implicit class ListOps[T](val s: Seq[T]) {
def takeWhileDistinct: Seq[T] = {
s.indexWhere(x => { s.count(x==) > 1 }) match {
case ind if (ind > 0) => s.take(
s.indexWhere(x => { s.count(x==) > 1 }, ind + 1) + ind).distinct
case _ => s
}
}
}
val ex = Seq(3, 5, 4, 5, 7, 1)
val ex2 = Seq(3, 5, 4, 1, 5, 7, 1, 5)
println(ex.takeWhileDistinct.mkString(", ")) // 3, 4, 5
println(ex2.takeWhileDistinct.mkString(", ")) // 3, 4, 5, 1
Look here for live results.
Interesting problem. Here's an alternative. First, let's get the stream of s so we can avoid unnecessary work (though the overhead is likely to be greater than the saved work, sadly).
val s = Seq(3, 5, 4, 5, 7, 1)
val ss = s.toStream
Now we can build s again, but keeping track of whether there are repetitions or not, and stopping at the first one:
val newS = ss.scanLeft(Seq[Int]() -> false) {
case ((seen, stop), current) =>
if (stop || (seen contains current)) (seen, true)
else ((seen :+ current, false))
}
Now all that's left is take the last element without repetition, and drop the flag:
val noRepetitionsS = newS.takeWhile(!_._2).last._1
A variation on Rex's (though I prefer his...)
This one is functional throughout, using the little-seen scanLeft method.
val prevs = xs.scanLeft(Set.empty[Int])(_ + _)
(xs zip prevs) takeWhile { case (x,prev) => !prev(x) } map {_._1}
UPDATE
And a lazy version (using iterators, for moar efficiency):
val prevs = xs.iterator.scanLeft(Set.empty[Int])(_ + _)
(prevs zip xs.iterator) takeWhile { case (prev,x) => !prev(x) } map {_._2}
Turn the resulting iterator back to a sequence if you want, but this'll also work nicely with iterators on both the input AND the output.
The problem is simpler than the std lib function I was looking for (takeWhileConditionOverListOfAllAlreadyTraversedItems):
scala> val s = Seq(3, 5, 4, 1, 5, 7, 1, 2)
scala> s.zip(s.distinct).takeWhile{case(a,b)=>a==b}.map(_._1)
res20: Seq[Int] = List(3, 5, 4, 1)
I would like to filter collection, so distance between adjacent elements would be at least 5.
So List(1, 2, 3, 4, 5, 6, 7, 11, 20) will become List(1, 6, 11, 20).
Is it possible to achieve in one pass using filter? What would be scala-way?
How about this one-liner:
scala> l.foldLeft(Vector(l.head)) { (acc, item) => if (item - acc.last >= 5) acc :+ item else acc }
res7: scala.collection.immutable.Vector[Int] = Vector(1, 6, 11, 20)
Try with foldLeft():
val input = List(1, 2, 3, 4, 5, 6, 7, 11, 20)
input.tail.foldLeft(List(input.head))((out, cur) =>
if(cur - out.head >= 5) cur :: out else out
).reverse
If it's not obvious:
Algorithm starts with first element (probably you need some edge cases handled) in the output collection
It iterates over all remaining elements from the input. If the difference between this element (cur) and first element of input is greater than or equal than 5, prepend to input. Otherwise skip and proceed
input was built by prepending and examining head to get better performance. .reverse is needed in the end
This is basically how you would implement this in imperative way, but with more concise syntax.