Related
Given the following list:
List(List(1,2,3), List(4,5))
I would like to generate all the possible combinations. Using yield, it can be done as follows:
scala> for (x <- l.head; y <- l.last) yield (x,y)
res17: List[(Int, Int)] = List((1,4), (1,5), (2,4), (2,5), (3,4), (3,5))
But the problem I have is that the List[List[Int]] is not fixed; it can grow and shrink in size, so I never know how many for loops I will need in advance. What I would like is to be able to pass that list into a function which will dynamically generate the combinations regardless of the number of lists I have, so:
def generator (x : List[List[Int]) : List[List[Int]]
Is there a built-in library function that can do this. If not how do I go about doing this. Any pointers and hints would be great.
UPDATE:
The answer by #DNA blows the heap with the following (not so big) nested List structure:
List(
List(0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, 300),
List(0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300),
List(0, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300),
List(0, 50, 100, 150, 200, 250, 300),
List(0, 100, 200, 300),
List(0, 200),
List(0)
)
Calling the generator2 function as follows:
generator2(
List(
List(0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, 300),
List(0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300),
List(0, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300),
List(0, 50, 100, 150, 200, 250, 300),
List(0, 100, 200, 300),
List(0, 200),
List(0)
)
)
Is there a way to generate the cartesian product without blowing the heap?
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at scala.LowPriorityImplicits.wrapRefArray(LowPriorityImplicits.scala:73)
at recfun.Main$.recfun$Main$$generator$1(Main.scala:82)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.flatMap(TraversableLike.scala:251)
at scala.collection.AbstractTraversable.flatMap(Traversable.scala:105)
at recfun.Main$.recfun$Main$$generator$1(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.flatMap(TraversableLike.scala:251)
at scala.collection.AbstractTraversable.flatMap(Traversable.scala:105)
at recfun.Main$.recfun$Main$$generator$1(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.flatMap(TraversableLike.scala:251)
at scala.collection.AbstractTraversable.flatMap(Traversable.scala:105)
at recfun.Main$.recfun$Main$$generator$1(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.flatMap(TraversableLike.scala:251)
Here's a recursive solution:
def generator(x: List[List[Int]]): List[List[Int]] = x match {
case Nil => List(Nil)
case h :: _ => h.flatMap(i => generator(x.tail).map(i :: _))
}
which produces:
val a = List(List(1, 2, 3), List(4, 5))
val b = List(List(1, 2, 3), List(4, 5), List(6, 7))
generator(a) //> List(List(1, 4), List(1, 5), List(2, 4),
//| List(2, 5), List(3, 4), List(3, 5))
generator(b) //> List(List(1, 4, 6), List(1, 4, 7), List(1, 5, 6),
//| List(1, 5, 7), List(2, 4, 6), List(2, 4, 7),
//| List(2, 5, 6), List(2, 5, 7), Listt(3, 4, 6),
//| List(3, 4, 7), List(3, 5, 6), List(3, 5, 7))
Update: the second case can also be written as a for comprehension, which may be a little clearer:
def generator2(x: List[List[Int]]): List[List[Int]] = x match {
case Nil => List(Nil)
case h :: t => for (j <- generator2(t); i <- h) yield i :: j
}
Update 2: for larger datasets, if you run out of memory, you can use Streams instead (if it makes sense to process the results incrementally). For example:
def generator(x: Stream[Stream[Int]]): Stream[Stream[Int]] =
if (x.isEmpty) Stream(Stream.empty)
else x.head.flatMap(i => generator(x.tail).map(i #:: _))
// NB pass in the data as Stream of Streams, not List of Lists
generator(input).take(3).foreach(x => println(x.toList))
>List(0, 0, 0, 0, 0, 0, 0)
>List(0, 0, 0, 0, 0, 200, 0)
>List(0, 0, 0, 0, 100, 0, 0)
Feels like your problem can be described in terms of recursion:
If you have n lists of int:
list1 of size m and list2, ... list n
generate the X combinations for list2 to n (so n-1 lists)
for each combination, you generate m new ones for each value of list1.
the base case is a list of one list of int, you just split all the elements in singleton lists.
so with List(List(1,2), List(3), List(4, 5))
the result of your recursive call is List(List(3,4),List(3,5)) and for each you add 2 combinations: List(1,3,4), List(2,3,4), List(1,3,5), List(2,3,5).
Ezekiel has exactly what I was looking for. This is just a minor tweak of it to make it generic.
def generateCombinations[T](x: List[List[T]]): List[List[T]] = {
x match {
case Nil => List(Nil)
case h :: _ => h.flatMap(i => generateCombinations(x.tail).map(i :: _))
}
}
Here is another solution based on Ezekiel's one. More verbose, but it's tail recursion (stack-safe).
def generateCombinations[A](in: List[List[A]]): List[List[A]] =
generate(in, List.empty)
#tailrec
private def generate[A](in: List[List[A]], acc: List[List[A]]): List[List[A]] = in match {
case Nil => acc
case head :: tail => generate(tail, generateAcc(acc, head))
}
private def generateAcc[A](oldAcc: List[List[A]], as: List[A]): List[List[A]] = {
oldAcc match {
case Nil => as.map(List(_))
case nonEmptyAcc =>
for {
a <- as
xs <- nonEmptyAcc
} yield a :: xs
}
}
I realize this is old, but it seems like no other answer provided the non-recursive solution with fold.
def generator[A](xs: List[List[A]]): List[List[A]] = xs.foldRight(List(List.empty[A])) { (next, combinations) =>
for (a <- next; as <- combinations) yield a +: as
}
output : prime numbers
2
3
()
5
()
7
()
()
i want as
2
3
5
7
def primeNumber(range: Int): Unit ={
val primeNumbers: immutable.IndexedSeq[AnyVal] =
for (number :Int <- 2 to range) yield{
var isPrime = true
for(checker : Int <- 2 to Math.sqrt(number).toInt if number%checker==0 if isPrime) isPrime = false
if(isPrime) number
}
println("prime numbers")
for(prime <- primeNumbers)
println(prime)
}
so the underlying problem here is that your yield block effectively will return an Int or a Unit depending on isPrime this leads your collection to be of type AnyVal because that's pretty much the least upper bound that can represent both types. Unit is a type only inhabited by one value which is represented as an empty set of round brackets in scala () so that's what you see in your list.
As Puneeth Reddy V said you can use collect to filter out all the non-Int values but I think that is a suboptimal approach (partial functions are often considered a code-smell depending on what type of scala-style you do). More idiomatic would be to rethink your loop (such for loops are scarcely used in scala) and this could be definitely be done using a foldLeft operation maybe even something else.
You can use collect on your output
primeNumbers.collect{
case i : Int => i
}
res2: IndexedSeq[Int] = Vector(2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97)
The reason is if else returning two types of value one is prime value and another is empty. You could return 0 in else case and filter it before printing it.
scala> def primeNumber(range: Int): Unit ={
|
| val primeNumbers: IndexedSeq[Int] =
|
| for (number :Int <- 2 to range) yield{
|
| var isPrime = true
|
| for(checker : Int <- 2 to Math.sqrt(number).toInt if number%checker==0 if isPrime) isPrime = false
|
| if(isPrime) number
| else
| 0
| }
|
| println("prime numbers" + primeNumbers)
| for(prime <- primeNumbers.filter(_ > 0))
| println(prime)
| }
primeNumber: (range: Int)Unit
scala> primeNumber(10)
prime numbersVector(2, 3, 0, 5, 0, 7, 0, 0, 0)
2
3
5
7
But we should not write the code the way you have written it. You are using mutable variables. Try to write code in an immutable way.
For example
scala> def isPrime(number: Int) =
| number > 1 && !(2 to number - 1).exists(e => e % number == 0)
isPrime: (number: Int)Boolean
scala> def generatePrimes(starting: Int): Stream[Int] = {
| if(isPrime(starting))
| starting #:: generatePrimes(starting + 1)
| else
| generatePrimes(starting + 1)
| }
generatePrimes: (starting: Int)Stream[Int]
scala> generatePrimes(2).take(100).toList
res12: List[Int] = List(2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101)
I'm very new to Scala and I have to find primes for numbers in range [i;j].
This is how I fill stream:
val stream = (i to j).toStream
and then I call this function which supposed to find primes:
def findPrimes(s: Stream[Int]): Stream[Int] = {
println("HEAD: " + s.head)
return findPrimes(s.tail)
// s.head #:: findPrimes(s.tail.filter( _ % s.head != 0 ))
}
Outputting stream to console, I am surprised why am I getting exception:
Exception in thread "main" java.util.NoSuchElementException: head of empty stream
at scala.collection.immutable.Stream$Empty$.head(Stream.scala:1104)
at scala.collection.immutable.Stream$Empty$.head(Stream.scala:1102)
at com.example.anna.app.HelloWorld$.findPrimes(HelloWorld.scala:43)
STREAM SIZE IS 100
at com.example.anna.app.HelloWorld$.prime(HelloWorld.scala:32)
HEAD: 1
at com.example.anna.app.HelloWorld$.delayedEndpoint$com$example$anna$app$HelloWorld$1(HelloWorld.scala:11)
HEAD: 2
HEAD: 3
at com.example.anna.app.HelloWorld$delayedInit$body.apply(HelloWorld.scala:3)
HEAD: 4
at scala.Function0.apply$mcV$sp(Function0.scala:34)
HEAD: 5
HEAD: 6
at scala.Function0.apply$mcV$sp$(Function0.scala:34)
HEAD: 7
at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
HEAD: 8
HEAD: 9
at scala.App.$anonfun$main$1$adapted(App.scala:76)
HEAD: 10
HEAD: 11
at scala.collection.immutable.List.foreach(List.scala:389)
HEAD: 12
at scala.App.main(App.scala:76)
HEAD: 13
at scala.App.main$(App.scala:74)
HEAD: 14
HEAD: 15
at com.example.anna.app.HelloWorld$.main(HelloWorld.scala:3)
HEAD: 16
at com.example.anna.app.HelloWorld.main(HelloWorld.scala)
HEAD: 17
starting from 17 up to end there's no single exception.
And also this commented line doesn't work - it throws same exception.
Any help & advice would be appreciated.
Stream that you pass to findPrimes is of limited size and is getting exhausted when you call findPrimes.
Create infinite stream using Stream.from method.
Scala REPL
scala> :paste
// Entering paste mode (ctrl-D to finish)
def primeStream(s: Stream[Int]): Stream[Int] =
Stream.cons(s.head, primeStream(s.tail filter { _ % s.head != 0 }))
val primes = primeStream(Stream.from(2))
// Exiting paste mode, now interpreting.
primeStream: (s: Stream[Int])Stream[Int]
primes: Stream[Int] = Stream(2, ?)
scala> primes.take(100).toList
res0: List[Int] = List(2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541)
Problem with your code.
scala> val primes = primeStream((1 to 100).toStream)
primes: Stream[Int] = Stream(1, ?)
scala> primes.take(100).toList
java.util.NoSuchElementException: head of empty stream
at scala.collection.immutable.Stream$Empty$.head(Stream.scala:1104)
at scala.collection.immutable.Stream$Empty$.head(Stream.scala:1102)
at .primeStream(<console>:12)
at .$anonfun$primeStream$1(<console>:12)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1169)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1159)
at scala.collection.immutable.Stream.$anonfun$take$2(Stream.scala:789)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1169)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1159)
at scala.collection.generic.Growable.loop$1(Growable.scala:54)
at scala.collection.generic.Growable.$plus$plus$eq(Growable.scala:58)
at scala.collection.generic.Growable.$plus$plus$eq$(Growable.scala:50)
at scala.collection.mutable.ListBuffer.$plus$plus$eq(ListBuffer.scala:186)
at scala.collection.mutable.ListBuffer.$plus$plus$eq(ListBuffer.scala:44)
at scala.collection.TraversableLike.to(TraversableLike.scala:590)
at scala.collection.TraversableLike.to$(TraversableLike.scala:587)
at scala.collection.AbstractTraversable.to(Traversable.scala:104)
at scala.collection.TraversableOnce.toList(TraversableOnce.scala:294)
at scala.collection.TraversableOnce.toList$(TraversableOnce.scala:294)
at scala.collection.AbstractTraversable.toList(Traversable.scala:104)
... 28 elided
Why this throws an exception? The reason for this is the head method called on empty stream in the println statement.
So we can pattern-match this stream to return empty stream when the tail is empty as follows (there are better ways to traverse stream, but I tried not to change your code very much):
def findPrimes(s: Stream[Int]): Stream[Int] = s match {
case Stream.Empty =>
println("END") //consider removing println statements
Stream.Empty
case h #:: tl =>
println(s"HEAD: $h")
h #:: findPrimes(tl)
}
Below sample data
val combineList = List(("A",12),("B",11),("C",12),("D",14),("E",23),("F",12),("D",53),("C",23),("B",12),("A",22),("E",21),("F",12),("C",21),("B",34),("A",34),("G",67),("D",23),("E",21),("F",12),("D",31),("B",41),("E",14),("F",15),("G",18),("A",11),("C",10),("D",9),("A",13),("E",1),("F",14))
and
val X = 98
Now want final output as below,
first group by all values as below
val groupKey = List(Map("A"->List(12,22,34,11,13)),Map("B"->List(11,12,34,41)),Map("C"->List(12,23,21,10)),Map("D"->List(14,53,23,31,9)),
Map("E"->List(23,21,21,14,1)),Map("F"->List(12,12,12,15,14)),Map("G"->List(67,18)))
Second substract X from groupKey List values here X always gretter than List values so second output will be as
val substrackValues = List(Map("A"->List(86,76,34,87,85)),Map("B"->List(87,86,34,57)),Map("C"->List(86,75,77,88)),Map("D"->List(84,45,75,31,89)),
Map("E"->List(75,77,77,84,97)),Map("F"->List(86,86,86,15,84)),Map("G"->List(31,80)))
Consider
combineList.groupBy(_._1).mapValues(xs => xs.map(v => X-v._2))
which delivers
Map(E -> List(75, 77, 77, 84, 97), F -> List(86, 86, 86, 83, 84), A -> List(86, 76, 64, 87, 85), G -> List(31, 80), B -> List(87, 86, 64, 57), C -> List(86, 75, 77, 88), D -> List(84, 45, 75, 67, 89))
Note the embedded maps in groupKey above are singleton maps which can well be represented with tuples of [(String,List[Int])] or even better agglomerated into one map.
In the solution proposed here after grouping by first tuple element, we transform each element in each list by the value of X.
Given the following list:
List(List(1,2,3), List(4,5))
I would like to generate all the possible combinations. Using yield, it can be done as follows:
scala> for (x <- l.head; y <- l.last) yield (x,y)
res17: List[(Int, Int)] = List((1,4), (1,5), (2,4), (2,5), (3,4), (3,5))
But the problem I have is that the List[List[Int]] is not fixed; it can grow and shrink in size, so I never know how many for loops I will need in advance. What I would like is to be able to pass that list into a function which will dynamically generate the combinations regardless of the number of lists I have, so:
def generator (x : List[List[Int]) : List[List[Int]]
Is there a built-in library function that can do this. If not how do I go about doing this. Any pointers and hints would be great.
UPDATE:
The answer by #DNA blows the heap with the following (not so big) nested List structure:
List(
List(0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, 300),
List(0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300),
List(0, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300),
List(0, 50, 100, 150, 200, 250, 300),
List(0, 100, 200, 300),
List(0, 200),
List(0)
)
Calling the generator2 function as follows:
generator2(
List(
List(0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, 300),
List(0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300),
List(0, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300),
List(0, 50, 100, 150, 200, 250, 300),
List(0, 100, 200, 300),
List(0, 200),
List(0)
)
)
Is there a way to generate the cartesian product without blowing the heap?
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at scala.LowPriorityImplicits.wrapRefArray(LowPriorityImplicits.scala:73)
at recfun.Main$.recfun$Main$$generator$1(Main.scala:82)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.flatMap(TraversableLike.scala:251)
at scala.collection.AbstractTraversable.flatMap(Traversable.scala:105)
at recfun.Main$.recfun$Main$$generator$1(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.flatMap(TraversableLike.scala:251)
at scala.collection.AbstractTraversable.flatMap(Traversable.scala:105)
at recfun.Main$.recfun$Main$$generator$1(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.flatMap(TraversableLike.scala:251)
at scala.collection.AbstractTraversable.flatMap(Traversable.scala:105)
at recfun.Main$.recfun$Main$$generator$1(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at recfun.Main$$anonfun$recfun$Main$$generator$1$1.apply(Main.scala:83)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.flatMap(TraversableLike.scala:251)
Here's a recursive solution:
def generator(x: List[List[Int]]): List[List[Int]] = x match {
case Nil => List(Nil)
case h :: _ => h.flatMap(i => generator(x.tail).map(i :: _))
}
which produces:
val a = List(List(1, 2, 3), List(4, 5))
val b = List(List(1, 2, 3), List(4, 5), List(6, 7))
generator(a) //> List(List(1, 4), List(1, 5), List(2, 4),
//| List(2, 5), List(3, 4), List(3, 5))
generator(b) //> List(List(1, 4, 6), List(1, 4, 7), List(1, 5, 6),
//| List(1, 5, 7), List(2, 4, 6), List(2, 4, 7),
//| List(2, 5, 6), List(2, 5, 7), Listt(3, 4, 6),
//| List(3, 4, 7), List(3, 5, 6), List(3, 5, 7))
Update: the second case can also be written as a for comprehension, which may be a little clearer:
def generator2(x: List[List[Int]]): List[List[Int]] = x match {
case Nil => List(Nil)
case h :: t => for (j <- generator2(t); i <- h) yield i :: j
}
Update 2: for larger datasets, if you run out of memory, you can use Streams instead (if it makes sense to process the results incrementally). For example:
def generator(x: Stream[Stream[Int]]): Stream[Stream[Int]] =
if (x.isEmpty) Stream(Stream.empty)
else x.head.flatMap(i => generator(x.tail).map(i #:: _))
// NB pass in the data as Stream of Streams, not List of Lists
generator(input).take(3).foreach(x => println(x.toList))
>List(0, 0, 0, 0, 0, 0, 0)
>List(0, 0, 0, 0, 0, 200, 0)
>List(0, 0, 0, 0, 100, 0, 0)
Feels like your problem can be described in terms of recursion:
If you have n lists of int:
list1 of size m and list2, ... list n
generate the X combinations for list2 to n (so n-1 lists)
for each combination, you generate m new ones for each value of list1.
the base case is a list of one list of int, you just split all the elements in singleton lists.
so with List(List(1,2), List(3), List(4, 5))
the result of your recursive call is List(List(3,4),List(3,5)) and for each you add 2 combinations: List(1,3,4), List(2,3,4), List(1,3,5), List(2,3,5).
Ezekiel has exactly what I was looking for. This is just a minor tweak of it to make it generic.
def generateCombinations[T](x: List[List[T]]): List[List[T]] = {
x match {
case Nil => List(Nil)
case h :: _ => h.flatMap(i => generateCombinations(x.tail).map(i :: _))
}
}
Here is another solution based on Ezekiel's one. More verbose, but it's tail recursion (stack-safe).
def generateCombinations[A](in: List[List[A]]): List[List[A]] =
generate(in, List.empty)
#tailrec
private def generate[A](in: List[List[A]], acc: List[List[A]]): List[List[A]] = in match {
case Nil => acc
case head :: tail => generate(tail, generateAcc(acc, head))
}
private def generateAcc[A](oldAcc: List[List[A]], as: List[A]): List[List[A]] = {
oldAcc match {
case Nil => as.map(List(_))
case nonEmptyAcc =>
for {
a <- as
xs <- nonEmptyAcc
} yield a :: xs
}
}
I realize this is old, but it seems like no other answer provided the non-recursive solution with fold.
def generator[A](xs: List[List[A]]): List[List[A]] = xs.foldRight(List(List.empty[A])) { (next, combinations) =>
for (a <- next; as <- combinations) yield a +: as
}