Returning values which compute sum

Question

Here is a Scala stream to calculate the Fibonacci sequence :

    import scala.math.BigInt

    object fib extends App {

      val fibs: Stream[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(
        fibs.tail).map(n => {
          n._1 + n._2
        })
    }
  fibs take 5 foreach println
  fibs take 6 foreach println

src : http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Stream

Instead of taking the first n numbers in the sequence how to sum the first n numbers using a stream , and return this sum ?

I could introduce a var sum like this :

object fib extends App {

    var sum = 3;
  val fibs: Stream[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(fibs.tail).map(n => {

      sum = n._1.toInt + n._2.toInt
      n._1 + n._2

    })

  fibs take 5 foreach println

}

And then somehow check this summed value. But this is not a good solution.

Update : I'm attempting to provide the summed value and return values that compute this sum, not sum the actual stream.

So takeSum(7) will return '0,1,1,2,3'

Assumption is that the n value sum will contain a subset of the fib sequence

My favorite fib stream: `def fib: Stream[BigInt] = 0 #:: fib.scan(1:BigInt)(_+_)` — jwvh, Sep 09 '16 at 09:39
what happens if the parameter passed to takeSum has a value that is not the sum of some prefix of the fib stream (e.g. 8)? — The Archetypal Paul, Sep 09 '16 at 09:56
@TheArchetypalPaul assumption is that the n value sum will contain a subset of the fib sequence — blue-sky, Sep 09 '16 at 10:00
So your question is "how to return the prefix of the stream whose sum is a specified value"? — The Archetypal Paul, Sep 09 '16 at 10:01

score 4 · Answer 1 · answered Sep 09 '16 at 09:29

4

You can just use the sum-method on Stream. It will work with all types that have an implicit Numeric[T] implementation available (such as Int, Long, Double, BigInt and so on).

fibs.take(5).sum

answered Sep 09 '16 at 09:29

marstran

26,413
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I'm attempting to provide the summed value and return values that compute this sum, not sum the actual stream. I've updated question, apologies for ambiguity – blue-sky Sep 09 '16 at 09:51

The Archetypal Paul · Answer 2 · 2016-09-09T10:32:38.883

def takeSum(n:Int) = fib.scanLeft(List[BigInt]()){(b,a)=> a::b}
                        .find(_.sum==n)
                        .get
                        .reverse
 takeSum(7)  //> res0: List[BigInt] = List(0, 1, 1, 2, 3)

The scanLeft builds prefixes of the stream (with the prefix in reverse order), then it's just a matter of finding the one with the right sum. If your assumption is that the n value will be the sum of a subset of the fib sequence is not valid, it will not terminate! You could change it to >= if that's important

jwvh · Accepted Answer · 2016-09-09T10:47:42.520

0

Sounds like you want something along the lines of...

fibs.takeWhile(f => fibs.takeWhile(_ <= f).sum <= n)

...where n is the target sum.

Not a terribly efficient implementation but I think it gets the job done.

Or, alternatively.

fibs.take( fibs.tail.scan(0:BigInt)(_+_).indexWhere(_>n) )

A little more verbose but a little less repetition of effort.

edited Sep 09 '16 at 10:47

answered Sep 09 '16 at 10:27

jwvh

50,871
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Returning values which compute sum

3 Answers3