Suppose I would like to calculate Pi with Monte Carlo simulation as an exercise.
I am writing a function, which picks a point in a square (0, 1), (1, 0) at random and tests if the point is inside the circle.
import scala.math._
import scala.util.Random
def circleTest() = {
val (x, y) = (Random.nextDouble, Random.nextDouble)
sqrt(x*x + y*y) <= 1
}
Then I am writing a function, which takes as arguments the test function and the number of trials and returns the fraction of the trials in which the test was found to be true.
def monteCarlo(trials: Int, test: () => Boolean) =
(1 to trials).map(_ => if (test()) 1 else 0).sum * 1.0 / trials
... and I can calculate Pi
monteCarlo(100000, circleTest) * 4
Now I wonder if monteCarlo function can be improved. How would you write monteCarlo efficient and readable ?
For example, since the number of trials is large is it worth using a view or iterator instead of Range(1, trials) and reduce instead of map and sum ?
It's worth noting that Random.nextDouble is side-effecting—when you call it it changes the state of the random number generator. This may not be a concern to you, but since there are already five answers here I figure it won't hurt anything to add one that's purely functional.
First you'll need a random number generation monad implementation. Luckily NICTA provides a really nice one that's integrated with Scalaz. You can use it like this:
import com.nicta.rng._, scalaz._, Scalaz._
val pointInUnitSquare = Rng.choosedouble(0.0, 1.0) zip Rng.choosedouble(0.0, 1.0)
val insideCircle = pointInUnitSquare.map { case (x, y) => x * x + y * y <= 1 }
def mcPi(trials: Int): Rng[Double] =
EphemeralStream.range(0, trials).foldLeftM(0) {
case (acc, _) => insideCircle.map(_.fold(1, 0) + acc)
}.map(_ / trials.toDouble * 4)
And then:
scala> val choosePi = mcPi(10000000)
choosePi: com.nicta.rng.Rng[Double] = com.nicta.rng.Rng$$anon$3#16dd554f
Nothing's been computed yet—we've just built up a computation that will generate our value randomly when executed. Let's just execute it on the spot in the IO monad for the sake of convenience:
scala> choosePi.run.unsafePerformIO
res0: Double = 3.1415628
This won't be the most performant solution, but it's good enough that it may not be a problem for many applications, and the referential transparency may be worth it.
Stream based version, for another alternative. I think this is quite clear.
def monteCarlo(trials: Int, test: () => Boolean) =
Stream
.continually(if (test()) 1.0 else 0.0)
.take(trials)
.sum / trials
(the sum isn't specialised for streams but the implementation (in TraversableOnce) just calls foldLeft that is specialised and "allows GC to collect along the way." So the .sum won't force the stream to be evaluated and so won't keep all the trials in memory at once)
I see no problem with the following recursive version:
def monteCarlo(trials: Int, test: () => Boolean) = {
def bool2double(b: Boolean) = if (b) 1.0d else 0.0d
#scala.annotation.tailrec
def recurse(n: Int, sum: Double): Double =
if (n <= 0) sum / trials
else recurse(n - 1, sum + bool2double(test()))
recurse(trials, 0.0d)
}
And a foldLeft version, too:
def monteCarloFold(trials: Int, test: () => Boolean) =
(1 to trials).foldLeft(0.0d)((s,i) => s + (if (test()) 1.0d else 0.0d)) / trials
This is more memory efficient than the map version in the question.
Using tail recursion might be an idea:
def recMonteCarlo(trials: Int, currentSum: Double, test:() => Boolean):Double = trials match {
case 0 => currentSum
case x =>
val nextSum = currentSum + (if (test()) 1.0 else 0.0)
recMonteCarlo(trials-1, nextSum, test)
def monteCarlo(trials: Int, test:() => Boolean) = {
val monteSum = recMonteCarlo(trials, 0, test)
monteSum / trials
}
Using aggregate on a parallel collection, like this,
def monteCarlo(trials: Int, test: () => Boolean) = {
val pr = (1 to trials).par
val s = pr.aggregate(0)( (a,_) => a + (if (test()) 1 else 0), _ + _)
s * 4.0 / trials
}
where partial results are summed up in parallel with other test calculations.
Related
Here is a problem that involves a factorial. For a given number, n, find the answer to the following:
(1 / n!) * (1! + 2! + 3! + ... + n!)
The iterative solution in Scala is very easy – a simple for loop suffices.
object MyClass {
def fsolve(n: Int): Double = {
var a: Double = 1
var cum: Double = 1
for (i <- n to 2 by -1) {
a = a * (1.0/i.toDouble)
cum += a
}
scala.math.floor(cum*1000000) / 1000000
}
def main(args: Array[String]) {
println(fsolve(7)) // answer 1.173214
}
}
I want to get rid of the for loop and use a foldLeft operation. Since the idea is to reduce a list of numbers to a single result, a foldLeft, or a similar instruction ought to do the job. How? I’m struggling to find a good Scala example I can follow. The code below illustrates where I am struggling to make the leap to more idiomatic Scala.
object MyClass {
def fsolve(n: Int) = {
(n to 2 by -1).foldLeft(1.toDouble) (_*_)
// what comes next????
}
def main(args: Array[String]) {
println(fsolve(7))
}
}
Any suggestions or pointers to a solution?
The result is returned from foldLeft, like this:
val cum = (n to 2 by -1).foldLeft(1.toDouble) (_*_)
Only in your case the function needs to be different, as the fold above would multiply all i values together. You will pass both cum and a values for the folding:
def fsolve(n: Int): Double = {
val (cum, _) = (n to 2 by -1).foldLeft(1.0, 1.0) { case ((sum, a),i) =>
val newA = a * (1.0/i.toDouble)
(sum + newA, newA)
}
scala.math.floor(cum*1000000) / 1000000
}
The formula you have provided maps very nicely to the scanLeft function. It works sort of like a combination of foldLeft and map, running the fold operation but storing each generated value in the output list. The following code generates all of the factorials from 1 to n, sums them up, then divides by n!. Note that by performing a single floating point division at the end, instead of at every intermediate step, you reduce the odds of floating point errors.
def fsolve(n: Int): Double =
{
val factorials = (2 to n).scanLeft(1)((cum: Int, value: Int) => value*cum)
scala.math.floor(factorials.reduce(_+_)/factorials.last.toDouble*1000000)/1000000
}
I'll try to implement a solution without filling in the blanks but by proposing a different approach.
def fsolve(n: Int): Double = {
require(n > 0, "n must be positive")
def f(n: Int): Long = (1 to n).fold(1)(_ * _)
(1.0 / f(n)) * ((1 to n).map(f).sum)
}
In the function I make sure to fail for invalid input with require, I define factorial (as f) and then use it by simply writing down the function in the closest possible way to the original expression we wanted to implement:
(1.0 / f(n)) * ((1 to n).map(f).sum)
If you really want to fold explicitly you can rewrite this expression as follows:
(1.0 / f(n)) * ((1 to n).map(f).fold(0L)(_ + _))
Also, please note that since all operations you are executing (sums and multiplications) are commutative, you can use fold instead of foldLeft: using the former doesn't prescribe an order in which operation should run, allowing a specific implementation of the collection to run the computation in parallel.
You can play around with this code here on Scastie.
I have this function to compute the distance between two n-dimensional points using Pythagoras' theorem.
def computeDistance(neighbour: Point) = math.sqrt(coordinates.zip(neighbour.coordinates).map {
case (c1: Int, c2: Int) => math.pow(c1 - c2, 2)
}.sum)
The Point class (simplified) looks like:
class Point(val coordinates: List[Int])
I'm struggling to refactor the method so it's a little easier to read, can anybody help please?
Here's another way that makes the following three assumptions:
The length of the list is the number of dimensions for the point
Each List is correctly ordered, i.e. List(x, y) or List(x, y, z). We do not know how to handle List(x, z, y)
All lists are of equal length
def computeDistance(other: Point): Double = sqrt(
coordinates.zip(other.coordinates)
.flatMap(i => List(pow(i._2 - i._1, 2)))
.fold(0.0)(_ + _)
)
The obvious disadvantage here is that we don't have any safety around list length. The quick fix for this is to simply have the function return an Option[Double] like so:
def computeDistance(other: Point): Option[Double] = {
if(other.coordinates.length != coordinates.length) {
return None
}
return Some(sqrt(coordinates.zip(other.coordinates)
.flatMap(i => List(pow(i._2 - i._1, 2)))
.fold(0.0)(_ + _)
))
I'd be curious if there is a type safe way to ensure equal list length.
EDIT
It was politely pointed out to me that flatMap(x => List(foo(x))) is equivalent to map(foo) , which I forgot to refactor when I was originally playing w/ this. Slightly cleaner version w/ Map instead of flatMap :
def computeDistance(other: Point): Double = sqrt(
coordinates.zip(other.coordinates)
.map(i => pow(i._2 - i._1, 2))
.fold(0.0)(_ + _)
)
Most of your problem is that you're trying to do math with really long variable names. It's almost always painful. There's a reason why mathematicians use single letters. And assign temporary variables.
Try this:
class Point(val coordinates: List[Int]) { def c = coordinates }
import math._
def d(p: Point) = {
val delta = for ((a,b) <- (c zip p.c)) yield pow(a-b, dims)
sqrt(delta.sum)
}
Consider type aliases and case classes, like this,
type Coord = List[Int]
case class Point(val c: Coord) {
def distTo(p: Point) = {
val z = (c zip p.c).par
val pw = z.aggregate(0.0) ( (a,v) => a + math.pow( v._1-v._2, 2 ), _ + _ )
math.sqrt(pw)
}
}
so that for any two points, for instance,
val p = Point( (1 to 5).toList )
val q = Point( (2 to 6).toList )
we have that
p distTo q
res: Double = 2.23606797749979
Note method distTo uses aggregate on a parallelised collection of tuples, and combines the partial results by the last argument (summation). For high dimensional points this may prove more efficient than the sequential counterpart.
For simplicity of use, consider also implicit classes, as suggested in a comment above,
implicit class RichPoint(val c: Coord) extends AnyVal {
def distTo(d: Coord) = Point(c) distTo Point(d)
}
Hence
List(1,2,3,4,5) distTo List(2,3,4,5,6)
res: Double = 2.23606797749979
My code is equivalent to this:
def iterate(prev: Vector[Int], acc: Int): Vector[Int] = {
val next = (for { i <- 1.to(1000000) }
yield (prev(Random.nextInt(i))) ).toVector
if (acc < 20) iterate(next, acc + 1)
else next
}
iterate(1.to(1000000).toVector, 1)
For a large number of iterations, it does an operation on a collection, and yields the value. At the end of the iterations, it converts everything to a vector. Finally, it proceeds to the next recursive self-call, but it cannot proceed until it has all the iterations done. The number of the recursive self-calls is very small.
I want to paralellize this, so I tried to use .par on the 1.to(1000000) range. This used 8 processes instead of 1, and the result was only twice faster! .toParArray was only slightly faster than .par. I was told it could be much faster if I used something different, like maybe ThreadPool - this makes sense, because all of the time is spent in constructing next, and I assume that concatenating the outputs of different processes onto shared memory will not result in huge slowdowns, even for very large outputs (this is a key assumption and it might be wrong). How can I do it? If you provide code, paralellizing the code I gave will be sufficient.
Note that the code I gave is not my actual code. My actual code is much more long and complex (Held-Karp algorithm for TSP with constraints, BitSets and more stuff), and the only notable difference is that in my code, prev's type is ParMap, instead of Vector.
Edit, extra information: the ParMap has 350k elements on the worst iteration at the biggest sample size I can handle, and otherwise it's typically 5k-200k (that varies on a log scale). If it inherently needs a lot of time to concatenate the results from the processes into one single process (I assume this is what's happening), then there is nothing much I can do, but I rather doubt this is the case.
Implemented few versions after the original, proposed in the question,
rec0 is the original with a for loop;
rec1 uses par.map instead of for loop;
rec2 follows rec1 yet it employs parallel collection ParArray for lazy builders (and fast access on bulk traversal operations);
rec3 is a non-idiomatic non-parallel version with mutable ArrayBuffer.
Thus
import scala.collection.mutable.ArrayBuffer
import scala.collection.parallel.mutable.ParArray
import scala.util.Random
// Original
def rec0() = {
def iterate(prev: Vector[Int], acc: Int): Vector[Int] = {
val next = (for { i <- 1.to(1000000) }
yield (prev(Random.nextInt(i))) ).toVector
if (acc < 20) iterate(next, acc + 1)
else next
}
iterate(1.to(1000000).toVector, 1)
}
// par map
def rec1() = {
def iterate(prev: Vector[Int], acc: Int): Vector[Int] = {
val next = (1 to 1000000).par.map { i => prev(Random.nextInt(i)) }.toVector
if (acc < 20) iterate(next, acc + 1)
else next
}
iterate(1.to(1000000).toVector, 1)
}
// ParArray par map
def rec2() = {
def iterate(prev: ParArray[Int], acc: Int): ParArray[Int] = {
val next = (1 to 1000000).par.map { i => prev(Random.nextInt(i)) }.toParArray
if (acc < 20) iterate(next, acc + 1)
else next
}
iterate((1 to 1000000).toParArray, 1).toVector
}
// Non-idiomatic non-parallel
def rec3() = {
def iterate(prev: ArrayBuffer[Int], acc: Int): ArrayBuffer[Int] = {
var next = ArrayBuffer.tabulate(1000000){i => i+1}
var i = 0
while (i < 1000000) {
next(i) = prev(Random.nextInt(i+1))
i = i + 1
}
if (acc < 20) iterate(next, acc + 1)
else next
}
iterate(ArrayBuffer.tabulate(1000000){i => i+1}, 1).toVector
}
Then a little testing on averaging elapsed times,
def elapsed[A] (f: => A): Double = {
val start = System.nanoTime()
f
val stop = System.nanoTime()
(stop-start)*1e-6d
}
val times = 10
val e0 = (1 to times).map { i => elapsed(rec0) }.sum / times
val e1 = (1 to times).map { i => elapsed(rec1) }.sum / times
val e2 = (1 to times).map { i => elapsed(rec2) }.sum / times
val e3 = (1 to times).map { i => elapsed(rec3) }.sum / times
// time in ms.
e0: Double = 2782.341
e1: Double = 2454.828
e2: Double = 3455.976
e3: Double = 1275.876
shows that the non-idiomatic non-parallel version proves the fastest in average. Perhaps for larger input data, the parallel, idiomatic versions may be beneficial.
I've looked over a few implementations of Fibonacci function in Scala starting from a very simple one, to the more complicated ones.
I'm not entirely sure which one is the fastest. I'm leaning towards the impression that the ones that uses memoization is faster, however I wonder why Scala doesn't have a native memoization.
Can anyone enlighten me toward the best and fastest (and cleanest) way to write a fibonacci function?
The fastest versions are the ones that deviate from the usual addition scheme in some way. Very fast is the calculation somehow similar to a fast binary exponentiation based on these formulas:
F(2n-1) = F(n)² + F(n-1)²
F(2n) = (2F(n-1) + F(n))*F(n)
Here is some code using it:
def fib(n:Int):BigInt = {
def fibs(n:Int):(BigInt,BigInt) = if (n == 1) (1,0) else {
val (a,b) = fibs(n/2)
val p = (2*b+a)*a
val q = a*a + b*b
if(n % 2 == 0) (p,q) else (p+q,p)
}
fibs(n)._1
}
Even though this is not very optimized (e.g. the inner loop is not tail recursive), it will beat the usual additive implementations.
for me the simplest defines a recursive inner tail function:
def fib: Stream[Long] = {
def tail(h: Long, n: Long): Stream[Long] = h #:: tail(n, h + n)
tail(0, 1)
}
This doesn't need to build any Tuple objects for the zip and is easy to understand syntactically.
Scala does have memoization in the form of Streams.
val fib: Stream[BigInt] = 0 #:: 1 #:: fib.zip(fib.tail).map(p => p._1 + p._2)
scala> fib take 100 mkString " "
res22: String = 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 ...
Stream is a LinearSeq so you might like to convert it to an IndexedSeq if you're doing a lot of fib(42) type calls.
However I would question what your use-case is for a fibbonaci function. It will overflow Long in less than 100 terms so larger terms aren't much use for anything. The smaller terms you can just stick in a table and look them up if speed is paramount. So the details of the computation probably don't matter much since for the smaller terms they're all quick.
If you really want to know the results for very big terms, then it depends on whether you just want one-off values (use Landei's solution) or, if you're making a sufficient number of calls, you may want to pre-compute the whole lot. The problem here is that, for example, the 100,000th element is over 20,000 digits long. So we're talking gigabytes of BigInt values which will crash your JVM if you try to hold them in memory. You could sacrifice accuracy and make things more manageable. You could have a partial-memoization strategy (say, memoize every 100th term) which makes a suitable memory / speed trade-off. There is no clear anwser for what is the fastest: it depends on your usage and resources.
This could work. it takes O(1) space O(n) time to calculate a number, but has no caching.
object Fibonacci {
def fibonacci(i : Int) : Int = {
def h(last : Int, cur: Int, num : Int) : Int = {
if ( num == 0) cur
else h(cur, last + cur, num - 1)
}
if (i < 0) - 1
else if (i == 0 || i == 1) 1
else h(1,2,i - 2)
}
def main(args: Array[String]){
(0 to 10).foreach( (x : Int) => print(fibonacci(x) + " "))
}
}
The answers using Stream (including the accepted answer) are very short and idiomatic, but they aren't the fastest. Streams memoize their values (which isn't necessary in iterative solutions), and even if you don't keep the reference to the stream, a lot of memory may be allocated and then immediately garbage-collected. A good alternative is to use an Iterator: it doesn't cause memory allocations, is functional in style, short and readable.
def fib(n: Int) = Iterator.iterate(BigInt(0), BigInt(1)) { case (a, b) => (b, a+b) }.
map(_._1).drop(n).next
A little simpler tail Recursive solution that can calculate Fibonacci for large values of n. The Int version is faster but is limited, when n > 46 integer overflow occurs
def tailRecursiveBig(n :Int) : BigInt = {
#tailrec
def aux(n : Int, next :BigInt, acc :BigInt) :BigInt ={
if(n == 0) acc
else aux(n-1, acc + next,next)
}
aux(n,1,0)
}
This has already been answered, but hopefully you will find my experience helpful. I had a lot of trouble getting my mind around scala infinite streams. Then, I watched Paul Agron's presentation where he gave very good suggestions: (1) implement your solution with basic Lists first, then if you are going to generify your solution with parameterized types, create a solution with simple types like Int's first.
using that approach I came up with a real simple (and for me, easy to understand solution):
def fib(h: Int, n: Int) : Stream[Int] = { h #:: fib(n, h + n) }
var x = fib(0,1)
println (s"results: ${(x take 10).toList}")
To get to the above solution I first created, as per Paul's advice, the "for-dummy's" version, based on simple lists:
def fib(h: Int, n: Int) : List[Int] = {
if (h > 100) {
Nil
} else {
h :: fib(n, h + n)
}
}
Notice that I short circuited the list version, because if i didn't it would run forever.. But.. who cares? ;^) since it is just an exploratory bit of code.
The code below is both fast and able to compute with high input indices. On my computer it returns the 10^6:th Fibonacci number in less than two seconds. The algorithm is in a functional style but does not use lists or streams. Rather, it is based on the equality \phi^n = F_{n-1} + F_n*\phi, for \phi the golden ratio. (This is a version of "Binet's formula".) The problem with using this equality is that \phi is irrational (involving the square root of five) so it will diverge due to finite-precision arithmetics if interpreted naively using Float-numbers. However, since \phi^2 = 1 + \phi it is easy to implement exact computations with numbers of the form a + b\phi for a and b integers, and this is what the algorithm below does. (The "power" function has a bit of optimization in it but is really just iteration of the "mult"-multiplication on such numbers.)
type Zphi = (BigInt, BigInt)
val phi = (0, 1): Zphi
val mult: (Zphi, Zphi) => Zphi = {
(z, w) => (z._1*w._1 + z._2*w._2, z._1*w._2 + z._2*w._1 + z._2*w._2)
}
val power: (Zphi, Int) => Zphi = {
case (base, ex) if (ex >= 0) => _power((1, 0), base, ex)
case _ => sys.error("no negative power plz")
}
val _power: (Zphi, Zphi, Int) => Zphi = {
case (t, b, e) if (e == 0) => t
case (t, b, e) if ((e & 1) == 1) => _power(mult(t, b), mult(b, b), e >> 1)
case (t, b, e) => _power(t, mult(b, b), e >> 1)
}
val fib: Int => BigInt = {
case n if (n < 0) => 0
case n => power(phi, n)._2
}
EDIT: An implementation which is more efficient and in a sense also more idiomatic is based on Typelevel's Spire library for numeric computations and abstract algebra. One can then paraphrase the above code in a way much closer to the mathematical argument (We do not need the whole ring-structure but I think it's "morally correct" to include it). Try running the following code:
import spire.implicits._
import spire.algebra._
case class S(fst: BigInt, snd: BigInt) {
override def toString = s"$fst + $snd"++"φ"
}
object S {
implicit object SRing extends Ring[S] {
def zero = S(0, 0): S
def one = S(1, 0): S
def plus(z: S, w: S) = S(z.fst + w.fst, z.snd + w.snd): S
def negate(z: S) = S(-z.fst, -z.snd): S
def times(z: S, w: S) = S(z.fst * w.fst + z.snd * w.snd
, z.fst * w.snd + z.snd * w.fst + z.snd * w.snd)
}
}
object Fibo {
val phi = S(0, 1)
val fib: Int => BigInt = n => (phi pow n).snd
def main(arg: Array[String]) {
println( fib(1000000) )
}
}
Right now, I am trying to learn Scala . I've started small, writing some simple algorithms . I've encountered some problems when I wanted to implement the Sieve algorithm from finding all all prime numbers lower than a certain threshold .
My implementation is:
import scala.math
object Sieve {
// Returns all prime numbers until maxNum
def getPrimes(maxNum : Int) = {
def sieve(list: List[Int], stop : Int) : List[Int] = {
list match {
case Nil => Nil
case h :: list if h <= stop => h :: sieve(list.filterNot(_ % h == 0), stop)
case _ => list
}
}
val stop : Int = math.sqrt(maxNum).toInt
sieve((2 to maxNum).toList, stop)
}
def main(args: Array[String]) = {
val ap = printf("%d ", (_:Int));
// works
getPrimes(1000).foreach(ap(_))
// works
getPrimes(100000).foreach(ap(_))
// out of memory
getPrimes(1000000).foreach(ap(_))
}
}
Unfortunately it fails when I want to computer all the prime numbers smaller than 1000000 (1 million) . I am receiving OutOfMemory .
Do you have any idea on how to optimize the code, or how can I implement this algorithm in a more elegant fashion .
PS: I've done something very similar in Haskell, and there I didn't encountered any issues .
I would go with an infinite Stream. Using a lazy data structure allows to code pretty much like in Haskell. It reads automatically more "declarative" than the code you wrote.
import Stream._
val primes = 2 #:: sieve(3)
def sieve(n: Int) : Stream[Int] =
if (primes.takeWhile(p => p*p <= n).exists(n % _ == 0)) sieve(n + 2)
else n #:: sieve(n + 2)
def getPrimes(maxNum : Int) = primes.takeWhile(_ < maxNum)
Obviously, this isn't the most performant approach. Read The Genuine Sieve of Eratosthenes for a good explanation (it's Haskell, but not too difficult). For real big ranges you should consider the Sieve of Atkin.
The code in question is not tail recursive, so Scala cannot optimize the recursion away. Also, Haskell is non-strict by default, so you can't hardly compare it to Scala. For instance, whereas Haskell benefits from foldRight, Scala benefits from foldLeft.
There are many Scala implementations of Sieve of Eratosthenes, including some in Stack Overflow. For instance:
(n: Int) => (2 to n) |> (r => r.foldLeft(r.toSet)((ps, x) => if (ps(x)) ps -- (x * x to n by x) else ps))
The following answer is about a 100 times faster than the "one-liner" answer using a Set (and the results don't need sorting to ascending order) and is more of a functional form than the other answer using an array although it uses a mutable BitSet as a sieving array:
object SoE {
def makeSoE_Primes(top: Int): Iterator[Int] = {
val topndx = (top - 3) / 2
val nonprms = new scala.collection.mutable.BitSet(topndx + 1)
def cullp(i: Int) = {
import scala.annotation.tailrec; val p = i + i + 3
#tailrec def cull(c: Int): Unit = if (c <= topndx) { nonprms += c; cull(c + p) }
cull((p * p - 3) >>> 1)
}
(0 to (Math.sqrt(top).toInt - 3) >>> 1).filterNot { nonprms }.foreach { cullp }
Iterator.single(2) ++ (0 to topndx).filterNot { nonprms }.map { i: Int => i + i + 3 }
}
}
It can be tested by the following code:
object Main extends App {
import SoE._
val top_num = 10000000
val strt = System.nanoTime()
val count = makeSoE_Primes(top_num).size
val end = System.nanoTime()
println(s"Successfully completed without errors. [total ${(end - strt) / 1000000} ms]")
println(f"Found $count primes up to $top_num" + ".")
println("Using one large mutable1 BitSet and functional code.")
}
With the results from the the above as follows:
Successfully completed without errors. [total 294 ms]
Found 664579 primes up to 10000000.
Using one large mutable BitSet and functional code.
There is an overhead of about 40 milliseconds for even small sieve ranges, and there are various non-linear responses with increasing range as the size of the BitSet grows beyond the different CPU caches.
It looks like List isn't very effecient space wise. You can get an out of memory exception by doing something like this
1 to 2000000 toList
I "cheated" and used a mutable array. Didn't feel dirty at all.
def primesSmallerThan(n: Int): List[Int] = {
val nonprimes = Array.tabulate(n + 1)(i => i == 0 || i == 1)
val primes = new collection.mutable.ListBuffer[Int]
for (x <- nonprimes.indices if !nonprimes(x)) {
primes += x
for (y <- x * x until nonprimes.length by x if (x * x) > 0) {
nonprimes(y) = true
}
}
primes.toList
}