CODEWITHSUNDEEP
javascala
Jacky Wong
Jacky Wong

Reputation: 521

how can i use Scala's for with index change?

i got a problem in Scala's loop: in java i can do this:

for(int i=1;i<list.length;i=i*2

but how could i do the same in scala? Scala's for index cannot change

Upvotes: 1

Views: 347

Answers (3)

adamwy
adamwy

Reputation: 1239

scanLeft method might be helpful there:

def exps(n: Int, p: Int) = (0 until n).scanLeft(1)((acc, _) => acc * p)

val list = List(2, 3, 5, 7)
for (i <- exps(list.size, 2)) println(i)
1
2
4
8
16

Or using infinite streams:

def expStream(p: Int, i: Int = 1): Stream[Int] = i #:: expStream(p, i * p)

for (i <- expStream(2).take(list.size)) println(i)

Also you can use Stream.iterate (which is somewhat more concise) as Gyro Gearless has suggested in their answer:

def expStream(p: Int) = Stream.iterate(1)(_ * p)

Upvotes: 4

Gyro Gearless
Gyro Gearless

Reputation: 5279

An idiomatic way would be to create an Stream from the function "*2":

import Stream._
val potsOfTwo = Stream.iterate (1) (_ * 2).takeWhile (_ < 1000)
potsOfTwo.foreach { println _ }

scala> potsOfTwo.foreach { println _ } 1 2 4 8 16 32 64 128 256 512

Upvotes: 3

Pavel
Pavel

Reputation: 1539

Well. On practice I would probably go for next solution:

val m = 100
val r = (0 to m).map(x => if (x * 2 < m) x * 2 else  -1).filter(t => t != -1 )
println(r)

Here is documentation and examples you need to look at. I would suspect you looking for ranges:

val r = 1 to 10
r: scala.collection.immutable.Range.Inclusive = Range(1, 2, 3, 4, 5,
   6, 7, 8, 9, 10)

scala> val r = 1 to 10 by 2
r: scala.collection.immutable.Range = Range(1, 3, 5, 7, 9)

scala> val r = 1 to 10 by 3
r: scala.collection.immutable.Range = Range(1, 4, 7, 10)

http://docs.scala-lang.org/tutorials/tour/sequence-comprehensions.html

https://www.safaribooksonline.com/library/view/scala-cookbook/9781449340292/ch02s09.html

Upvotes: 0

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