Decreasing for loop in Scala?

Question

First day and first attempt at using Scala - so go easy on me! I\'m trying to rewrite some old Java code I have which is simply a function which takes two numbers and prints

Answer 1

for (i <- (6 to 3 by -1)) {println ("i: " + i)}
i: 6
i: 5
i: 4
i: 3

If you happen to forget by -1, you can move up and use a function, to revert the result:

for (i <- (3 to 6)) {println ("i: " + ((6+3) - i))}

To exclude the second boundary, use until:

for (i <- (6 until 3 by -1)) {println ("i: " + i)}
i: 6
i: 5
i: 4

Alternatively, you could define an Iterator for your purpose. Extending an Iterator is easy; just implement 'hasNext:Boolean' and 'Next:[T]', where T is the type to handle - in our case Int or maybe Long or BigInt:

class FromToIterator (start: Int, stop: Int) extends Iterator [Int] { 
  var current = start
  //                        3       6       3         6         6       3       6         3
  def hasNext : Boolean = ((start < stop && current <= stop) || (start > stop && current >= stop)) 
  def next: Int = { 
    val res = current
    if (start < stop) current += 1 else current -= 1
    res
  } 
}
val it = new FromToIterator (3, 6)
val ti = new FromToIterator (6, 3)

for (i <-it) println (i)
for (i <-ti) println (i)

Answer 2

def printInDecreasingOrder(start : Int, end : Int){
  if(start > end ){
     for(i <- start to end by -1){
       println(s"Current value (decreasing from $start to $end) is $i")
     }
   }else{
     println("first num is smaller than second")
   }
}

method call:

printInDecreasingOrder(10, 2)

Result:

Current value (decreasing from 10 to 2) is 10

Current value (decreasing from 10 to 2) is 9

Current value (decreasing from 10 to 2) is 8

Current value (decreasing from 10 to 2) is 7

Current value (decreasing from 10 to 2) is 6

Current value (decreasing from 10 to 2) is 5

Current value (decreasing from 10 to 2) is 4

Current value (decreasing from 10 to 2) is 3

Current value (decreasing from 10 to 2) is 2

printInDecreasingOrder(1, 10)

Result:

first num is smaller than second

Answer 3

A point to add to this answer, if you do something like this

for(j <- finish to start){......}

sbt does not even show the error. So if you want a decrementing for loop you need to do this

for(j <- finish to start by -1){.......}

Answer 4

This way you can use Decreasing for loop in Scala.

    object Example extends App {


      for(i <- 20 to 2 by -2){


        println("Value of i = "+ i)

      }
    }
------------------
O/P
------------------
Value of i = 20
Value of i = 18
Value of i = 16
Value of i = 14
Value of i = 12
Value of i = 10
Value of i = 8
Value of i = 6
Value of i = 4
Value of i = 2

Answer 5

highnum to lownum by -1 (switch with other negative or positive step to change stepping)

def decrement(start: Int, finish: Int) = {
  for (i <- start to finish by -1) {
     println("Current value (decreasing from "+start+" to "+finish+") is "+i)
  }
}

I think this is a dupe of Scala downwards or decreasing for loop?

Answer 6

Here is a global increment/decrement solution inspired by Scala downwards or decreasing for loop?:

def goThrough(start: Int, finish: Int) = {     
  val d = if(start<=finish) 1 else -1
  for (i <- start to finish by d) {
    println("Current value (increasing from "+start+" to "+finish+") is "+i)
  } 
}