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 out the numbers from x to y. For example, i have the increment function:

    def increment(start: Int, finish: Int) = {
      for (i <- start to finish) {
         println("Current value (increasing from "+start+" to "+finish+") is "+i)
      }
    }

However, im struggling writting a corresponding decrement function which will decrease from start to finish? I have read Scala downwards or decreasing for loop? but am still unsure

Thank you

dacwe · Accepted Answer · 2012-04-02 13:04:42Z

18

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

scala> decrement(10, 1)
Current value (decreasing from 10 to 1) is 10
Current value (decreasing from 10 to 1) is 9
Current value (decreasing from 10 to 1) is 8
Current value (decreasing from 10 to 1) is 7
Current value (decreasing from 10 to 1) is 6
Current value (decreasing from 10 to 1) is 5
Current value (decreasing from 10 to 1) is 4
Current value (decreasing from 10 to 1) is 3
Current value (decreasing from 10 to 1) is 2
Current value (decreasing from 10 to 1) is 1

answered Apr 2, 2012 at 13:04

dacwe

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6 Comments

rwb Over a year ago

Perfect thanks. I now have no idea how i didnt see that from stackoverflow.com/questions/2617513/… Definitely not thinking straight!

rwb Over a year ago

One more thing - sorry. How would i get the value of the iterator in the println?

dacwe Over a year ago

start to finish by -1 will return a Range which you can iterate over.

rwb Over a year ago

Is there anyway to write this function in functional form?

dacwe Over a year ago

def dec(start: Int, finish: Int) = { start to finish by -1 }; dec(10, 1).foreach( i => println("decrementing " + i) );

|

user unknown · Accepted Answer · 2012-04-02 13:37:43Z

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)

Nilesh Shinde · Accepted Answer · 2017-11-07 10:27:00Z

2

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

answered Nov 7, 2017 at 10:27

Nilesh Shinde

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Comments

Community · Accepted Answer · 2017-05-23 12:02:59Z

1

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?

edited May 23, 2017 at 12:02

CommunityBot

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answered Apr 2, 2012 at 13:14

Roman A. Taycher

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Comments

Community · Accepted Answer · 2017-05-23 12:26:42Z

0

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)
  } 
}

edited May 23, 2017 at 12:26

CommunityBot

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answered Apr 2, 2012 at 13:07

Christopher Chiche

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Comments

Tunaki · Accepted Answer · 2016-01-17 18:03:12Z

0

object Test extends App{

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

  decrement(5,0,-1)
}

This is also a method. but may not be the best

edited Jan 17, 2016 at 18:03

Tunaki

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answered Jan 17, 2016 at 17:58

StandForTheRight

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Comments

Azam Khan · Accepted Answer · 2017-10-24 16:33:44Z

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

CV_Ruddha · Accepted Answer · 2020-10-09 22:24:42Z

0

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){.......}

answered Oct 9, 2020 at 22:24

CV_Ruddha

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