Friday, August 30, 2013

Breakable loops in Scala

Overview
Contrary to C++ and Java, Scala does not allow developer to exit an iterative execution prematurely using a syntax equivalent to break.

var sum = 0

for( i <- 0 until 100) {
  sum += i
  if( sum > 400) 
    break   // won't compile!                                  
}

Scala purists tend to stay away from this type of constructs and use higher order collection methods such as
   exists( p: (T) => Boolean)
   find( p: (T) => Boolean)
   takeWhile( p: (T) => Boolean)
However these methods are not available outside collections. There are cases where a `break` construct may be a simpler solution.
This post review the different options to 'break' from a loop according to a predefined condition.

Breakable control
Although breaking out of a loop may not be appealing to "pure" functional developer, the Scala language provides former Java and C++ developer with the option to use break and continue statements. The breakable statement define the scope for which break is valid.

import scala.util.control.Breaks._                            

var sum = 0
breakable { 
  for (i <- 0 until 100 ) {
    sum += i
    if( sum > 55) 
      break
  }
}

Any experienced developer would be quite reluctant to use such an idiom: beside the fact that the accumulator is defined as a variable, the implementation is unnecessary convoluted adding a wrapper around the loop. Let's try to find a more functional like approach to breaking out of any loop

scan & fold to the rescue
Luckily, Scala provides collections with functional traversal patterns that can be used and chained to break, elegantly from a loop. The following code snippets introduce applies some of those traversal patterns to an array and a associative map to illustrate the overall "functional" approach to iteration.
Let's consider the problem of extracting the elements of an array or map before a predefined condition is met for an element. For instance, let's extract the elements of an array or a hash map until one element has the value 0. The Scala programming language provide us with the takeWhile method (lines 7 & 10) that allows to to end traversing a collection and return a elements of the collection visited when a condition is met.

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val values = Array[Int](9, 45, 11, 0, 67, 33)

val mappedValues = Map[String, Int](
  "A"->2, "B"->45, "C"->67, "D"->0, "E"->56
)
 // Extract a subarray of values until the condition is met
values takeWhile( _ != 0) // -> Array(9, 45, 11)
 
  // Extract a submap until the condition is met
mappedValues takeWhile( _._2 != 0) 
  // -> Map(E -> 56, A -> 2, B -> 45, C -> 67)

The second case consists in accumulating values until a condition on the accumulator is met. Contrary to fold and reduce methods which apply a function (i.e summation) to all the elements of a collection to return a single value, scan, scanLeft and scanRight methods return a collection of values processed by the function.

In the following example, the invocation of the higher order method scanLeft (lines 4 generates an array and a hash map with the cumulative values. The takeWhile method i(lines 5 & 8) s then applied to the resulting array or map to return a collection of cumulative values until the accumulator exceeds 56. The same invocation can be used to return the element which push the accumulator beyond the threshold.

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values.scanLeft(0)(_ + _).takeWhile (_ < 56) 
   //-> Array(0, 9, 54)
 
mappedValues.scanLeft(0) (_ + _._2)
   .takeWhile( _ < 56)
 
val indexTargetEl = values.scanLeft(0)(_ + _)
      .takeWhile (_ < 56).size -1            
val targetEl = values(indexTargetEl)



Tail recursion
A third and elegant alternative to exit from a loop is using the tail recursion which is supported natively in the Scala language. The first method, findValue exits the loop when a condition on the value is reached (line 5). The second method, findCummulative, is implemented as a closure and exits when the sum of elements exceeds a predefined value (line 15).

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val values = Array[Int](9, 45, 11, 0, 67, 33)
 
@scala.annotation.tailrec
def findValue(values: Array[Int], index: Int) : Int = {
   if( values(index) == 0 || index >= values.size)
     index
   else
     findValue(values, index + 1)
}
val newValues = values.slice(0, findValue(values, 0))   

val MAX_VALUE :Int = 86

@scala.annotation.tailrec
def findCumulative(sum: Int, index: Int): Int = {
   if( index >= values.size ||sum >= MAX_VALUE)
     index -1
   else
     findCumulative(sum + values(index), index + 1)
}

val newValues2 = values.slice(0, findCumulative(0, 0))

This concludes our review of three different Scala constructs available to developer to exit prematurely from a loop
  • breakable construct
  • Higher order method such as exists, find, takeWhile....
  • tail recursion

References
github.com/prnicolas
Programming in Scala M. Odersky, L. Spoon, B. Venners - Artima 2010

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