Target audience: Beginner
Estimated reading time: 10'
Contrary to C++ and Java, Scala does not allow developer to exit an iterative execution prematurely using a syntax equivalent to break.
Scala purists tend to stay away from this type of constructs and use higher order collection methods such as
This post review the different options to 'break' from a loop according to a predefined condition.
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.
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
Tail recursion
References
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.
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.
1 2 3 4 5 6 7 8 9 10 11 | 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.
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.
1 2 3 4 5 6 7 8 9 | 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).
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | 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
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