Convert Java loop to Scala

Question

I'm attempting to convert a Selection sort from Java to Scala and im unsure how to convert this loop to Scala :

for (int j = i + 1; j < N; j++)

Here is a larger Java code sample and its Scala equivalent :

Java :

sort(Comparable[] a)
{
int N = a.length;
for (int i = 0; i < N; i++)
{
   int min = i
   for (int j = i + 1; j < N; j++)
}

Scala :

  def sort(a : Array[Ordered[Any]]) = {
    var N = a.length

    for (i <- 0 until N) {
        var min = i

        for(j <- until j < N){

        }
    }

  }

How do I convert the inner loop to Scala ?

for (int j = i + 1; j < N; j++)

I dont know how to do assignment while iterating...

om-nom-nom · Accepted Answer · 2013-04-23 20:08:46Z

7

Here you go:

def sort(a : Array[Ordered[Any]]) = {
    val N = a.length

    for (i <- 0 until N) {
        var min = i

        for(j <- i + 1 until N){

        }
    }

  }

Moreover, in Scala you can define values inside for comprehension, and merge multiple blocks into one:

def sort(a : Array[Ordered[Any]]) = {
        val n = a.length

        for(i <- 0 until n; min = i; j <- i + 1 until n) { // min here is val, not var
          // do something with i, j and min
        }
}

Sometimes, this may lead to cleaner code

edited Apr 23, 2013 at 20:08

answered Apr 23, 2013 at 20:02

om-nom-nom

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Comments

BeniBela · Accepted Answer · 2013-04-23 20:10:33Z

3

Unfortunately, the standard for loops are quite slow on Scala (especially with older version).

An alternative is the classical while loop, even if it is not so clear:

  def sort(a : Array[Ordered[Any]]) = {
    val N = a.length

    var i = 0;
    while (i < N) {
        var min = i

        var j = i + 1;
        while (j < N) {
          j += 1;
        }
        i += 1;
    }

}

Or tail recursive functions:

  def sort(a : Array[Ordered[Any]]) = {
    val N = a.length

    def l1(i: Int){
      def l2(j: Int, min: Int){
        if (j < N) 
          l2(j+1, min)
      }
      if (i < N) {
         l2(i+1, i);
         l1(i+1);
      }
    }
}

Or the cfor of spire:

  def sort(a : Array[Ordered[Any]]) = {
    val N = a.length

    cfor(0)(_ < N, _ + 1) { i => 
        var min = i
        cfor(i+1)(_ < N, _ + 1) { j => 
        }
    }

}

answered Apr 23, 2013 at 20:10

BeniBela

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2 Comments

om-nom-nom Over a year ago

See also this question on scala for comprehensions performance.

blue-sky Over a year ago

@BeniBela by 'standard for loops' are you including '.foreach' ?

ferk86 · Accepted Answer · 2013-04-24 00:56:16Z

0

Using for loops (not exactly a functional/idiomatic scala way) for iterating will closely resemble the java code. This will get you through.

def sort(a: Array[Comparable]) {
  val N = a.length
  for (
    i <- 0 until N;
    min = i;
    j <- (i + 1) until N
  ) {
    // more code..
  }
}

Here's your java code for reference:

sort(Comparable[] a)
{
int N = a.length;
for (int i = 0; i < N; i++)
{
   int min = i
   for (int j = i + 1; j < N; j++)
}

answered Apr 24, 2013 at 0:56

ferk86

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