CODEWITHSUNDEEP
scalavariable-assignmentprintlnfor-comprehension
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Reputation: 36229

println in scala for-comprehension

In a for-comprehension, I can't just put a print statement:

def prod (m: Int) = {
  for (a <- 2 to m/(2*3);
    print (a + "  ");
    b <- (a+1) to m/a;
    c = (a*b) 
    if (c < m)) yield c
}

but I can circumvent it easily with a dummy assignment:

def prod (m: Int) = {
  for (a <- 2 to m/(2*3);
    dummy = print (a + "  ");
    b <- (a+1) to m/a;
    c = (a*b) 
    if (c < m)) yield c
}

Being a side effect, and only used (so far) in code under development, is there a better ad hoc solution?

Is there a serious problem why I shouldn't use it, beside being a side effect?

update showing the real code, where adapting one solution is harder than expected:

From the discussion with Rex Kerr, the necessity has risen to show the original code, which is a bit more complicated, but did not seem to be relevant for the question (2x .filter, calling a method in the end), but when I tried to apply Rex' pattern to it I failed, so I post it here:

  def prod (p: Array[Boolean], max: Int) = {
    for (a <- (2 to max/(2*3)).
        filter (p);
      dummy = print (a + "  ");
      b <- (((a+1) to max/a).
         filter (p));
      if (a*b <= max)) 
        yield (em (a, b, max)) }

Here is my attempt -- (b * a).filter is wrong, because the result is an int, not a filterable collection of ints:

  // wrong: 
  def prod (p: Array[Boolean], max: Int) = {
    (2 to max/(2*3)).filter (p).flatMap { a =>
      print (a + " ")
      ((a+1) to max/a).filter (p). map { b => 
        (b * a).filter (_ <= max).map (em (a, b, max))
      }
    }
  }

Part II belongs to the comments, but can't be read, if written there - maybe I delete it in the end. Please excuse.

Ok - here is Rex last answer in code layout:

  def prod (p: Array[Boolean], max: Int) = {
    (2 to max/(2*3)).filter (p).flatMap { a =>
      print (a + " ")
      ((a+1) to max/a).filter (b => p (b) 
        && b * a < max).map { b => (m (a, b, max))
      }
    }
  }

Upvotes: 33

Views: 17556

Answers (4)

Xavier Guihot
Xavier Guihot

Reputation: 61646

Starting Scala 2.13, the chaining operation tap, has been included in the standard library, and can be used with minimum intrusiveness wherever we need to print some intermediate state of a pipeline:

import util.chaining._

def prod(m: Int) =
  for {
    a <- 2 to m / (2 * 3)
    b <- (a + 1) to (m / a.tap(println)) // <- a.tap(println)
    c =  a * b
    if c < m
 } yield c

prod(20)
// 2
// 3
// res0: IndexedSeq[Int] = Vector(6, 8, 10, 12, 14, 16, 18, 12, 15, 18)

The tap chaining operation applies a side effect (in this case println) on a value (in this case a) while returning the value (a) untouched:

def tap[U](f: (A) => U): A

It's very convenient when debugging as you can use a bunch of taps without having to modify the code:

def prod(m: Int) =
  for {
    a <- (2 to m.tap(println) / (2 * 3)).tap(println)
    b <- (a + 1) to (m / a.tap(println))
    c = (a * b).tap(println)
    if c < m
 } yield c

Upvotes: 5

Luigi Plinge
Luigi Plinge

Reputation: 51099

It doesn't seem like good style to put a side-effecting statement within a for-comprehension (or indeed in the middle of any function), execept for debugging in which case it doesn't really matter what you call it ("debug" seems like a good name).

If you really need to, I think you'd be better separating your concerns somewhat by assigning an intermediate val, e.g. (your original laid out more nicely):

  def prod (p: Array[Boolean], max: Int) = {
    for {
      a <- (2 to max / (2 * 3)) filter p
      debug = print (a + "  ")
      b <- ((a + 1) to max / a) filter p
      if a * b <= max
    } yield em(a, b, max) 
  }

becomes

  def prod2 (p: Array[Boolean], max: Int) = {
    val as = (2 to max / (2 * 3)) filter p

    for(a <- as) print(a + "  ")

    as flatMap {a => 
      for {
        b <- ((a + 1) to max / a) filter p
        if a * b <= max
      } yield em(a, b, max)
    }
  }

Upvotes: 2

missingfaktor
missingfaktor

Reputation: 92016

This is how you need to write it:

scala> def prod(m: Int) = {
     |   for {
     |     a <- 2 to m / (2 * 3)
     |     _ = print(a + " ")
     |     b <- (a + 1) to (m / a)
     |     c = a * b
     |     if c < m
     |   } yield c
     | }
prod: (m: Int)scala.collection.immutable.IndexedSeq[Int]

scala> prod(20)
2 3 res159: scala.collection.immutable.IndexedSeq[Int] = Vector(6, 8, 10, 12, 14
, 16, 18, 12, 15, 18)

Upvotes: 50

Rex Kerr
Rex Kerr

Reputation: 167871

I generally find that style of coding rather difficult to follow, since loops and intermediate results and such get all mixed in with each other. I would, instead of a for loop, write something like

def prod(m: Int) = {
  (2 to m/(2*3)).flatMap { a =>
    print(a + " ")
    ((a+1) to m/a).map(_ * a).filter(_ < m)
  }
}

This also makes adding print statements and such easier.

Upvotes: 2

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