Scala Pattern Matching with tuple

Question

I'm working on the first assignment of the Coursera class.

For the following code, I'm stuck on the compile-time errors.

    object pascal {
        def main(c: Int, r: Int) = {
                pascal(c, r)
            }
            def pascal(c: Int, r: Int) = {
                (c, r) match {
                    case ((r < 0) || (c < 0)) => throw new Exception
                                                 ("r and c must be > 0")
                    case r == 0 => 1
                    case r == 1 => 1
                    case c == 0 => 1
                    case r == c => 1
                    case _ => pascal(r-1,c-1) + pascal(r-1,c-1)
            }
        }
     }

vagrant@precise64:/vagrant/Workspace/Scala/hw1$ scalac pascal.scala
pascal.scala:7: error: not found: value ||
                        case ((r < 0) || (c < 0)) => throw ...
                                      ^
pascal.scala:8: error: value == is not a case class constructor, nor 
does it have an unapply/unapplySeq method
                        case r == 0 => 1

Please advise.

Thanks.

score 3 · Accepted Answer · edited May 23 '17 at 10:30

3

This is invalid syntax in scala: you need to use so called guards to check for boolean properties:

def pascal(c: Int, r: Int) = {
   (c, r) match {
     case (c,r) if ((r < 0) || (c < 0)) => throw new Exception
                                                 ("r and c must be > 0")
     ...
}

By the way much more idiomatic to write code like this:

def pascal(c: Int, r: Int) = {
   require((r >= 0) && (c >= 0), "r and c must be >= 0")
   (c, r) match {
     case (_, 1) => 1
     ...
}

And you have to change error message to "r and c must be >= 0"

edited May 23 '17 at 10:30

Community

1
1

answered Dec 20 '12 at 05:53

om-nom-nom

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Not sure if the assignment explicitly requires working with pattern matching, but isn't this code packing c and r (which are already separate parameters) into a tuple just to unpack them again in the case? I would have used just a simple if – Paolo Falabella Dec 20 '12 at 07:27
Your last piece of code is wrong: `case r == 0` won't compile – oxbow_lakes Dec 20 '12 at 08:26
How can I do: case "where c == r"? – Kevin Meredith Dec 22 '12 at 18:32
1

@Kevin try `(c,r) match { case(\`c\`,\`c\`) => ... }` -- this instructs pattern matching to do a comparison by value – om-nom-nom Dec 22 '12 at 18:35
Thanks. Once I compile using "scalac Pascal.scala," how can I even run this program? – Kevin Meredith Dec 22 '12 at 21:02
1

@Kevin http://www.scala-lang.org/node/166 and please, [ask a different question](http://stackoverflow.com/questions/ask) if it is unrelated to this one – om-nom-nom Dec 22 '12 at 21:07
Is using the tick ` to force comparison by value hacky? – Kevin Meredith Aug 27 '14 at 20:22

Scala Pattern Matching with tuple

1 Answers1