shaktimaan - 1 year ago 85
Scala Question

# Displaying nested arithmetic expressions as a string using Scala pattern matching

I am learning the concepts of pattern matching in Scala. Following is an exercise for the same. The task is to define a show function that outputs an expression as a

`String`
. Following are the definitions involved:

``````object Test {
trait Expr
case class Number(n: Int) extends Expr
case class Sum(e1: Expr, e2: Expr) extends Expr
case class Prod(e1: Expr, e2: Expr) extends Expr
case class Var(name: String) extends Expr

def nestOperations(e1: Expr, e2: Expr): String = {
val l = e1 match {
case Number(n) => n.toString
case Prod(x, y) => nestOperations(x, y)
case Var(s) => s
case Sum(x, y) => "(" + show(x) + " + " + show(y) + ")"
}
val r = e2 match {
case Number(n) => n.toString
case Prod(x, y) => nestOperations(x, y)
case Var(s) => s
case Sum(x, y) => "(" + show(x) + " + " + show(y) + ")"
}
l + " * " + r
}

def show(e: Expr): String = e match {
case Number(n) => n.toString
case Sum(e1, e2) => show(e1) + " + " + show(e2)`
case Prod(e1, e2) => nestOperations(e1, e2)
case Var(s) => s
}
show(Sum(Number(1), Number(2)))
show(Sum(Prod(Number(2), Var("x")), Var("y")))
show(Prod(Sum(Number(2), Var("x")), Var("y")))
}
``````

The expectation is that:
`show(Sum(Number(1), Number(2)))`
outputs
`1 + 2`

`show(Sum(Prod(Number(2), Var("x")), Var("y")))`
outputs
`2 * x + y`

`show(Prod(Sum(Number(2), Var("x")), Var("y")))`
outputs
`(2 + x) * y`

The program achieves all of the above. I want to know if the
`nestOperations`
method can be simplified? Looks like a lot of repetitive code in there.

You're right. That repeated code in `nestOperations()` can be eliminated.

``````def nestOperations(es: Expr*): String =
es.map {
case Number(n) => n.toString
case Prod(x, y) => nestOperations(x, y)
case Var(s) => s
case Sum(x, y) => "(" + show(x) + " + " + show(y) + ")"
}.mkString(" * ")
``````

But why stop there? You can pull much the same stunt with `show()` and eliminate `nestOperations()` altogether.

``````def show(es: Expr*): String = es.map{
case Number(n) => n.toString
case Sum(e1, e2) => if (es.length > 1) "(" + show(e1) + " + " + show(e2) + ")"
else show(e1) + " + " + show(e2)
case Prod(e1, e2) => show(e1, e2)
case Var(s) => s
}.mkString(" * ")
``````
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