Assign value to variable in Scala Parser

Question

i have a question about the Parser in Scala. I'll just post the important part here so that there would not be too much codes. For the eval Function:

def eval(t: Term): Double = t match {
    case Addition(l, r) => eval(l) + eval(r)
    case Multiplication(l, r) => eval(l) * eval(r)
    case Numeric(i) => i
    case Variable("X") => 3
    }

And the calculate function:

def calculate(arg: String): Double = {
    return eval(parseAll(term, arg).get)
  }

now i should overload the function "calculate" so that it takes an extra Parameter tup : (String, Double) and assign the value for this String. For example ("Y",2) then Y = 2 in the Parser. And then calculate the parser. But i don't know how to assign the value for this String. I had a stupid idea and tried this but it didn't work.

def calculate(arg: String, tup : (String, Double)) : Double = {
    tup match {
      case (a,b) => {
        def eval(t : Term): Double = t match {
          case Variable(a) => b
        }
        return eval(parseAll(term, arg).get)
      }
    }

can you guys pls help me out ? Thank you !!

Cyrille Corpet · Accepted Answer · 2017-06-06 12:10:21Z

4

You're almost there, you just need to tell the compiler that the a in your Variable pattern is actually the a from your (a, b) pattern. By default, what you do is called shadowing of the variable name a (in the scope of this pattern match, a is the value extracted in Variable, and the other a is forgotten).

What you want is something like

...
  case Variable(`a`) => b
...

or, if your expression gets a little more complicated, you should rather use a guard:

...
  case Variable(v) if v == a => b
...

EDIT However, now your eval function is not well defined. You need to put it all at once:

def eval(t: Term, varAssignement: (String, Double)): Double = t match {
  case Addition(l, r) => eval(l) + eval(r)
  case Multiplication(l, r) => eval(l) * eval(r)
  case Numeric(i) => i
  case Variable(a) if a == varAssignment._1 => varAssignment._2
}

Or, if you want to have multiple variables:

def eval(t: Term, assignments: Map[String, Double]): Double = t match {
  case Addition(l, r) => eval(l) + eval(r)
  case Multiplication(l, r) => eval(l) * eval(r)
  case Numeric(i) => i
  case Variable(a) if assignments.exists(a) => assignments(a)
}

Beware that you'll still get MatchErrors whenever an unassigned variable is used.

edited Jun 6, 2017 at 12:10

answered Jun 6, 2017 at 9:57

Cyrille Corpet

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2 Comments

user7959714 Over a year ago

i tried to use your method but when i call println(calculate("3+5*A",("A",2))), it throws exception like this Exception in thread "main" scala.MatchError: Addition(Numeric(3.0),Multiplication(Numeric(5.0),Variable(A))) so i guess the value 2 was not assigned to "A". The error is in the eval function where i do the shadowing in my pattern match

Cyrille Corpet Over a year ago

Oh, right. Your eval only matched if you have a Variable(``a``), you'll probably need all the cases. I'll edit my answer.

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