CODEWITHSUNDEEP
javascalacompiler-errors
Night JonSnow King
Night JonSnow King

Reputation: 43

To insert scala code for two methods

Consider the LL1 grammer: ::= | | num ::= + ::= *

Now consider the set of scala cases classes. Below code is given and I only need to add code inside the expr() and times() method

sealed abstract class Token
case object PlusToken extends Token
case object TimesToken extends Token
case class NumToken(value:Int) extends Token

def isExpr(tokens:List[Token]):Boolean = {    
var remaining = tokens
   object ParseError extends Exception

   def expr():Unit = // ** add code here

   def plus():Unit = {
     if (remaining.headOption != Some(PlusToken)) throw ParseError
     remaining = remaining.tail
     expr()
     expr()
   }

   def times():Unit = // ** add code here

 try {
 expr()
 if (remaining.isEmpty)
    true
 else
    false
} catch {
 case ParseError => false
 }
}

I need to add code for the expr() and times() method in such a way that isExpr() returns whether the list of tokens passed to it can be derived from according to the grammar.

Output should be in following ways: 

isExpr(List(PlusToken, NumToken(5), NumToken(3)))               => true
isExpr(List(PlusToken, PlusToken))                              => false
isExpr(List(PlusToken, NumToken(1)))                            => false
isExpr(List(PlusToken, NumToken(9), NumToken(8), NumToken(7)))  => false

Following is my code for expr() and times() method:

sealed abstract class Token
case object PlusToken extends Token
case object TimesToken extends Token
case class NumToken(value:Int) extends Token

def isExpr(tokens:List[Token]):Boolean = 
{
 var remaining = tokens
 object ParseError extends Exception

 def expr():Unit = // ** add code here
 {
     if(remaining.HeadOption == Some(PlusToken))
       plus()
       else if(remaining.headOption == Some(TimesToken))
       times()
     else remaining = remaining.tail
 }

   def plus():Unit = 
   {
     if (remaining.headOption != Some(PlusToken)) throw ParseError
     remaining = remaining.tail
     expr()
     expr()
   } 

   def times():Unit = // ** add code here
   {
     if (remaining.headOption != Some(TimesToken)) throw ParseError
     remaining = remaining.tail
     expr()
     expr()
   }
   try 
   {
     expr()
     if (remaining.isEmpty)
       true
     else
    false
   } catch 
   {
     case ParseError => false
   }
def main(argv: Array[String]) {}
}

For my code, I am getting compile time error. Error is: excepted class or object definition. Also, I am not sure how to use the main class to print whatever is expected. Please look into my scala code as I am completely new to it and provide your input.

Upvotes: 0

Views: 57

Answers (1)

Justin Pihony
Justin Pihony

Reputation: 67075

You are writing your code outside of an object (class, trait, or object) as the error is stating. So you need to wrap the code inside of one of those structures.

Example for object

object Main {
  def someMethod....
  def main(argv: Array[String]) {
    ...main entry point of your system...
  }
}

Upvotes: 1

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