Question

I have the following function and cannot figure out why it is not working.

Parameters are a set of Nonterminals and a vector of GrammarSymbol* (a word). Nonterminal is a subclass of GrammarSymbol. The function is supposed to filter all Nonterminals that are contained in the word as well as in the Nonterminal set and return them in a set.

std::set<Nonterminal> Filter(const std::set<Nonterminal>& symbolSet, const std::vector<GrammarSymbol*> w){

  //resulting set
  std::set<Nonterminal> rSet;

  std::vector<GrammarSymbol*>::const_iterator wit;
  std::set<Nonterminal>::const_iterator ntit;

  //iterate over all symbols of the word
  for(wit = w.begin(); wit != w.end(); wit++){

    //test if current symbol is a nonterminal
    const Nonterminal* nt = dynamic_cast<const Nonterminal*>(*wit);
    if(nt != NULL){
      std::cout << "current symbol " << nt->Str() << " is nonterminal" << std::endl;

      for(ntit = symbolSet.begin(); ntit != symbolSet.end(); ntit++){
        std::cout << ntit->Str() << "  " << (!(*ntit < *nt) && !(*nt < *ntit))<< std::endl;
      }
      //look for the symbol in the nonterminal set
      ntit = symbolSet.find(*nt);

      //if the symbol was found, insert it into resulting set
      if(ntit != symbolSet.end()){
        rSet.insert(*ntit);
        std::cout << "inserted " << ntit->Str() << "into set, size: " << rSet.size() << std::endl;
      }
      else{
        std::cout << "not found in symbolSet" << std::endl;
      }
    }
  }
  return rSet;
}

This yields the output

current symbol (1, 2, 2) is nonterminal
(1, 2, 2)  1
(2, 3, 3)  0
(3, 2)  0
(4, 3)  0
(5, 3, 1)  0
not found in symbolSet

It works just fine if I don’t rely on the filter function and filter on my own:

std::set<Nonterminal> Filter(const std::set<Nonterminal>& symbolSet, const std::vector<GrammarSymbol*> w){

  //resulting set
  std::set<Nonterminal> rSet;

  std::vector<GrammarSymbol*>::const_iterator wit;
  std::set<Nonterminal>::const_iterator ntit;

  //iterate over all symbols of the word
  for(wit = w.begin(); wit != w.end(); wit++){

    //test if current symbol is a nonterminal
    const Nonterminal* nt = dynamic_cast<const Nonterminal*>(*wit);
    if(nt != NULL){
      std::cout << "current symbol " << nt->Str() << " is nonterminal" << std::endl;

      for(ntit = symbolSet.begin(); ntit != symbolSet.end(); ntit++){
        std::cout << ntit->Str() << "  " << (!(*ntit < *nt) && !(*nt < *ntit))<< std::endl;
        if(!(*ntit < *nt) && !(*nt < *ntit)){
          rSet.insert(*ntit);
        }
      }
    }
  }
  return rSet;
}

Can anyone explain to me what happens here? As far as I know, std::set is supposed to compare elements with the operator<. Comparing seems to work just fine, as shown by the output.

I would just continue with the selfmade filter, but I fear there is a bigger underlying problem.

Thanks!

Edit: Nonterminal and operator< for Nonterminal:

class Nonterminal : public GrammarSymbol{

  public: 

  /** The start state*/
  Idx mStartState;
  /** The stack symbol*/
  Idx mOnStack;
   /** The end state */
  Idx mEndState;
  //...
  }

Idx is just a typedef for an int.

bool Nonterminal::operator<(const GrammarSymbol& other) const{
  if(typeid(*this) != typeid(other)) return true; //other is a terminal
  const Nonterminal& nt = dynamic_cast<const Nonterminal&>(other); //other is a nonterminal
  if (mStartState < nt.StartState()) return true;
  if (mOnStack < nt.OnStack()) return true;
  if (mEndState < nt.EndState()) return true;
  return false;    
}

Answer 1

You operator < is incorrect

Consider

Nonterminal nt1 (1,2,3);
Nonterminal nt2 (3,2,1);

bool b1 = nt1 < nt2;
bool b2 = nt2 < nt1;

For nt1 < nt2 comparison:

• 1 < 3 immediatelly yelds true;

For nt2 < nt1:

• 3 < 1 doesn’t hold, so you proceed to
• 2 < 2 which doesn’t hold, so you proceed to
• 1 < 3 which holds

Thus both b1 and b2 will be true, which is nonsense

As for your second variant of filter, it works becase of logic error

for(ntit = symbolSet.begin(); ntit != symbolSet.end(); ntit++){
        std::cout << ntit->Str() << "  " << (!(*ntit < *nt) && !(*nt < *ntit))<< std::endl;
        if(!(*ntit < *nt) && !(*nt < *ntit)){
          rSet.insert(*ntit); 
        }

here rSet.insert(*ntit); will be called every time if(!(*ntit < *nt) && !(*nt < *ntit)) doesn’t hold, not once as it should.