CODEWITHSUNDEEP
bisonyaccparser-generator
rameshmani
rameshmani

Reputation: 825

yacc/bison identify result in action

I am using Jflex, byacc, java for parsing statement which looks something like

where expr,expr having expr

and the grammer looks like

%token WHERE HAVING COMMA
%%

stmt : whereclause havingclause

whereclause : WHERE conditionlist { move tmpList to whereClause};
havingclause : HAVING conditionlist { move tmpList to havingClause};

conditionlist : condition | condition COMMA conditionlist;

condition : expr { tmpList.add($1); 
                  /How do I know this is being executed for whereclause or havingclause /};

expr : ....

I am not able to differentiate if condition is part of whereclause or havingclause, So I am storing conditions in temporary list and then moving to the right clause. What is the correct way of doing this?

Upvotes: 0

Views: 108

Answers (1)

Chris Dodd
Chris Dodd

Reputation: 126378

Most commonly, you build the data structures in the rule actions, assigning pointers to them to $$ and then read them in the higher level rules. Something like:

%token WHERE HAVING COMMA
%union {
    class List *listptr;
    class Expr *exprptr;
}
%type<listptr> whereclasue havingclause conditionlist
%type<exprptr> condition expr
%destructor { delete $$; } <listptr>
%destructor { delete $$; } <exprptr>
%%

stmt : whereclause havingclause { do something with $1 and $2... }

whereclause : WHERE conditionlist { $$ = $2; };
havingclause : HAVING conditionlist { $$ = $2 };

conditionlist : condition { $$ = new List($1); }
              | condition COMMA conditionlist { $$ = $2->prepend($1); };

condition : expr { $$ = $1; }

expr : ....

Note the %destructor actions are a bison extension, and are needed to avoid memory leaks when there are syntax errors.

Upvotes: 1

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