Flex和野牛中的逻辑表达式语法

Question

I am writing a simple Grammar for logical expression in flex and bison. The expression is a postfix expression. This is my yacc code.

float_logic_expr:
                float_logic_expr
                | float_logic_exprs
                ;

float_logic_exprs:
                float_reln_expr
            | float_reln_expr float_reln_expr PP_AND                    
            | float_reln_expr float_reln_expr PP_OR                 
            ;

string_logic_expr:
              string_reln_expr
            | string_reln_expr string_reln_expr PP_AND              
            | string_reln_expr string_reln_expr PP_OR               
            ;

The above rule works fine for the below statement.

if #a 40 >  #b 20 == && then

But if i give one more condition to the logical expression, it fails.

if #a 40 >  #b 20 == && #b 30 == && then

Parse error.

What should I change the grammar to work for multiple relational expression?

Answer 1

It's doing what you told it to do, no?

float_logic_exprs
            : float_reln_expr
            | float_reln_expr float_reln_expr PP_AND                    
            | float_reln_expr float_reln_expr PP_OR                 
            ;

That says that a logic expression is a single relation expression or two (exactly) relation expressions followed by an AND or OR token. But the example you provide shows that you expect a logic expression to be able to take another logic expression as an argument, not just a relation expression.

Also, this is pointless:

float_logic_expr:
                float_logic_expr
                | float_logic_exprs
                ;

Since the production float_logic_expr: float_logic_expr cannot ever be useful (and it must trigger some kind of bison warning).

All that should suggest that you want:

float_logic_expr
            : float_reln_expr
            | float_logic_expr float_logic_expr PP_AND                    
            | float_logic_expr float_logic_expr PP_OR                 
            ;