Is there a shift/reduce error in this yacc code?

Question

I'm getting a message from yacc saying that there is a shift/reduce conflict. I think it's coming from this part of the yacc file.

statement : expression_stmt
          | compound_stmt
          | selection_stmt
          | iteration_stmt
          | return_stmt ;

selection_stmt : IF '(' expression ')' statement
               | IF '(' expression ')' statement ELSE statement ;

expression : var '=' expression | simple_expression ;

Can you see a conflict? How can it be fixed?

No, I don't see a conflict, but your second question seems to imply that you do. Anyway, this can't be your full grammar (you're missing the rules for expression_stmt, compound_stmt, etc.). If you're having an actual conflict with your grammar, please post all the relevant bits so that we can try and reproduce it. — Arthur Reutenauer
– Arthur Reutenauer, Commented Nov 18, 2009 at 9:49
@arthur: the full grammar is here: stackoverflow.com/questions/1752185/whats-wrong-with-my-grammar — Adrien Plisson
– Adrien Plisson, Commented Nov 18, 2009 at 10:03
@arthur: no it's just that our OP is not able to do his homework himself. each error pointed in an answer gives way to a new question where he wants us to correct his mistakes... using this iterative approach, he might be able to answer his assignment in some weeks, and even manage to get a pretty decent mark without doing anything... — Adrien Plisson
– Adrien Plisson, Commented Nov 18, 2009 at 10:16
No, I am learning dialectically. Dialectical learning is one of the best ways to learn. — neuromancer
– neuromancer, Commented Nov 18, 2009 at 10:22

Arthur Reutenauer · Accepted Answer · 2009-11-18 10:35:52Z

1

Yes, I'm seeing a conflict. The selection_statement rule matches expressions like

IF(<expression 1>)
THEN
    IF(<expression 2>)
    THEN <expression statement 1>
    ELSE <expression statement 2>

But that's ambiguous. It could also be

IF(<expression 1>)
THEN
    IF(<expression 2>)
    THEN <expression statement 1>
ELSE <expression statement 2>

answered Nov 18, 2009 at 10:35

Arthur Reutenauer

2,6381 gold badge17 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Arthur Reutenauer Over a year ago

Hey, I had to discover it by hand! I inspected the grammar for that.

neuromancer Over a year ago

Yeah, I thought it was in there. Just needed to confirm. So how can it be fixed?

Arthur Reutenauer Over a year ago

@Phenom: you have asked this precise question in a number of different ways over the last few days. Just read the answers! Your solution is there.

Collectives™ on Stack Overflow

Is there a shift/reduce error in this yacc code?

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related