Attributes of semantic rules

Question

I've these rules:

S -> I#A
I -> a
I -> b
A -> aA
A -> bA
A -> EPSILON

This grammar produces a sequence of a's and b's. They are preceded by a single a (or a single b) and an #. I need to define semantic rules which gives me the number of a's or b's (it depends on which one I choose before the #) after the #. I don't know how to start because the value of A (in the first rule) depends on the value of I. How do I pass 'a' or 'b' to A?

A context free grammar doesn't "give" you anything except a bunch of rules for deriving strings in a language. What do you want to *do* with the numbers of a's and of b's once you get them? Or, what system are you working in that adds "semantic rules" to context free grammars? — Patrick87, Jan 04 '18 at 19:52

score 0 · Accepted Answer · answered Mar 13 '18 at 17:32

I'm not 100% sure what the question is, but as in the comment the grammar is used to read in a string. You can look at the rules as if they are describing a tree where the left hand side is the parent and the right hand side are the children.

So in your system there seems to be an alphabet which consists of {a, b, EPSILON, #}. Therefore if you have the input string of a#b you would get the following tree

S ---  I -- a
  ---  #
  ---  A -- b
         -- A -- EPSILON

So S has the children of I, # and A ( S -> I # A), I has a child of a (I -> a). A has two children b and A (A -> b A) and the last A has a single child EPSILON

After this you can then add in semantic rules such as conditions to your grammar also known as an attribute grammar.

Attributes of semantic rules

1 Answers1