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S – attributed and L – attributed SDTs in Syntax directed translation

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  • Difficulty Level : Easy
  • Last Updated : 03 May, 2019
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Before coming up to S-attributed and L-attributed SDTs, here is a brief intro to Synthesized or Inherited attributes

Types of attributes –
Attributes may be of two types – Synthesized or Inherited.

  1. Synthesized attributes –
    A Synthesized attribute is an attribute of the non-terminal on the left-hand side of a production. Synthesized attributes represent information that is being passed up the parse tree. The attribute can take value only from its children (Variables in the RHS of the production).

    For eg. let’s say A -> BC is a production of a grammar, and A’s attribute is dependent on B’s attributes or C’s attributes then it will be synthesized attribute.

  2. Inherited attributes –
    An attribute of a nonterminal on the right-hand side of a production is called an inherited attribute. The attribute can take value either from its parent or from its siblings (variables in the LHS or RHS of the production).

    For example, let’s say A -> BC is a production of a grammar and B’s attribute is dependent on A’s attributes or C’s attributes then it will be inherited attribute.

Now, let’s discuss about S-attributed and L-attributed SDT.

  1. S-attributed SDT :
    • If an SDT uses only synthesized attributes, it is called as S-attributed SDT.
    • S-attributed SDTs are evaluated in bottom-up parsing, as the values of the parent nodes depend upon the values of the child nodes.
    • Semantic actions are placed in rightmost place of RHS.
  2. L-attributed SDT:
    • If an SDT uses both synthesized attributes and inherited attributes with a restriction that inherited attribute can inherit values from left siblings only, it is called as L-attributed SDT.
    • Attributes in L-attributed SDTs are evaluated by depth-first and left-to-right parsing manner.
    • Semantic actions are placed anywhere in RHS.
    • For example,

      A -> XYZ {Y.S = A.S, Y.S = X.S, Y.S = Z.S} 

      is not an L-attributed grammar since Y.S = A.S and Y.S = X.S are allowed but Y.S = Z.S violates the L-attributed SDT definition as attributed is inheriting the value from its right sibling.

      Note – If a definition is S-attributed, then it is also L-attributed but NOT vice-versa.

      Example – Consider the given below SDT.

      P1: S -> MN  {S.val= M.val + N.val}
      P2: M -> PQ  {M.val = P.val * Q.val  and P.val =Q.val} 

      Select the correct option.
      A. Both P1 and P2 are S attributed.
      B. P1 is S attributed and P2 is L-attributed.
      C. P1 is L attributed but P2 is not L-attributed.
      D. None of the above

      Explanation –
      The correct answer is option C as, In P1, S is a synthesized attribute and in L-attribute definition synthesized is allowed. So P1 follows the L-attributed definition. But P2 doesn’t follow L-attributed definition as P is depending on Q which is RHS to it.

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