Reduction & Handle Pruning

Reduction & Handle Pruning

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  Course Code- CSE-3201 1
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  Moderator : Member 1: Member 2 : Member 3 : Member 4 : Member 5 : 2
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   What isreduction?  Key decision for reduction  Example of reduction  What is Handle?  What is Handle Pruning?  Example of Handle Pruning 3
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  Bottom up parsingis the process of “reducing” a string ‘w’ to the start symbol of the grammar. At each reduction step , a specific substring matching the body of a production is replaced by the nonterminal at the head of the production. 4
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  5 The key concernduring bottom-up parsing are about when to reduce and about what production to apply, as the parse proceeds. It will be discuss in example section.
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  GRAMMAR: E -> E+ T | T T -> T * F | F F -> ( E ) | id STRING: W = id * id 6
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  id * id F* id [ F -> id ] T * id [ T -> F] T * F [ F -> id ] T [T -> T*F ] E [ E -> T ] The Parse completes with the reduction of string W to the start symbol E. 7
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  A “Handle” isa substring that matches the body of a production and whose reduction represents one step along the reverse of a rightmost derivation. If, A -> α Then α is the handle 8
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  9 Removing the childrenof left hand side non terminal from the parse tree is called Handle Pruning. A right most derivation in reverse can be obtained by Handle Pruning. If S=>αAw=>αβw ◦ then A->β in the position following α is a handle of αβw The handle of right-sentential form γ is a production A->β and a position of γ where β may be found , such that replacing β at that position by A produces the previous right sentential form in a rightmost derivation of γ.
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  Steps to Follow: Start with a string of terminals ‘w’ that is to be parsed.  Let w is a string of the grammar.  w=γn where γn is the n-th right-sentential form of some unknown rightmost derivation.  S=γ0=>γ1=>γ2=>… =>γn-1=>γn =w. 10
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  Steps to Follow: To rebuild this derivation in reverse order ◦ locate handle βn in γn by production of An -> βn to get right-sentential form γn-1 ◦ handles must be found with specific methods ◦ repeat the process until the start symbol S is found ◦ reverse of reductions = rightmost derivation 11
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  GRAMMAR: E -> E+ T | T T -> T * F | F F -> ( E ) | id STRING: W = id * id 12
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  Right Sentential FormHandle Reducing Production id1 * id2 id1 F -> id F * id2 F T -> F T * id2 id2 F -> id T * F T * F E -> T * F E * Adding subscripts to the tokens ‘id’ for clarity. 13
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  Thanks All 14