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by: Vito Quigley


Vito Quigley
GPA 3.55

David Whalley

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About this Document

David Whalley
Class Notes
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This 5 page Class Notes was uploaded by Vito Quigley on Thursday September 17, 2015. The Class Notes belongs to COP 5621 at Florida State University taught by David Whalley in Fall. Since its upload, it has received 65 views. For similar materials see /class/205472/cop-5621-florida-state-university in Computer Programming at Florida State University.




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Date Created: 09/17/15
Concepts Introduced in Chapter 4 Grammars G N T P S Grammars l N is a nite set of nonterminal symbols COHtBXt Free Grammam 2 T is a finite set of terminal symbols Derivations and Parse Trees 3 P is a nite subset of NUTNNUTgtltNUT Ambiguity Precedence and Associativity Top Down Parsing Recursive Descent LL An element 0 B G P is written as Bottom Up Parsing o B Operator Precedence SLR LR LALR and is called a production Yam 4 S is a distinguished symbol in N and is called the Error Handling start symbol Example of a Grammar Advantages of Using Grammars bleak b egl n Opt Stmts m Provides a precise syntactic specification of a optistmts stmtilist I 6 programming language stmtilist stmtilist stmt I stmt For some classes of grammars tools exist that can automatically construct an efficient parser These tools can also detect syntactic ambiguities and other problems automatically A compiler based on a grammatical description of a language is more easily maintained and updated Role of a Parser in a Compiler Detects and reports any syntax errors 39 Produces a parse tree from which intermediate code can be generated Conventions for Specifying Grammars in the Text cont grammar symbols nonterminals or terminals upper case letters late in the alphabet X Y Z strings of terminals lower case letters late in the alphabet u V m z 39 sentential form string of grammar symbols lower case Greek letters 0 B Y Conventions for Specifying Grammars in the Text terminals lower case letters early in the alphabet a b c punctuation and operator symbols 7 digits boldface words if then nonterminals uppercase letters early in the alphabet A B C S is the start symbol lower case words Chomsky Hierarchy A grammar is said to be 1 Wif it is where each production in P has the form a rightlinear A wB or A w b leftlinear A Bw or A w where A B G N andw G T Chomsky Hierarchy cont 2 contextfree if each production in P is of the form A gtoc whereAeNand 06 NUT 3 contextsensitive if each production in P is of the form 6 3 where Iorl S IBI 4 unrestricted if each production in P is of the form 6 3 where 0 8 Derivation cont leftmost derivation each step replaces the leftmost nonterminal Derive id id id using leftmost derivation Egt EEgt idEgt idEEgt ididEgt id id id LG language generated by the grammar G sentence ofG ifS gt w where w is a string of terminals in LG sentential form if S gt 0 where or may contain nonterminals Derivation Derivation a sequence of replacements from the start symbol in a grammar by applying productions Example E E E E gtEE E gtE E gt E E gtid Derive id from the grammar Egt Egt Egt id thus Edeiives id or Egt id Parse Tree A parse tree pictorially shows how the start symbol of a grammar derives a speci c string in the language Given a contextfree grammar a parse tree has the H bwtx properties The root is labeled by the start symbol Each leaf is labeled by a token or 5 Each interior node is labeled by a nontenninal If A is a nonterminal labeling some interior node and X1 X2 X3 Xn are the labels of the children of that node from left to right then A X1 X2 X3 Xn is a production of the grammar Example of a Parse Tree list list digit list digit digit 9 5 2 list gt list digit list digit digit Example of an Ambiguous Grammar string gt string string string gt string string string gt0ll23456789 string string l l string string string string l l l string string 2 9 string string l l l l 9 5 5 2 a string gt string string gt string string string gt 9 stringstring gt 9 5string 9 52 b string gt string string gt 9 string gt 9 stringstring gt 9 5string 9 52 Parse Tree cont Yield the leaves of the parse tree read from left to right or the string derived from the nonterminal at the root of the parse tree An ambiguous grammar is one that can generate two or more parse trees that yield the same string Precedence By convention 9 5 2 has higher precedence than because it takes its operands before expr gt expr term term expr term gt term digit I digit l expr term l l term term digit l l digit digit


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