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#REDIRECT [[Simple precedence grammar#Simple precedence parser]]
In [[computer science]], a '''simple precedence parser''' is a type of [[bottom-up parser]] for [[context-free grammars]] that can be used only by [[simple precedence grammar]]s.
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The implementation of the parser is quite similar to the generic [[bottom-up parser]]. A stack is used to store a [[viable prefix]] of a [[sentential form]] from a [[rightmost derivation]]. The symbols ⋖, ≐ and ⋗ are used to identify the '''pivot''', and to know when to '''Shift''' or when to '''Reduce'''.
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==Implementation==
* Compute the [[Wirth–Weber precedence relationship]] table for a grammar with initial symbol S.
* Initialize a stack with the '''starting marker''' $.
* Append an '''ending marker''' $ to the string being parsed ('''Input''').
* Until Stack equals "$ S" and Input equals "$"
** Search the table for the relationship between Top(stack) and NextToken(Input)
** if the relationship is ⋖ or ≐
*** '''Shift''':
*** Push(Stack, relationship)
*** Push(Stack, NextToken(Input))
*** RemoveNextToken(Input)
** if the relationship is ⋗
*** '''Reduce''':
*** SearchProductionToReduce(Stack)
*** Remove the Pivot from the Stack
*** Search the table for the relationship between the nonterminal from the production and first symbol in the stack (Starting from top)
*** Push(Stack, relationship)
*** Push(Stack, Non terminal)
SearchProductionToReduce (Stack)
* Find the topmost ⋖ in the stack; this and all the symbols above it are the '''Pivot'''.
* Find the production of the grammar which has the '''Pivot''' as its right side.
==Example==
Given the language:
<pre>
E --> E + T' | T'
T' --> T
T --> T * F | F
F --> ( E' ) | num
E' --> E
</pre>
'''num''' is a terminal, and the [[lexer (computer science)|lexer]] parse any integer as '''num'''.
and the Parsing table:
{| class="wikitable"
! ||E|| E' || T || T' || F || + ||*||(||)||num || $
|-
! E
| || || || || ||≐|| || ||⋗ || ||⋗
|-
! E'
| || || || || || || || ||≐|| ||
|-
! T
| || || || || ||⋗||≐|| ||⋗|| ||⋗
|-
! T'
| || || || || ||⋗|| || ||⋗|| ||⋗
|-
! F
| || || || || ||⋗||⋗|| ||⋗|| ||⋗
|-
! +
| || ||⋖⋖||≐||⋖|| || ||⋖|| ||⋖ ||
|-
! *
| || || || ||≐|| || ||⋖|| ||⋖ ||
|-
! (
|⋖||≐||⋖||⋖||⋖|| || ||⋖|| ||⋖ ||
|-
! )
| || || || || ||⋗||⋗|| ||⋗|| ||⋗
|-
! num
| || || || || ||⋗||⋗||||⋗|| ||⋗
|-
! $
|⋖|| ||⋖||⋖||⋖|| || ||⋖|| ||⋖ ||
|}
<pre>
STACK PRECEDENCE INPUT ACTION
$ < 2 * ( 1 + 3 )$ SHIFT
$ < 2 > * ( 1 + 3 )$ REDUCE (F -> num)
$ < F > * ( 1 + 3 )$ REDUCE (T -> F)
$ < T = * ( 1 + 3 )$ SHIFT
$ < T = * < ( 1 + 3 )$ SHIFT
$ < T = * < ( < 1 + 3 )$ SHIFT
$ < T = * < ( < 1 > + 3 )$ REDUCE 4 times (F -> num) (T -> F) (T' -> T) (E ->T ')
$ < T = * < ( < E = + 3 )$ SHIFT
$ < T = * < ( < E = + < 3 )$ SHIFT
$ < T = * < ( < E = + < 3 > )$ REDUCE 3 times (F -> num) (T -> F) (T' -> T)
$ < T = * < ( < E = + = T > )$ REDUCE 2 times (E -> E + T) (E' -> E)
$ < T = * < ( < E' = )$ SHIFT
$ < T = * < ( = E' = ) > $ REDUCE (F -> ( E' ))
$ < T = * = F > $ REDUCE (T -> T * F)
$ < T > $ REDUCE 2 times (T' -> T) (E -> T')
$ < E > $ ACCEPT
</pre>
==References==
* Alfred V. Aho, Jeffrey D. Ullman (1977). ''[[Principles of Compiler Design]]''. 1st Edition. Addison–Wesley.
* William A. Barrett, John D. Couch (1979). ''Compiler construction: Theory and Practice''. Science Research Associate.
* Jean-Paul Tremblay, P. G. Sorenson (1985). ''The Theory and Practice of Compiler Writing''. McGraw–Hill.
{{Parsers}}
{{DEFAULTSORT:Simple Precedence Parser}}
[[Category:Parsing algorithms]]
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