Editing LR parser (section)

=== Bottom-up parse steps for example A * 2 + 1 ===

{| class="wikitable"
|-
! <small>Step</small> !! <small>Parse Stack</small>  !!  <small>Unparsed</small> !! <small>Shift/Reduce</small>
|-
| 0 || ''empty'' || align="right" | A * 2 + 1 || shift
|-
| 1 || ''id'' || align="right" | * 2 + 1 || Value → ''id''  
|-
| 2 || Value || align="right" | * 2 + 1 || Products → Value 
|-
| 3 || Products || align="right" | * 2 + 1 || shift 
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| 4 || Products * || align="right" | 2 + 1 || shift
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| 5 || Products * ''int'' || align="right" | + 1 || Value → ''int''
|-
| 6 || Products * Value || align="right" | + 1 ||  Products → Products * Value
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| 7 || Products || align="right" | + 1 || Sums → Products 
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| 8 || Sums || align="right" | + 1 || shift  
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| 9 || Sums + || align="right" | 1 || shift 
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| 10 || Sums + ''int'' || ''eof'' || Value → ''int'' 
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| 11 || Sums + Value || ''eof'' || Products → Value
|-
| 12 || Sums + Products || ''eof'' ||  Sums → Sums + Products 
|-
| 13 || Sums || ''eof'' || accept 
|-
|}

Step 6 applies a grammar rule with multiple parts:
: Products → Products * Value
This matches the stack top holding the parsed phrases "... Products * Value".  The reduce step replaces this instance of the rule's right hand side, "Products * Value" by the rule's left hand side symbol, here a larger Products.  If the parser builds complete parse trees, the three trees for inner Products, *, and Value are combined by a new tree root for Products.  Otherwise, [[Semantics#Programming languages|semantic]]{{Broken anchor|date=2025-04-28|bot=User:Cewbot/log/20201008/configuration|target_link=Semantics#Programming languages|reason=Anchor "Semantics#Programming languages" links to a specific web page: "Programming languages". The anchor (Programming languages) [[Special:Diff/1208968924|has been deleted]].|diff_id=1208968924}} details from the inner Products and Value are output to some later compiler pass, or are combined and saved in the new Products symbol.<ref>
Crafting and Compiler, by Charles Fischer, Ron Cytron, and Richard LeBlanc, Addison Wesley 2009.</ref>