Overview
LL Parsing predicts grammar productions from the current nonterminal and lookahead tokens.
CoreSource checked
LL Parsing
Parses input from left to right using leftmost derivations.
Quick Facts
Compact metadata for implementation and discovery.
- Category
- Parsing
- Difficulty
- Intermediate
- Family
- Top-Down Parsing
- Tier
- Core
- Status
- Verified
- Review
- Source Checked
- Sources
- 1
- Formulas
- 0
- Relationships
- 3
- Code example
- Ready
- Visualization
- Ready
- Aliases
- LL Parser
Core Concepts
parse statetop-down expansiongrammar productiontop-down parsingtoken streamgrammar rule
Quick facts
Complexity
Runtime and memory behavior at a glance.
Best
O(n)
Average
O(n)
Worst
O(n)
Space
O(d)
When to Use
- Use LL Parsing when you need to expand grammar rules top-down.
- Use it for hand-written parsers with inputs like token stream.
- Use it when text or tokens must be turned into a reliable structured representation.
- Use it when the Top-Down Parsing tradeoff is acceptable for the problem size.
When Not to Use
- Avoid it when the grammar is ambiguous or left-recursive in ways the parser cannot handle.
- Avoid it when the required output is not parse tree.
What It Does
- Expands grammar rules top-down
- Uses lookahead to choose productions
Use Cases
- Hand-written parsers
- Simple language grammars
- Parser education
Inputs
- Token stream
- LL grammar
- Parse table or predictive functions
Outputs
- Parse tree
- Syntax error
Pseudocode
- 1.Start with the grammar start symbol
- 2.Look at the next input token
- 3.Choose a production from the parse table
- 4.Expand nonterminals
- 5.Match terminals until input is consumed
Canonical Example
- Input
- tokens = ['2', '+', '3', '*', '4']
- Output
- structured expression tree or evaluated result
- Explanation
- LL Parsing: The parser uses grammar or precedence rules to recover structure from tokens.
Implementation Notes
- Keep the input contract explicit: Token stream; LL grammar; Parse table or predictive functions.
- Keep tokenization separate from parsing unless the algorithm explicitly combines them.
- Keep lexical analysis separate from parsing unless the method explicitly combines them.
Common Pitfalls
- LL grammars cannot contain direct left recursion.
- Small grammar changes can introduce ambiguity or precedence mistakes.
- Producing unclear errors that do not point to the token or rule that failed.
- Handle empty, invalid, and boundary inputs before trusting parse tree.
Interactive Mechanism Walkthrough
Sequence Window & Automaton AlignmentShow a parse stack expanding grammar symbols from the top down.
Input:ABABABCABAB
Window:ABAB
Step 1 of 5: Start with the grammar start symbol
predictive-parse-stackNotes
LL Parsing operates in O(n) time and O(d) space within the Top-Down Parsing family. Often compared with Recursive Descent Parsing and LR Parsing.
Sources (1)
What each reference supports for this entry.
Compilers: Principles, Techniques, and Tools
BookCompiler construction reference for LL Parsing grammar transformation, parser construction, or syntax-tree workflow.
Explore Next
Good next jumps from the relationship graph, ranked for useful discovery.
Relationships
View full relationship pageAlternative toLR Parsing
LL parsing is top-down predictive parsing, while LR parsing is bottom-up shift-reduce parsing.
Alternative toRecursive Descent Parsing
LL Parsing is an alternative to Recursive Descent Parsing within the Top-Down Parsing family.
Related Algorithms
Tags
ParsingTop-Down ParsingGrammar