1 / 9

Lecture 8

Predictive Parsing. Lecture 8. Predictive – LL(1) Parsing. Dragon section 4.4, Fischer chapter 5 Deterministic parsers for a subset of CFG languages. Similar to general parsers, but when TOS is a non-terminal parser looks at input to deterministically choose production.

aladdin-francis
Download Presentation

Lecture 8

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Predictive Parsing Lecture 8

  2. Predictive – LL(1) Parsing • Dragon section 4.4, Fischer chapter 5 • Deterministic parsers for a subset of CFG languages. • Similar to general parsers, but when TOS is a non-terminal parser looks at input to deterministically choose production. • Parsers run O(n) time • Only parses the class for grammars called LL(1) Scan Input Left-to-Right Left most non-terminal expanded Token look ahead

  3. Example • S → (S) → [s] → λ • Input Stack Action 0 S ? • Choose the action by looking at next input character: ‘(‘ • Only first production S → (S) can produce a leading left parenthesis • May need more look ahead, e.g LL(2)S → (S) [S] ( ) [ ] Either production could reduce a ‘(‘ But, if the next char is ‘)’, then only the second one is possible

  4. LL Grammars • Some languages are not LL(k) for any k • S → (S) | SS | ( ) • Don’t know to apply 1st or 2nd rule until we see )( CFG LL(2) LL(k) LL(1)

  5. Testing for LL(1) • A grammar is LL(1) if its parse table has disjoint predictive sets for productions with a common LHS • A predictive set contains the terminal symbols that occur first in the strings produced by a production • Use sets to decide which production to apply • Grammar cannot have productions X → α and X → β such that α and β can begin with the same terminal. • Parser cannot decide to apply first or second rule by looking at input • Some non-LL(1) grammars can be “fixed” • Write a LL(1) grammar that produces the same set of strings

  6. Left-Recursive Grammars • Left-recursive grammars are not LL(1) • X→Xα • e.g E → E + F | F • A finite recursive grammar must contain at least two productions • X → Xα | β • Terminals occurring first in the non-recursive production also occur first in recursive production. • Can remove immediate left-recursion • A → Aα | β • A → βA` • A` → αA`|λ Terminals and non-terminals that don’t begin with A Terminals and non-terminals Right-recursive

  7. Example of Recursion Removal • E → E + F | F • E → F E` E`→ +F E` | λ • Cannot directly build parse trees from new grammar Old New E E E F F + E` E + F 4 2 F + E` F 3 3 F E` + 2 4 λ

  8. Common Prefixes • Another common non-LL(1) grammar has two productions whose RHS have a common prefix • e.g S → (S) | () • Cannot choose production by looking at next token • Easy to remove with left-factoring • A → α B1 | α B2 • A → α A` • A`→ B1 | B2 • e.g S → (S` S` → S) | )

  9. Double Example • S → (S) | SS | 0 • First, remove immediate left recursion • S → (S) S` | ()S` • S` → SS` | λ • Next, eliminate common prefix • S → (TT → S)S` | )S`S` → SS` | λ • Wouldn’t want to write this grammar originally

More Related