# Building a Parser III. CS164 3:30-5:00 TT 10 Evans. Prof. Bodik CS 164 Lecture 6 1

Size: px
Start display at page:

Download "Building a Parser III. CS164 3:30-5:00 TT 10 Evans. Prof. Bodik CS 164 Lecture 6 1"

Transcription

1 Building a Parser III CS164 3:30-5:00 TT 10 Evans 1

2 Overview Finish recursive descent parser when it breaks down and how to fix it eliminating left recursion reordering productions Predictive parsers (aka LL(1) parsers) computing FIRST, FOLLOW table-driven, stack-manipulating version of the parser 3

3 Review: grammar for arithmetic expressions Simple arithmetic expressions: E n id ( E ) E + E E * E Some elements of this language: id n ( n ) n + id id * ( id + id ) 4

4 Review: derivation Grammar: E n id ( E ) E + E E * E a derivation: E rewrite E with ( E ) ( E ) rewrite E with n ( n ) this is the final string of terminals another derivation (written more concisely): E ( E ) ( E * E ) ( E + E * E ) ( n + E * E ) ( n + id * E ) ( n + id * id ) this is left-most derivation (remember it) always expand the left-most non-terminal can you guess what s right-most derivation? 5

5 Recursive Descent Parsing Consider the grammar E T + E T T int int * T ( E ) Token stream is: int 5 * int 2 Start with top-level non-terminal E Try the rules for E in order 6

6 Recursive-Descent Parsing Parsing: given a string of tokens t 1 t 2... t n, find its parse tree Recursive-descent parsing: Try all the productions exhaustively At a given moment the fringe of the parse tree is: t 1 t 2 t k A Try all the productions for A: if A BC is a production, the new fringe is t 1 t 2 t k B C Backtrack when the fringe doesn t match the string Stop when there are no more non-terminals 7

7 When Recursive Descent Does Not Work Consider a production S S a: In the process of parsing S we try the above rule What goes wrong? A fix? S must have a non-recursive production, say S b expand this production before you expand S S a Problems remain performance (steps needed to parse baaaaa ) termination (parse the error input c ) 8

8 Solutions First, restrict backtracking backtrack just enough to produce a sufficiently powerful r.d. parser Second, eliminate left recursion transformation that produces a different grammar the new grammar generates same strings but does it give us same parse tree as old grammar? Let s see the restricted r.d. parser first 9

9 A Recursive Descent Parser (1) Define boolean functions that check the token string for a match of A given token terminal bool term(token tok) { return in[next++] == tok; } A given production of S (the n th ) bool S n () { } Any production of S: bool S() { } These functions advance next 10

10 A Recursive Descent Parser (2) For production E T + E bool E 1 () { return T() && term(plus) && E(); } For production E T bool E 2 () { return T(); } For all productions of E (with backtracking) bool E() { int save = next; return (next = save, E 1 ()) (next = save, E 2 ()); } 11

11 A Recursive Descent Parser (3) Functions for non-terminal T bool T 1 () { return term(open) && E() && term(close); } bool T 2 () { return term(int) && term(times) && T(); } bool T 3 () { return term(int); } bool T() { int save = next; return (next = save, T 1 ()) (next = save, T 2 ()) (next = save, T 3 ()); } 12

12 Recursive Descent Parsing. Notes. To start the parser Initialize next to point to first token Invoke E() Notice how this simulates our backtracking example from lecture Easy to implement by hand Predictive parsing is more efficient 13

13 Now back to left-recursive grammars Does this style of r.d. parser work for our left-recursive grammar? the grammar: S S a b what happens when S S a is expanded first? what happens when S b is expanded first? 14

14 Left-recursive grammars A left-recursive grammar has a non-terminal S S + S for some Recursive descent does not work in such cases It goes into an loop Notes: : a shorthand for any string of terminals, nonterminals symbol + is a shorthand for can be derived in one or more steps : S + S is same as S S 15

15 Elimination of Left Recursion Consider the left-recursive grammar S S S generates all strings starting with a and followed by a number of Can rewrite using right-recursion S S S S 16

16 Elimination of Left-Recursion. Example Consider the grammar S 1 S 0 ( = 1 and = 0 ) can be rewritten as S 1 S S 0 S 17

17 More Elimination of Left-Recursion In general S S 1 S n 1 m All strings derived from S start with one of 1,, m and continue with several instances of 1,, n Rewrite as S 1 S m S S 1 S n S 18

18 General Left Recursion The grammar S A A S is also left-recursive because S + S This left-recursion can also be eliminated See [ASU], Section 4.3 for general algorithm 19

19 Summary of Recursive Descent simple parsing strategy left-recursion must be eliminated first but that can be done automatically unpopular because of backtracking thought to be too inefficient in practice, backtracking is (sufficiently) eliminated by restricting the grammar so, it s good enough for small languages careful, though: order of productions important even after left-recursion eliminated try to reverse the order of E T + E T what goes wrong? 20

20 Motivation Wouldn t it be nice if the r.d. parser just knew which production to expand next? Idea: replace with return (next = save, E1()) (next = save, E2()); } switch ( something ) { case L1: return E1(); case L2: return E2(); otherwise: print syntax error ; } what s something, L1, L2? the parser will do lookahead (look at next token) 22

21 Predictive Parsers Like recursive-descent but parser can predict which production to use By looking at the next few tokens No backtracking Predictive parsers accept LL(k) grammars L means left-to-right scan of input L means leftmost derivation k means predict based on k tokens of lookahead In practice, LL(1) is used 23

22 LL(1) Languages In recursive-descent, for each non-terminal and input token there may be a choice of production LL(1) means that for each non-terminal and token there is only one production that could lead to success Can be specified as a 2D table One dimension for current non-terminal to expand One dimension for next token A table entry contains one production 24

23 Predictive Parsing and Left Factoring Recall the grammar E T + E T T int int * T ( E ) Impossible to predict because For T two productions start with int For E it is not clear how to predict A grammar must be left-factored before use predictive parsing 26

24 Left-Factoring Example Recall the grammar E T + E T T int int * T ( E ) Factor out common prefixes of productions E T X X + E T ( E ) int Y Y * T 27

25 LL(1) parser to simplify things, instead of switch ( something ) { case L1: return E1(); case L2: return E2(); otherwise: print syntax error ; } we ll use a LL(1) table and a parse stack the LL(1) table will replace the switch the parse stack will replace the call stack 29

26 LL(1) Parsing Table Example Left-factored grammar E T X T ( E ) int Y The LL(1) parsing table: X + E Y * T int * + ( ) \$ T int Y ( E ) E T X T X X + E Y * T 30

27 LL(1) Parsing Table Example (Cont.) Consider the [E, int] entry When current non-terminal is E and next input is int, use production E T X This production can generate an int in the first place Consider the [Y,+] entry When current non-terminal is Y and current token is +, get rid of Y We ll see later why this is so 31

28 LL(1) Parsing Tables. Errors Blank entries indicate error situations Consider the [E,*] entry There is no way to derive a string starting with * from non-terminal E 32

29 Using Parsing Tables Method similar to recursive descent, except For each non-terminal S We look at the next token a And choose the production shown at [S,a] We use a stack to keep track of pending nonterminals We reject when we encounter an error state We accept when we encounter end-of-input 33

30 LL(1) Parsing Algorithm initialize stack = < S \$ > and next ( pointer to tokens) repeat case stack of < X, rest> : if T [ X, * next] = Y 1 Y n then stack < Y 1 Y n rest> ; else error ( ) ; < t, rest> : if t = = * next + + then stack < rest> ; else error ( ) ; until stack = = < > 34

31 LL(1) Parsing Example Stack Input Action E \$ int * int \$ T X T X \$ int * int \$ int Y int Y X \$ int * int \$ terminal Y X \$ * int \$ * T * T X \$ * int \$ terminal T X \$ int \$ int Y int Y X \$ int \$ terminal Y X \$ \$ X \$ \$ \$ \$ ACCEPT 35

32 Constructing Parsing Tables LL(1) languages are those defined by a parsing table for the LL(1) algorithm No table entry can be multiply defined We want to generate parsing tables from CFG 36

33 Constructing Predictive Parsing Tables Consider the state S * A With b the next token Trying to match b There are two possibilities: 1. b belongs to an expansion of A Or Any A can be used if b can start a string derived from In this case we say that b First( ) 37

34 Constructing Predictive Parsing Tables (Cont.) 2. b does not belong to an expansion of A The expansion of A is empty and b belongs to an expansion of Means that b can appear after A in a derivation of the form S * Ab We say that b Follow(A) in this case What productions can we use in this case? Any A can be used if can expand to We say that First(A) in this case 38

35 First Sets. Example Recall the grammar E T X X + E T ( E ) int Y Y * T First sets First( ( ) = { ( } First( T ) = {int, ( } First( ) ) = { ) } First( E ) = {int, ( } First( int) = { int } First( X ) = {+, } First( + ) = { + } First( Y ) = {*, } First( * ) = { * } 40

36 Computing First Sets Definition 1. First(b) = { b } First(X) = { b X * b } { X * } 2. For all productions X A 1 A n Add First(A 1 ) { } to First(X). Stop if First(A 1 ) Add First(A 2 ) { } to First(X). Stop if First(A 2 ) Add First(A n ) { } to First(X). Stop if First(A n ) Add to First(X) 3. Repeat step 2 until no First set grows 41

37 Follow Sets. Example Recall the grammar E T X X + E T ( E ) int Y Y * T Follow sets Follow( + ) = { int, ( } Follow( * ) = { int, ( } Follow( ( ) = { int, ( } Follow( E ) = {), \$} Follow( X ) = {\$, ) } Follow( T ) = {+, ), \$} Follow( ) ) = {+, ), \$} Follow( Y ) = {+, ), \$} Follow( int) = {*, +, ), \$} 42

38 Computing Follow Sets Definition Follow(X) = { b S * X b } 1. Compute the First sets for all non-terminals first 2. Add \$ to Follow(S) (if S is the start non-terminal) 3. For all productions Y X A 1 A n Add First(A 1 ) { } to Follow(X). Stop if First(A 1 ) Add First(A 2 ) { } to Follow(X). Stop if First(A 2 ) Add First(A n ) { } to Follow(X). Stop if First(A n ) Add Follow(Y) to Follow(X) 4. Repeat step 3 until no Follow set grows 43

39 Constructing LL(1) Parsing Tables Construct a parsing table T for CFG G For each production A in G do: For each terminal b First( ) do T[A, b] = If *, for each b Follow(A) do T[A, b] = If * and \$ Follow(A) do T[A, \$] = 45

40 Constructing LL(1) Tables. Example Recall the grammar E T X X + E T ( E ) int Y Y * T Where in the line of Y we put Y * T? In the lines of First( *T) = { * } Where in the line of Y we put Y? In the lines of Follow(Y) = { \$, +, ) } 46

41 Notes on LL(1) Parsing Tables If any entry is multiply defined then G is not LL(1) If G is ambiguous If G is left recursive If G is not left-factored And in other cases as well Most programming language grammars are not LL(1) There are tools that build LL(1) tables 47

42 Top-Down Parsing. Review Top-down parsing expands a parse tree from the start symbol to the leaves Always expand the leftmost non-terminal E T + E int * int + int 49

43 Top-Down Parsing. Review Top-down parsing expands a parse tree from the start symbol to the leaves int Always expand the leftmost non-terminal E T * T + E int * int + int The leaves at any point form a string A contains only terminals The input string is b The prefix matches The next token is b 50

44 Top-Down Parsing. Review Top-down parsing expands a parse tree from the start symbol to the leaves int Always expand the leftmost non-terminal E T * T int + E T int * int + int The leaves at any point form a string A contains only terminals The input string is b The prefix matches The next token is b 51

45 Top-Down Parsing. Review Top-down parsing expands a parse tree from the start symbol to the leaves int Always expand the leftmost non-terminal E T * T int + E T int int * int + int The leaves at any point form a string A contains only terminals The input string is b The prefix matches The next token is b 52

46 Predictive Parsing. Review. A predictive parser is described by a table For each non-terminal A and for each token b we specify a production A When trying to expand A we use A if b follows next Once we have the table The parsing algorithm is simple and fast No backtracking is necessary 53

### Administrativia. WA1 due on Thu PA2 in a week. Building a Parser III. Slides on the web site. CS164 3:30-5:00 TT 10 Evans.

Administrativia Building a Parser III CS164 3:30-5:00 10 vans WA1 due on hu PA2 in a week Slides on the web site I do my best to have slides ready and posted by the end of the preceding logical day yesterday,

### CS1622. Today. A Recursive Descent Parser. Preliminaries. Lecture 9 Parsing (4)

CS1622 Lecture 9 Parsing (4) CS 1622 Lecture 9 1 Today Example of a recursive descent parser Predictive & LL(1) parsers Building parse tables CS 1622 Lecture 9 2 A Recursive Descent Parser. Preliminaries

### Building a Parser II. CS164 3:30-5:00 TT 10 Evans. Prof. Bodik CS 164 Lecture 6 1

Building a Parser II CS164 3:30-5:00 TT 10 Evans 1 Grammars Programming language constructs have recursive structure. which is why our hand-written parser had this structure, too An expression is either:

### Administrativia. PA2 assigned today. WA1 assigned today. Building a Parser II. CS164 3:30-5:00 TT 10 Evans. First midterm. Grammars.

Administrativia Building a Parser II CS164 3:30-5:00 TT 10 Evans PA2 assigned today due in 12 days WA1 assigned today due in a week it s a practice for the exam First midterm Oct 5 will contain some project-inspired

### Extra Credit Question

Top-Down Parsing #1 Extra Credit Question Given this grammar G: E E+T E T T T * int T int T (E) Is the string int * (int + int) in L(G)? Give a derivation or prove that it is not. #2 Revenge of Theory

### 8 Parsing. Parsing. Top Down Parsing Methods. Parsing complexity. Top down vs. bottom up parsing. Top down vs. bottom up parsing

8 Parsing Parsing A grammar describes syntactically legal strings in a language A recogniser simply accepts or rejects strings A generator produces strings A parser constructs a parse tree for a string

### Ambiguity, Precedence, Associativity & Top-Down Parsing. Lecture 9-10

Ambiguity, Precedence, Associativity & Top-Down Parsing Lecture 9-10 (From slides by G. Necula & R. Bodik) 9/18/06 Prof. Hilfinger CS164 Lecture 9 1 Administrivia Please let me know if there are continued

### Abstract Syntax Trees & Top-Down Parsing

Review of Parsing Abstract Syntax Trees & Top-Down Parsing Given a language L(G), a parser consumes a sequence of tokens s and produces a parse tree Issues: How do we recognize that s L(G)? A parse tree

### Compilers. Predictive Parsing. Alex Aiken

Compilers Like recursive-descent but parser can predict which production to use By looking at the next fewtokens No backtracking Predictive parsers accept LL(k) grammars L means left-to-right scan of input

### Abstract Syntax Trees & Top-Down Parsing

Abstract Syntax Trees & Top-Down Parsing Review of Parsing Given a language L(G), a parser consumes a sequence of tokens s and produces a parse tree Issues: How do we recognize that s L(G)? A parse tree

### Abstract Syntax Trees & Top-Down Parsing

Review of Parsing Abstract Syntax Trees & Top-Down Parsing Given a language L(G), a parser consumes a sequence of tokens s and produces a parse tree Issues: How do we recognize that s L(G)? A parse tree

### Top down vs. bottom up parsing

Parsing A grammar describes the strings that are syntactically legal A recogniser simply accepts or rejects strings A generator produces sentences in the language described by the grammar A parser constructs

### Ambiguity. Grammar E E + E E * E ( E ) int. The string int * int + int has two parse trees. * int

Administrivia Ambiguity, Precedence, Associativity & op-down Parsing eam assignments this evening for all those not listed as having one. HW#3 is now available, due next uesday morning (Monday is a holiday).

### Top-Down Parsing and Intro to Bottom-Up Parsing. Lecture 7

Top-Down Parsing and Intro to Bottom-Up Parsing Lecture 7 1 Predictive Parsers Like recursive-descent but parser can predict which production to use Predictive parsers are never wrong Always able to guess

### Top-Down Parsing and Intro to Bottom-Up Parsing. Lecture 7

Top-Down Parsing and Intro to Bottom-Up Parsing Lecture 7 1 Predictive Parsers Like recursive-descent but parser can predict which production to use Predictive parsers are never wrong Always able to guess

### LL(k) Parsing. Predictive Parsers. LL(k) Parser Structure. Sample Parse Table. LL(1) Parsing Algorithm. Push RHS in Reverse Order 10/17/2012

Predictive Parsers LL(k) Parsing Can we avoid backtracking? es, if for a given input symbol and given nonterminal, we can choose the alternative appropriately. his is possible if the first terminal of

### LL Parsing: A piece of cake after LR

LL Parsing: A piece of cake after LR Lecture 11 Dr. Sean Peisert ECS 142 Spring 2009 1 LL Parsing Still specified using a CFG Still reads left-to-right (Lx) Now is leftmost derivation (xl) rather than

### Note that for recursive descent to work, if A ::= B1 B2 is a grammar rule we need First k (B1) disjoint from First k (B2).

LL(k) Grammars We need a bunch of terminology. For any terminal string a we write First k (a) is the prefix of a of length k (or all of a if its length is less than k) For any string g of terminal and

### 4 (c) parsing. Parsing. Top down vs. bo5om up parsing

4 (c) parsing Parsing A grammar describes syntac2cally legal strings in a language A recogniser simply accepts or rejects strings A generator produces strings A parser constructs a parse tree for a string

### Types of parsing. CMSC 430 Lecture 4, Page 1

Types of parsing Top-down parsers start at the root of derivation tree and fill in picks a production and tries to match the input may require backtracking some grammars are backtrack-free (predictive)

### Chapter 3. Parsing #1

Chapter 3 Parsing #1 Parser source file get next character scanner get token parser AST token A parser recognizes sequences of tokens according to some grammar and generates Abstract Syntax Trees (ASTs)

### Error Handling Syntax-Directed Translation Recursive Descent Parsing

Error Handling Syntax-Directed Translation Recursive Descent Parsing Lecture 6 by Professor Vijay Ganesh) 1 Outline Recursive descent Extensions of CFG for parsing Precedence declarations Error handling

### 1 Introduction. 2 Recursive descent parsing. Predicative parsing. Computer Language Implementation Lecture Note 3 February 4, 2004

CMSC 51086 Winter 2004 Computer Language Implementation Lecture Note 3 February 4, 2004 Predicative parsing 1 Introduction This note continues the discussion of parsing based on context free languages.

### CSCI312 Principles of Programming Languages

Copyright 2006 The McGraw-Hill Companies, Inc. CSCI312 Principles of Programming Languages! LL Parsing!! Xu Liu Derived from Keith Cooper s COMP 412 at Rice University Recap Copyright 2006 The McGraw-Hill

### Parsing #1. Leonidas Fegaras. CSE 5317/4305 L3: Parsing #1 1

Parsing #1 Leonidas Fegaras CSE 5317/4305 L3: Parsing #1 1 Parser source file get next character scanner get token parser AST token A parser recognizes sequences of tokens according to some grammar and

### Syntactic Analysis. Top-Down Parsing

Syntactic Analysis Top-Down Parsing Copyright 2017, Pedro C. Diniz, all rights reserved. Students enrolled in Compilers class at University of Southern California (USC) have explicit permission to make

### CS Parsing 1

CS414-20034-03 Parsing 1 03-0: Parsing Once we have broken an input file into a sequence of tokens, the next step is to determine if that sequence of tokens forms a syntactically correct program parsing

### Error Handling Syntax-Directed Translation Recursive Descent Parsing. Lecture 6

Error Handling Syntax-Directed Translation Recursive Descent Parsing Lecture 6 1 Outline Extensions of CFG for parsing Precedence declarations (previous slide set) Error handling (slight digression) I.e.,

### Parsing. Handle, viable prefix, items, closures, goto s LR(k): SLR(1), LR(1), LALR(1)

TD parsing - LL(1) Parsing First and Follow sets Parse table construction BU Parsing Handle, viable prefix, items, closures, goto s LR(k): SLR(1), LR(1), LALR(1) Problems with SLR Aho, Sethi, Ullman, Compilers

### CA Compiler Construction

CA4003 - Compiler Construction David Sinclair A top-down parser starts with the root of the parse tree, labelled with the goal symbol of the grammar, and repeats the following steps until the fringe of

### Lexical and Syntax Analysis

Lexical and Syntax Analysis (of Programming Languages) Top-Down Parsing Lexical and Syntax Analysis (of Programming Languages) Top-Down Parsing Easy for humans to write and understand String of characters

### Parsing. Note by Baris Aktemur: Our slides are adapted from Cooper and Torczon s slides that they prepared for COMP 412 at Rice.

Parsing Note by Baris Aktemur: Our slides are adapted from Cooper and Torczon s slides that they prepared for COMP 412 at Rice. Copyright 2010, Keith D. Cooper & Linda Torczon, all rights reserved. Students

### Lexical and Syntax Analysis. Top-Down Parsing

Lexical and Syntax Analysis Top-Down Parsing Easy for humans to write and understand String of characters Lexemes identified String of tokens Easy for programs to transform Data structure Syntax A syntax

### CS2210: Compiler Construction Syntax Analysis Syntax Analysis

Comparison with Lexical Analysis The second phase of compilation Phase Input Output Lexer string of characters string of tokens Parser string of tokens Parse tree/ast What Parse Tree? CS2210: Compiler

### More Bottom-Up Parsing

More Bottom-Up Parsing Lecture 7 Dr. Sean Peisert ECS 142 Spring 2009 1 Status Project 1 Back By Wednesday (ish) savior lexer in ~cs142/s09/bin Project 2 Due Friday, Apr. 24, 11:55pm My office hours 3pm

### Syntax Analysis. Martin Sulzmann. Martin Sulzmann Syntax Analysis 1 / 38

Syntax Analysis Martin Sulzmann Martin Sulzmann Syntax Analysis 1 / 38 Syntax Analysis Objective Recognize individual tokens as sentences of a language (beyond regular languages). Example 1 (OK) Program

### 3. Parsing. Oscar Nierstrasz

3. Parsing Oscar Nierstrasz Thanks to Jens Palsberg and Tony Hosking for their kind permission to reuse and adapt the CS132 and CS502 lecture notes. http://www.cs.ucla.edu/~palsberg/ http://www.cs.purdue.edu/homes/hosking/

### Wednesday, September 9, 15. Parsers

Parsers What is a parser A parser has two jobs: 1) Determine whether a string (program) is valid (think: grammatically correct) 2) Determine the structure of a program (think: diagramming a sentence) Agenda

### Parsers. What is a parser. Languages. Agenda. Terminology. Languages. A parser has two jobs:

What is a parser Parsers A parser has two jobs: 1) Determine whether a string (program) is valid (think: grammatically correct) 2) Determine the structure of a program (think: diagramming a sentence) Agenda

### Monday, September 13, Parsers

Parsers Agenda Terminology LL(1) Parsers Overview of LR Parsing Terminology Grammar G = (Vt, Vn, S, P) Vt is the set of terminals Vn is the set of non-terminals S is the start symbol P is the set of productions

### Fall Compiler Principles Lecture 3: Parsing part 2. Roman Manevich Ben-Gurion University

Fall 2014-2015 Compiler Principles Lecture 3: Parsing part 2 Roman Manevich Ben-Gurion University Tentative syllabus Front End Intermediate Representation Optimizations Code Generation Scanning Lowering

### Parsing. Roadmap. > Context-free grammars > Derivations and precedence > Top-down parsing > Left-recursion > Look-ahead > Table-driven parsing

Roadmap > Context-free grammars > Derivations and precedence > Top-down parsing > Left-recursion > Look-ahead > Table-driven parsing The role of the parser > performs context-free syntax analysis > guides

### Compila(on (Semester A, 2013/14)

Compila(on 0368-3133 (Semester A, 2013/14) Lecture 4: Syntax Analysis (Top- Down Parsing) Modern Compiler Design: Chapter 2.2 Noam Rinetzky Slides credit: Roman Manevich, Mooly Sagiv, Jeff Ullman, Eran

### Part III : Parsing. From Regular to Context-Free Grammars. Deriving a Parser from a Context-Free Grammar. Scanners and Parsers.

Part III : Parsing From Regular to Context-Free Grammars Deriving a Parser from a Context-Free Grammar Scanners and Parsers A Parser for EBNF Left-Parsable Grammars Martin Odersky, LAMP/DI 1 From Regular

### Syntax Analysis, III Comp 412

COMP 412 FALL 2017 Syntax Analysis, III Comp 412 source code IR Front End Optimizer Back End IR target code Copyright 2017, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled in Comp

### CS502: Compilers & Programming Systems

CS502: Compilers & Programming Systems Top-down Parsing Zhiyuan Li Department of Computer Science Purdue University, USA There exist two well-known schemes to construct deterministic top-down parsers:

### Outline. Top Down Parsing. SLL(1) Parsing. Where We Are 1/24/2013

Outline Top Down Parsing Top-down parsing SLL(1) grammars Transforming a grammar into SLL(1) form Recursive-descent parsing 1 CS 412/413 Spring 2008 Introduction to Compilers 2 Where We Are SLL(1) Parsing

### Syntax Analysis. COMP 524: Programming Language Concepts Björn B. Brandenburg. The University of North Carolina at Chapel Hill

Syntax Analysis Björn B. Brandenburg The University of North Carolina at Chapel Hill Based on slides and notes by S. Olivier, A. Block, N. Fisher, F. Hernandez-Campos, and D. Stotts. The Big Picture Character

### CSE431 Translation of Computer Languages

CSE431 Translation of Computer Languages Top Down Parsers Doug Shook Top Down Parsers Two forms: Recursive Descent Table Also known as LL(k) parsers: Read tokens from Left to right Produces a Leftmost

### Syntax Analysis. The Big Picture. The Big Picture. COMP 524: Programming Languages Srinivas Krishnan January 25, 2011

Syntax Analysis COMP 524: Programming Languages Srinivas Krishnan January 25, 2011 Based in part on slides and notes by Bjoern Brandenburg, S. Olivier and A. Block. 1 The Big Picture Character Stream Token

### Compilers: CS31003 Computer Sc & Engg: IIT Kharagpur 1. Top-Down Parsing. Lect 5. Goutam Biswas

Compilers: CS31003 Computer Sc & Engg: IIT Kharagpur 1 Top-Down Parsing Compilers: CS31003 Computer Sc & Engg: IIT Kharagpur 2 Non-terminal as a Function In a top-down parser a non-terminal may be viewed

### Compilation Lecture 3: Syntax Analysis: Top-Down parsing. Noam Rinetzky

Compilation 0368-3133 Lecture 3: Syntax Analysis: Top-Down parsing Noam Rinetzky 1 Recursive descent parsing Define a function for every nonterminal Every function work as follows Find applicable production

### CMSC 330: Organization of Programming Languages

CMSC 330: Organization of Programming Languages Context Free Grammars and Parsing 1 Recall: Architecture of Compilers, Interpreters Source Parser Static Analyzer Intermediate Representation Front End Back

### Syntax Analysis, III Comp 412

Updated algorithm for removal of indirect left recursion to match EaC3e (3/2018) COMP 412 FALL 2018 Midterm Exam: Thursday October 18, 7PM Herzstein Amphitheater Syntax Analysis, III Comp 412 source code

### LL parsing Nullable, FIRST, and FOLLOW

EDAN65: Compilers LL parsing Nullable, FIRST, and FOLLOW Görel Hedin Revised: 2014-09- 22 Regular expressions Context- free grammar ATribute grammar Lexical analyzer (scanner) SyntacKc analyzer (parser)

### Fall Compiler Principles Lecture 2: LL parsing. Roman Manevich Ben-Gurion University of the Negev

Fall 2017-2018 Compiler Principles Lecture 2: LL parsing Roman Manevich Ben-Gurion University of the Negev 1 Books Compilers Principles, Techniques, and Tools Alfred V. Aho, Ravi Sethi, Jeffrey D. Ullman

### CMSC 330: Organization of Programming Languages

CMSC 330: Organization of Programming Languages Parsing CMSC 330 - Spring 2017 1 Recall: Front End Scanner and Parser Front End Token Source Scanner Parser Stream AST Scanner / lexer / tokenizer converts

### Compiler Design 1. Top-Down Parsing. Goutam Biswas. Lect 5

Compiler Design 1 Top-Down Parsing Compiler Design 2 Non-terminal as a Function In a top-down parser a non-terminal may be viewed as a generator of a substring of the input. We may view a non-terminal

### The procedure attempts to "match" the right hand side of some production for a nonterminal.

Parsing A parser is an algorithm that determines whether a given input string is in a language and, as a side-effect, usually produces a parse tree for the input. There is a procedure for generating a

### Parsing Part II (Top-down parsing, left-recursion removal)

Parsing Part II (Top-down parsing, left-recursion removal) Copyright 2003, Keith D. Cooper, Ken Kennedy & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit

### Syntax-Directed Translation. Lecture 14

Syntax-Directed Translation Lecture 14 (adapted from slides by R. Bodik) 9/27/2006 Prof. Hilfinger, Lecture 14 1 Motivation: parser as a translator syntax-directed translation stream of tokens parser ASTs,

### Parsing III. CS434 Lecture 8 Spring 2005 Department of Computer Science University of Alabama Joel Jones

Parsing III (Top-down parsing: recursive descent & LL(1) ) (Bottom-up parsing) CS434 Lecture 8 Spring 2005 Department of Computer Science University of Alabama Joel Jones Copyright 2003, Keith D. Cooper,

### CS 2210 Sample Midterm. 1. Determine if each of the following claims is true (T) or false (F).

CS 2210 Sample Midterm 1. Determine if each of the following claims is true (T) or false (F). F A language consists of a set of strings, its grammar structure, and a set of operations. (Note: a language

### Derivations vs Parses. Example. Parse Tree. Ambiguity. Different Parse Trees. Context Free Grammars 9/18/2012

Derivations vs Parses Grammar is used to derive string or construct parser Context ree Grammars A derivation is a sequence of applications of rules Starting from the start symbol S......... (sentence)

### Parsers. Xiaokang Qiu Purdue University. August 31, 2018 ECE 468

Parsers Xiaokang Qiu Purdue University ECE 468 August 31, 2018 What is a parser A parser has two jobs: 1) Determine whether a string (program) is valid (think: grammatically correct) 2) Determine the structure

### Compilers. Yannis Smaragdakis, U. Athens (original slides by Sam

Compilers Parsing Yannis Smaragdakis, U. Athens (original slides by Sam Guyer@Tufts) Next step text chars Lexical analyzer tokens Parser IR Errors Parsing: Organize tokens into sentences Do tokens conform

### DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING

KATHMANDU UNIVERSITY SCHOOL OF ENGINEERING DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING REPORT ON NON-RECURSIVE PREDICTIVE PARSER Fourth Year First Semester Compiler Design Project Final Report submitted

### PESIT Bangalore South Campus Hosur road, 1km before Electronic City, Bengaluru -100 Department of Computer Science and Engineering

TEST 1 Date : 24 02 2015 Marks : 50 Subject & Code : Compiler Design ( 10CS63) Class : VI CSE A & B Name of faculty : Mrs. Shanthala P.T/ Mrs. Swati Gambhire Time : 8:30 10:00 AM SOLUTION MANUAL 1. a.

### Parsing II Top-down parsing. Comp 412

COMP 412 FALL 2018 Parsing II Top-down parsing Comp 412 source code IR Front End Optimizer Back End IR target code Copyright 2018, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled

### Wednesday, August 31, Parsers

Parsers How do we combine tokens? Combine tokens ( words in a language) to form programs ( sentences in a language) Not all combinations of tokens are correct programs (not all sentences are grammatically

### CS 536 Midterm Exam Spring 2013

CS 536 Midterm Exam Spring 2013 ID: Exam Instructions: Write your student ID (not your name) in the space provided at the top of each page of the exam. Write all your answers on the exam itself. Feel free

### Lecture Notes on Top-Down Predictive LL Parsing

Lecture Notes on Top-Down Predictive LL Parsing 15-411: Compiler Design Frank Pfenning Lecture 8 1 Introduction In this lecture we discuss a parsing algorithm that traverses the input string from l eft

### Part 3. Syntax analysis. Syntax analysis 96

Part 3 Syntax analysis Syntax analysis 96 Outline 1. Introduction 2. Context-free grammar 3. Top-down parsing 4. Bottom-up parsing 5. Conclusion and some practical considerations Syntax analysis 97 Structure

### EDA180: Compiler Construc6on. Top- down parsing. Görel Hedin Revised: a

EDA180: Compiler Construc6on Top- down parsing Görel Hedin Revised: 2013-01- 30a Compiler phases and program representa6ons source code Lexical analysis (scanning) Intermediate code genera6on tokens intermediate

### Table-Driven Parsing

Table-Driven Parsing It is possible to build a non-recursive predictive parser by maintaining a stack explicitly, rather than implicitly via recursive calls [1] The non-recursive parser looks up the production

### Prelude COMP 181 Tufts University Computer Science Last time Grammar issues Key structure meaning Tufts University Computer Science

Prelude COMP Lecture Topdown Parsing September, 00 What is the Tufts mascot? Jumbo the elephant Why? P. T. Barnum was an original trustee of Tufts : donated \$0,000 for a natural museum on campus Barnum

### 1. Consider the following program in a PCAT-like language.

CS4XX INTRODUCTION TO COMPILER THEORY MIDTERM EXAM QUESTIONS (Each question carries 20 Points) Total points: 100 1. Consider the following program in a PCAT-like language. PROCEDURE main; TYPE t = FLOAT;

### JavaCC Parser. The Compilation Task. Automated? JavaCC Parser

JavaCC Parser The Compilation Task Input character stream Lexer stream Parser Abstract Syntax Tree Analyser Annotated AST Code Generator Code CC&P 2003 1 CC&P 2003 2 Automated? JavaCC Parser The initial

### Sometimes an ambiguous grammar can be rewritten to eliminate the ambiguity.

Eliminating Ambiguity Sometimes an ambiguous grammar can be rewritten to eliminate the ambiguity. Example: consider the following grammar stat if expr then stat if expr then stat else stat other One can

### Parsing Wrapup. Roadmap (Where are we?) Last lecture Shift-reduce parser LR(1) parsing. This lecture LR(1) parsing

Parsing Wrapup Roadmap (Where are we?) Last lecture Shift-reduce parser LR(1) parsing LR(1) items Computing closure Computing goto LR(1) canonical collection This lecture LR(1) parsing Building ACTION

### Parsing. Lecture 11: Parsing. Recursive Descent Parser. Arithmetic grammar. - drops irrelevant details from parse tree

Parsing Lecture 11: Parsing CSC 131 Fall, 2014 Kim Bruce Build parse tree from an expression Interested in abstract syntax tree - drops irrelevant details from parse tree Arithmetic grammar ::=

### Parsing Techniques. CS152. Chris Pollett. Sep. 24, 2008.

Parsing Techniques. CS152. Chris Pollett. Sep. 24, 2008. Outline. Top-down versus Bottom-up Parsing. Recursive Descent Parsing. Left Recursion Removal. Left Factoring. Predictive Parsing. Introduction.

### LECTURE 7. Lex and Intro to Parsing

LECTURE 7 Lex and Intro to Parsing LEX Last lecture, we learned a little bit about how we can take our regular expressions (which specify our valid tokens) and create real programs that can recognize them.

### Fall Compiler Principles Lecture 2: LL parsing. Roman Manevich Ben-Gurion University of the Negev

Fall 2016-2017 Compiler Principles Lecture 2: LL parsing Roman Manevich Ben-Gurion University of the Negev 1 Books Compilers Principles, Techniques, and Tools Alfred V. Aho, Ravi Sethi, Jeffrey D. Ullman

### Question Points Score

CS 453 Introduction to Compilers Midterm Examination Spring 2009 March 12, 2009 75 minutes (maximum) Closed Book You may use one side of one sheet (8.5x11) of paper with any notes you like. This exam has

### CSE 401 Midterm Exam Sample Solution 2/11/15

Question 1. (10 points) Regular expression warmup. For regular expression questions, you must restrict yourself to the basic regular expression operations covered in class and on homework assignments:

### Introduction to Parsing. Comp 412

COMP 412 FALL 2010 Introduction to Parsing Comp 412 Copyright 2010, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit permission to make

### ([1-9] 1[0-2]):[0-5][0-9](AM PM)? What does the above match? Matches clock time, may or may not be told if it is AM or PM.

What is the corresponding regex? [2-9]: ([1-9] 1[0-2]):[0-5][0-9](AM PM)? What does the above match? Matches clock time, may or may not be told if it is AM or PM. CS 230 - Spring 2018 4-1 More CFG Notation

### Compilerconstructie. najaar Rudy van Vliet kamer 140 Snellius, tel rvvliet(at)liacs(dot)nl. college 3, vrijdag 22 september 2017

Compilerconstructie najaar 2017 http://www.liacs.leidenuniv.nl/~vlietrvan1/coco/ Rudy van Vliet kamer 140 Snellius, tel. 071-527 2876 rvvliet(at)liacs(dot)nl college 3, vrijdag 22 september 2017 + werkcollege

### CS 321 Programming Languages and Compilers. VI. Parsing

CS 321 Programming Languages and Compilers VI. Parsing Parsing Calculate grammatical structure of program, like diagramming sentences, where: Tokens = words Programs = sentences For further information,

### Parsing III. (Top-down parsing: recursive descent & LL(1) )

Parsing III (Top-down parsing: recursive descent & LL(1) ) Roadmap (Where are we?) Previously We set out to study parsing Specifying syntax Context-free grammars Ambiguity Top-down parsers Algorithm &

SYNTAX ANALYSIS 1. Define parser. Hierarchical analysis is one in which the tokens are grouped hierarchically into nested collections with collective meaning. Also termed as Parsing. 2. Mention the basic

### Introduction to Parsing. Lecture 8

Introduction to Parsing Lecture 8 Adapted from slides by G. Necula Outline Limitations of regular languages Parser overview Context-free grammars (CFG s) Derivations Languages and Automata Formal languages

### Revisit the example. Transformed DFA 10/1/16 A B C D E. Start

Revisit the example ε 0 ε 1 Start ε a ε 2 3 ε b ε 4 5 ε a b b 6 7 8 9 10 ε-closure(0)={0, 1, 2, 4, 7} = A Trans(A, a) = {1, 2, 3, 4, 6, 7, 8} = B Trans(A, b) = {1, 2, 4, 5, 6, 7} = C Trans(B, a) = {1,

### Context-free grammars (CFG s)

Syntax Analysis/Parsing Purpose: determine if tokens have the right form for the language (right syntactic structure) stream of tokens abstract syntax tree (AST) AST: captures hierarchical structure of

### Syntax Analysis/Parsing. Context-free grammars (CFG s) Context-free grammars vs. Regular Expressions. BNF description of PL/0 syntax

Susan Eggers 1 CSE 401 Syntax Analysis/Parsing Context-free grammars (CFG s) Purpose: determine if tokens have the right form for the language (right syntactic structure) stream of tokens abstract syntax

### CSE P 501 Compilers. LR Parsing Hal Perkins Spring UW CSE P 501 Spring 2018 D-1

CSE P 501 Compilers LR Parsing Hal Perkins Spring 2018 UW CSE P 501 Spring 2018 D-1 Agenda LR Parsing Table-driven Parsers Parser States Shift-Reduce and Reduce-Reduce conflicts UW CSE P 501 Spring 2018

### LL(1) Grammars. Example. Recursive Descent Parsers. S A a {b,d,a} A B D {b, d, a} B b { b } B λ {d, a} D d { d } D λ { a }

LL(1) Grammars A context-free grammar whose Predict sets are always disjoint (for the same non-terminal) is said to be LL(1). LL(1) grammars are ideally suited for top-down parsing because it is always