Syntax analysis principle-Parsing

Introduction

Parsing Method category:
Universal: cocke-younger-kasami algorithm and Earley's algorithm.
Top-down: LL (k ).
Bottom-up: LR (k ).

Error recovery mode:
Panic-mode recovery:
Phrase-level recovery:
Error production:
Global correction:

Context-independent grammar:
Terminal: the basic symbol that forms a string.
Nonterminal: represents the syntax variables of the string set, which gives the key to syntax analysis and translation.
Start Symbol: In a grammar, A nonterminal is specified as the start symbol.
Production: A syntax generator that contains a head/left side, conversion symbol->/: =, and body/right side.

Derivation process:
Leftmost derivation (leftmost derivation): Always select the leftmost non-terminator for each sentence.
Rightmost derivation: select the rightmost non-terminator for each sentence. It is also called canonical derivation ).

Algorithm 1: Remove left recursion

Remove left recursion immediately:
For a-> A α 1 | A α 2 |... | A α M | β1 | β2 |... | β N, replace these a generative formula:
A-> β 1A '| β 2a' |... | β Na'
A'-> α 1A '| α 2a' |... | α Ma' | ε

Remove the left recursion of the Loop:
Input: grammar G without loops (a-> A) or ε (a-> ε.
Output: an equivalent non-left recursive syntax G', but G' may have an ε generative formula.
Method:

Eliminate the left recursive algorithm of loops: arrange non-terminologies into A1, A2 ,..., an. for I = (1 to n) {for J = (1 to i-1) {Replace the generative formula of each shape, such as AI-> AJ gamma, with the generative formula Ai-> delta 1 gamma | Delta 2 gamma |... | Delta 3 gamma, where AJ-> delta 1 | Delta 2 |... | Delta K is all AJ Generative Patterns;} removes the instant left recursion between AI Generative Patterns ;}

Algorithm 2: extract Left Factor

Purpose: To postpone the generative selection.
Input: syntax rule G.
Output: equivalent extract left factor syntax G '.
Method: Find the longest public prefix α for each non-terminator! = ε. For all a-> α-β 1 | α-β 1 |... | α-β n | γ, replace:
A-> α a' | Gamma
A'-> β1 | β1 |... | βn
Repeat this behavior until two production types without the same non-terminator contain a public prefix.

Top-down recursive-descent Parsing

Typical process of recursive downloading Parser:
Void A () {choose an-production, A: = x1x2... XK; for I = (I to k) {If (Xi is a nonterminal) Call procedure XI (); else if (XI equals the current input symbol) advance the input to the next symbol; else call error recovery procedure ;}}

 

First and follow Sets

First (α): α is any syntactic symbol. This set is the first terminator of a sentence derived from α.
During computing, the following rules are applied until no new terminator or ε can be added to any first set:
1) if X is a Terminator, first (x) = {x }.
2) If X is a non-Terminator and has a generative formula X: = y1y2... YK (k ≥ 1 ). If first (yi) contains a and first (Y1),... first (Yi-1) can be pushed to export ε, then add a to first (X. If all first (yi) contains ε, add ε to first (X.
3) If X: = ε exists, add it to first (X.

Follow (a): A is a non-Terminator, which is a set of Terminators that can be followed by the right side of a's sentence pattern.
During computing, the following rules are applied until no new Terminator is added to any follow set:
1) if S is the start symbol, add $ to follow (s), and $ is the end mark of the input string.
2) If the generative expression A: = α B β exists, all the symbols except ε in first (β) are in follow (B.
3) If formula A: = α B exists, or formula A: = α B Beta exists and first (Beta) contains ε, then follow (B) contains all the symbols in follow (.

 

Syntax analysis principle-Parsing