module 2 introduction to syntax analysis

module 2 introduction to syntax analysis

• 1.
  UNIT II SYNTAX ANALYSIS
• 2.
  SYLLABUS Role of Parser– Grammars – Error Handling – Context-free grammars – Writing a grammar –Top Down Parsing – General Strategies Recursive Descent Parser Predictive Parser-LL(1) Parser-Shift Reduce Parser-LR Parser – LR(0) Item Construction of SLR Parsing Table – Introduction to LALR Parser – Error Handling and Recovery in Syntax Analyzer-YACC.
• 3.
  Syntax Analysis • Bydesign , every programming language has precise rules that prescribe the syntactic structure of well formed programs • In c , for example , a program is made up of functions, a function out of declarations and statements, a statement out of expressions and so on. • The syntax of programming language constructs can be specified by context-free grammars or BNF( Backus-Naur-Form)notation. • The Parser determines the syntax or structure of a program. That is,it checks whether the input is syntactically correct or not.
• 4.
  Role of Parser •Parser also called syntax analyzer is the one which does parsing • Parsing is the process of getting tokens from the lexical analyzer and obtains a derivation for the sequence of tokens and builds a parse tree • Thus if the program is syntactically correct, the parse tree is generated • If a derivation for the sequence of tokens does not exist i.e., if the program is syntactically wrong, it results in syntax error and the parser displays the appropriate error messages. • The parse trees are very important in figuring out the meaning of a program or part of the program • The parse tree is also called the syntax tree or derivation tree.
• 5.
  Role of aparser • In compiler model, the parser obtains a string of tokens from the lexical analyzer and verifies that the string of token names can be generated by the grammar for the source language.
• 6.
  Contd.. A parser isa software component that takes input data and builds a data structure called parse tree. The parser reads the tokens from the lexical analyser and checks whether the sequence of tokens matches the grammar rules of the programming language and generate the parse tree. A syntax error is reported when there is mismatch that is if the token sequence does not match the grammar.
• 7.
  Types of parser Threetypes of parser 1. Universal parser – the Cocke-Youngster-Kasami algorithm and Earley’s algorithm can parse any grammar, 2. Top-down parser – a top-down parser starts with the root of the parse tree , labelled with the start or goal symbol of the grammar and proceeds down to the leaves. 3. Bottom-up parser – a bottom up parser starts with the leaves and moves to the start symbol or the root.
• 8.
  Error handling insyntax analysis The following are the common errors: 1. Lexical Error: such as misspelling an identifier, keyword or operator Example: misspelling as ‘whil’ or ‘caase’ 2. Syntactic Error: such as an arithmetic expression with unbalanced paranthesis Example: a=a%+s; or a=((a+b)/n)*100) 3. Semantic Error: such as operator applied to an incompatible operand. Example: c=true*8; this expression is syntactically right, but it does not has meaning. 4. Logical Error: such as infinitely recursive call. Example: for(i=0;i<n;i++);
• 9.
  Error recovering strategies Thefollowing are the common error recovering strategies: 1. Panic Mode 2. Phrase level Recovery 3. Error production 4. Global correction
• 10.
  1.Panic Mode • Whena parser encounters an error anywhere in the statement ,it ignores the rest of the statement by not processing input from erroneous input to delimiter. • This method often skips a considerable amount of input without checking it for additional errors. • It is an easiest way of error-recovery. • It prevents the parser from developing infinite loops
• 11.
  Contd.. Example: a=b+c a=b+c //after reach c parser discards input symbol one at a time d=e+f; The compiler will discard all subsequent token till the semicolon encountered. int a, 5abcd, sum, $2; // After int a, 5abcd , sum, $2 ; // parser discards input symbol one at a time.
• 12.
  Contd.. • Advantage: 1.It’s easyto use. 2.The program never falls into the loop. • Disadvantage: 1.This technique may lead to semantic error or runtime error in further stages.
• 13.
  2.Phrase Level Recovery •On discovering an error, a parser may perform local corrections on the remaining input • It may replace a prefix of the remaining input by some string that allows the parser to continue • Parser designers have to be careful here one wrong correction can lead to infinite loop Example: in case of an error like the previous ,it will report the error and generate the”;” and continue processing.
• 14.
  3.Error Production • Itrequires good knowledge of common errors that might get encountered, then we can augment the grammar for the corresponding language with error productions that generate the erroneous constructs. • If error production is used during parsing, we can generate an appropriate error message to indicate the error that has been recognized in the input. • This method is extremely difficult to maintain, because if we change grammar, then it becomes necessary to change the corresponding productions.
• 15.
  Contd.. Example: Suppose theinput string is abcd. Grammar: S-> A A-> aA | bA | a | b B-> cd The input string is not obtainable by the above grammar, so we need to add Augmented Grammar. Grammar: E->SB // AUGMENT THE GRAMMAR S-> A A-> aA| bA | a | b B-> cd Now, string abcd is possible to obtain.
• 16.
  Contd.. Advantages: • Syntactic phaseerrors are generally recovered by error productions. Disadvantages: • The method is very difficult to maintain because if we change the grammar then it becomes necessary to change the corresponding production. • It is difficult to maintain by the developers.
• 17.
  4.Global Correction • Weoften want such a compiler that makes very few changes in processing an incorrect input string to the correct input string. • Given an incorrect input string x and grammar G, the algorithm itself can find a parse tree for a related string y (Expected output string); such that a number of insertions, deletions, and changes of token require to transform x into y is as low as possible. • Global correction methods increase time & space requirements at parsing time. This is simply a theoretical concept.
• 18.
  CONTEXT FREE GRAMMAR
• 19.
  Context free Grammar(CFG) •Context free grammar is a formal grammar which is used to generate all possible strings in a given formal language. • Context free grammar G can be defined by four tuples as:G= (V, T, P, S) G  Describes a grammar V  set of variables or non-terminals T  set of terminal symbols P  set of production rules S  Start symbols