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Introduction to Syntax Analysis in Compiler Design

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  • Difficulty Level : Easy
  • Last Updated : 26 Jan, 2023
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When an input string (source code or a program in some language) is given to a compiler, the compiler processes it in several phases, starting from lexical analysis (scans the input and divides it into tokens) to target code generation.

Syntax Analysis or Parsing is the second phase, i.e. after lexical analysis. It checks the syntactical structure of the given input, i.e. whether the given input is in the correct syntax (of the language in which the input has been written) or not. It does so by building a data structure, called a Parse tree or Syntax tree. The parse tree is constructed by using the pre-defined Grammar of the language and the input string. If the given input string can be produced with the help of the syntax tree (in the derivation process), the input string is found to be in the correct syntax. if not, the error is reported by the syntax analyzer.

Syntax analysis, also known as parsing, is a process in compiler design where the compiler checks if the source code follows the grammatical rules of the programming language. This is typically the second stage of the compilation process, following lexical analysis.

The main goal of syntax analysis is to create a parse tree or abstract syntax tree (AST) of the source code, which is a hierarchical representation of the source code that reflects the grammatical structure of the program.

There are several types of parsing algorithms used in syntax analysis, including:

  • LL parsing: This is a top-down parsing algorithm that starts with the root of the parse tree and constructs the tree by successively expanding non-terminals. LL parsing is known for its simplicity and ease of implementation.
  • LR parsing: This is a bottom-up parsing algorithm that starts with the leaves of the parse tree and constructs the tree by successively reducing terminals. LR parsing is more powerful than LL parsing and can handle a larger class of grammars.
  • LR(1) parsing: This is a variant of LR parsing that uses lookahead to disambiguate the grammar.
  • LALR parsing: This is a variant of LR parsing that uses a reduced set of lookahead symbols to reduce the number of states in the LR parser.
  • Once the parse tree is constructed, the compiler can perform semantic analysis to check if the source code makes sense and follows the semantics of the programming language.
  • The parse tree or AST can also be used in the code generation phase of the compiler design to generate intermediate code or machine code.

The pushdown automata (PDA) is used to design the syntax analysis phase.

The Grammar for a Language consists of Production rules.

Example: Suppose Production rules for the Grammar of a language are:

  S -> cAd
  A -> bc|a
  And the input string is “cad”.

Now the parser attempts to construct a syntax tree from this grammar for the given input string. It uses the given production rules and applies those as needed to generate the string. To generate string “cad” it uses the rules as shown in the given diagram: syntaxAnalysis

In step (iii) above, the production rule A->bc was not a suitable one to apply (because the string produced is “cbcd” not “cad”), here the parser needs to backtrack, and apply the next production rule available with A which is shown in step (iv), and the string “cad” is produced.

Thus, the given input can be produced by the given grammar, therefore the input is correct in syntax. But backtrack was needed to get the correct syntax tree, which is really a complex process to implement.

There can be an easier way to solve this, which we shall see in the next article “Concepts of FIRST and FOLLOW sets in Compiler Design”.

 

Advantages :

  • Advantages of using syntax analysis in compiler design include:
  • Structural validation: Syntax analysis allows the compiler to check if the source code follows the grammatical rules of the programming language, which helps to detect and report errors in the source code.
  • Improved code generation: Syntax analysis can generate a parse tree or abstract syntax tree (AST) of the source code, which can be used in the code generation phase of the compiler design to generate more efficient and optimized code.
  • Easier semantic analysis: Once the parse tree or AST is constructed, the compiler can perform semantic analysis more easily, as it can rely on the structural information provided by the parse tree or AST.

Disadvantages:

  • Disadvantages of using syntax analysis in compiler design include:
  • Complexity: Parsing is a complex process, and the quality of the parser can greatly impact the performance of the resulting code. Implementing a parser for a complex programming language can be a challenging task, especially for languages with ambiguous grammars.
  • Reduced performance: Syntax analysis can add overhead to the compilation process, which can reduce the performance of the compiler.
  • Limited error recovery: Syntax analysis algorithms may not be able to recover from errors in the source code, which can lead to incomplete or incorrect parse trees and make it difficult for the compiler to continue the compilation process.
  • Inability to handle all languages: Not all languages have formal grammars, and some languages may not be easily parseable.
  • Overall, syntax analysis is an important stage in the compiler design process, but it should be balanced against the goals and

Quiz on Syntax Analysis

This article is compiled by Vaibhav Bajpai. Please write comments if you find anything incorrect, or if you want to share more information about the topic discussed above

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