Java programs implementing different concepts of compiler designing like lexical analysis, parsing, etc.

• In this phase of compilation, the source code is broken down into a series of lexemes and corresponding to each valid lexeme, a token is generated, removing any whitespace or comments in the source code.
• The validity of a lexeme is decided by certain grammar rules.
• Tokens are generated for each of the keywords, functions, headers, operators and special symbols.
• The lexical analyser also recognizes symbols (or identifiers) and literals (or constants) and generates tokens for them as well.
• If the lexical analyzer finds a token invalid, it generates an error. For example, in this program, an error will be generated for each invalid symbol declaration.
• An instance of how the program works is as shown below:

Tokens are generated for each of the valid lexemes present in the C program in the file 'test.txt' and further all the different tables are shown as well:

Here an error gets generated as an incorrect symbol declaration is made as '2i' as seen in 'test.txt':

• Parsing forms a part of Syntax Analysis that comes after Lexical Analysis.

• A Syntax Analyzer or Parser takes the input from a lexical analyzer in the form of token streams. The parser analyzes the source code (token stream) against a set of production rules to detect any errors in the code. The output of this phase is a parse tree.

• Recursive Descent Parser or RDP is a top-down parsing technique that constructs the parse tree from the top and the input is read from left to right.

• Here we have different procedures for each of the non-terminals and the input string is read character-by-character and correspondingly a procedure is chosen for the non-terminal encountered.

• The technique is recursive in the sense that if a certain procedure (with respect to a production rule) selected does not allow the string to be accepted, the control goes back to the parent procedure wherein a different production rule is chosen and the process continues until there are no more production rules left and the string will then be rejected completely as invalid.

• The program was written with respect to the following left recursion-free grammar:

    S->aAc|aAb
    A->bAb|ab|a
  

• Instances of how the program works are as shown below: