Simulation of the front end phase of a C compiler involving the If-Else and While constructs.
This includes:

1. Lexical Analysis- Scanning the input and formation of the symbol table.
   
2. Syntactic Analysis- Creation of an Abstract Syntax Tree.
   
3. Semantic Analysis- Verifying Type compatability and generating the Intermediate Code.
   
4. Machine Independent Code Optimization- Constant Folding, Common Sub-Expression Elimination.
   

To Run Program:

1. Place all files in the same directory.
   
2. lex lexer.l
   
3. yacc -d -v parser.y
   ss4) gcc y.tab.c -ll
   
4. ./a.out<input.txt
   

Here lexer.l is the lexical analyzer, yacc -d -v parser.y runs parser.y written in YACC and -d creates y.tab.h and -v creates y.output (debugger for parser).