Compiler HWK#4 Due date: 2:20pm, May 18, 2006

Write a simple compiler-interpreter for a simple PASCAL-like
expression language.

A sample program is as follows:

       PROGRAM program_name
       VAR %% beginning of variable declarations, required even no var's
          x, y, z: REAL;
       BEGIN
          READ(y);
          x := (y + 3.0 * 4.0) - 5.0;
          WRITE(y);
          WRITESP();
          WRITE(x);
          WRITELN();
       END

The definition of the language is as follows:
1. Case-sensitive with reserved words.
2. Assignment is ":=", no space is allowed between ":" and "=".
   the statement terminator is ";"
   each line has at most one statement
3. Variable must be declared before its usage.
   Variable/program names follow C style.
   Can have only the type of REAL (4-byte signed)
4. Can have only three different statements:
   a. assignment of expressions, where expressions
      can have variables, real-constant, (, ), +, -, *, / and
      minus "-".
      A real-constant is in the form of "digits.digits",
      where digits = [0-9]+
   b. input statement
         READ(single variable)
   c. output statements
         WRITE(single variable) ---  output a variable using "%f" format
         WRITESP()              ---  output a single space
         WRITELN()              ---  write a new line
   There is no space before and in between "()" for WRITESP() and
   WRITELN().
5. Anything after %% are comments until to the end of the line.
   Allow white space of tab and space.
   Can have empty lines.
   Cannot have empty statements, ie, a line with only a ";" is illegal.
6. Must detect the following error and reports its line number:
    a. invalid ID names
    b. undeclared variables
    c. illegal syntax
    d. a variable declared twice
7. Must run the program and produce outputs accordingly if there is no error.

Hints: (1) Assume <= 256 variables can be declared.
           Each variable name has at most 32 characters.
           First allocate 256*4 bytes and keep track of
           the address of each variable.
       (2) The semantics of a variable name are different
           on the LHS and RHS of an assignment.
              LHS: retrieve the storage address of a variable and store
                   a value from the RHS to it
              RHS: retrieve the value of the variable

Note:    You need to have LEX and YACC programs.
Your program code is in the file "program.p".
LEX takes input from program.p.
Your keyboard input becomes the standard input of your program.