Compilers:: Implementing Something Like Our Example Expression Grammar Calculator

here is a link to a simple calculator in lex and yacc, implementing something like our example expression grammar calculator
This is calc.lex and calc.yacc from Andrew Brinker - you may use these as youy starting point instead of the first link
1. Implement it
> 2. Add and exit/quit feature, so that you can enter one expression and then use the result as the start of another expression - for example:
2+5 => 7
/2 => 3.5
3. Add variables to the grammar - this requires modifying the grammar with an assign statement so you can set the values of a variable, and a print ot display statement so you can view the contents of the variable. You may implement it as an interpreter or a compiler 

*** This is link for 'calculator' :: https://thegeekuday.wordpress.com/2012/11/14/calculator-program-using-yacc-and-lex/ 

*** This is link for 'calc.lex' ::  http://www.cse.csusb.edu/egomez/calc.lex 

*** This is link for 'calc.yacc' :: http://www.cse.csusb.edu/egomez/calc.yacc