2.4 Case Study: The Calculator Scanner

As a running example for this and the subsequent sections, we will construct a simple interpreter for a calculator. It is very important that you download the code yourself on gamma and play with it:

1. Download the file calc.tar.gz in your gamma directory
2. Execute: gzip -d -c calc.tar.gz | tar -xvf -
3. Execute: cd calc; make
4. Now you are ready to run the calculator using: ./calc

If you have a home computer with Linux and you want to install the system there, you have to do the following steps before you compile the calculator:

1. Download the garbage collector, gc.tar.gz
2. Execute: gzip -d -c gc.tar.gz | tar -xvf -
3. Do a: cd gc and change the first lines of the Makefile to CC=gcc, CXX=g++, and

   CFLAGS= -O -DNO_SIGNALS -DALL_INTERIOR_POINTERS -DSILENT -DATOMIC_UNCOLLECTABLE -DREDIRECT_MALLOC=GC_malloc_uncollectable
   

   and execute: make; make c++ to create the C and C++ garbage collectors.
4. Change the line GCDIR = /public/cse/5317-501/gc/ in calc/Makefile to point to your own gc directory and then proceed with the instalation of calc.

All the files of the calculator example are given in the calc directory. They will be explained in detail later. For now, we are ready to look at the scanner, given in calc.lex. The upercase constants, such as INT, are defined as tokens in the parser and will be explained later. For now, just assume that these are different constants (integers). The order of lines is important in some cases: if we put the rule for {ID} before the keywords, then the keywords will never be recognized (this is the consequence of the rule priority law). The lexical constructs that need to be skipped, such as comments and whitespaces, should never return (we must return only when we find a complete token).