Parsing: (00) Some sample code to refer to

Some code you can run, compile, and consult as go through the tutorial

Acknowledgments: this series or articles is joint work, a collaboration between Kyle Dewey and Phill Conrad.

Some code

To ground all of the following discussion in some code, let’s start with a github repository that provides an example of the type of code we are talking about in this tutorial.

The following github repo contains an example of a program that evaluates arithmetic expressions:

• https://github.com/UCSB-CS56-pconrad/ucsb-cs56-parsing-arithmetic-expressions

Sample Output

Let’s look at a sample run of this program, so that you understand what kind of code we are trying to be able to write. You are encouraged to git clone this repo (or a fork of it) and experiment with it. As you read this tutorial, we may make reference to this code.

This program provides a “Read/Eval/Print Loop (REPL)” for evaluating integer expressions for the four main arithmetic operators: +,-,*,/, with / representing integer division as in C/C++/Java, the usual precedence and associativity rules, and parentheses (, ) for grouping.

The program will either evaluate the expression and return a result, or print an error message if the expression is not well-formed. Note that the usual precedence and associative rules are followed, so that:

• 2 + 3 * 5 is evaluated as 2 + (3 * 5) which is 17 rather than (2 + 3) * 5 which would be 25
• 3 - 3 * 3 - 3 is evaluated as (3 - (3 * 3)) - 3, which is -9 rather than 3 - ((3 * 3) - 3) which would be -3

So, here’s sample output.

-bash-4.3$ ant run
Buildfile: /cs/faculty/pconrad/github/UCSB-CS56-pconrad/ucsb-cs56-parsing-arithmetic-expressions/build.xml

compile:
    [mkdir] Created dir: /cs/faculty/pconrad/github/UCSB-CS56-pconrad/ucsb-cs56-parsing-arithmetic-expressions/build/classes
    [javac] Compiling 41 source files to /cs/faculty/pconrad/github/UCSB-CS56-pconrad/ucsb-cs56-parsing-arithmetic-expressions/build/classes

jar:
    [mkdir] Created dir: /cs/faculty/pconrad/github/UCSB-CS56-pconrad/ucsb-cs56-parsing-arithmetic-expressions/build/jar
      [jar] Building jar: /cs/faculty/pconrad/github/UCSB-CS56-pconrad/ucsb-cs56-parsing-arithmetic-expressions/build/jar/CS56Parser.jar

run:
     [java] Enter expressions, or q to quit.
2+2
     [java] 4
2 + 3 * 5
     [java] 17
3 - 3 * 3 - 3
     [java] -9
5 ** 5
     [java] Failed to parse: Extra tokens at the end
3 == 4 
     [java] Failed to parse: Extra tokens at the end
4 4 +
     [java] Failed to parse: Extra tokens at the end
+ 4 4 
     [java] Failed to parse: Expected primary expression; got: PlusToken
123-456                  
     [java] -333
These are bad tokens
     [java] Failed to parse: Expected primary expression; got: ErrorToken(T)
4 + x
     [java] Failed to parse: Extra tokens at the end
x + 4
     [java] Failed to parse: Expected primary expression; got: ErrorToken(x)
foo + bar
     [java] Failed to parse: Expected primary expression; got: ErrorToken(f)
q

BUILD SUCCESSFUL
Total time: 1 minute 46 seconds
-bash-4.3$ 

This repo is a “playground”, not the starting point for your assignment.

One more thing: the https://github.com/UCSB-CS56-pconrad/ucsb-cs56-parsing-arithmetic-expressions repo is here to be a “reusable” repo that corresponds to the text of this tutorial, which is also intended to be “reusable”, quarter-after-quarter. Note that if you have a programming assignment based on this repo, it may have starter code that is similar to this repo, but in a different place, and possibly with subtle but important differences.

So be sure to check your specific assignment for the proper starting point code. If you clone the https://github.com/UCSB-CS56-pconrad/ucsb-cs56-parsing-arithmetic-expressions repo and play with it, consider that a “playground” or a “sandbox” in which to try things out to learn about parsing, NOT as a place to work on code you are going to submit as part of your programming assignment.

For the proper starting point for your assignment, consult the lab instructions.