Programming Language Assignment

This repository is created to update files between Linux cluster and my PC more easily, and describing about the structure of this project.

Simple Assignable Calculator using ANTLR4

PA#1 is about implementing listener-based calculator that can assign a value to variables and refer to the assigned value while evaluating an expression.

In this version, it does not need to detect referring undefined variable; like JS.

Description about this repo

This project is managed by Apache Maven, so actual source files are located to src/main/java and testing sources are in src/test/java. On this README, test files are not covered.

Expr.g4

Unlike other source files, this file is located out of src, because VSCode ANTLR4 extension has an issue about producing output files on local .antlr folder, which can mess up the classpath.

There are four lexer tokens and parser rules defined in this file:

• Lexer Tokens
  • NUM : decimal notation of real numbers, include integers.
  • ID : possible name of variables. The rule is same to C.
  • WS : White spaces. It is a skipped token.
  • END : Semicolon. It indicates the end of a statement.
• Parser Rules
  • prog : Program. It is defined as a array of statements.
  • stmt : Statement. It is an expression or an assignment, terminated with a semicolon.
  • expr : Expression. It can be a number(NUM) or a variable(ID), or a valid composition of them (refer to the file). An expression always has a value.
  • assn : Assignment. It is assigning(=) a value(expr) to a variable(ID). It is not considered as an expression, so it does not produce any value.

Java Files

Some base classes that ANTLR4 produces from *.g4 file are not covered in this section. This is the list of those classes:

• ExprLexer (child class of Lexer)
• ExprParser (child class of Parser)
• ExprListener (child interface of ParseTreeListener)
• ExprBaseListener (child class of ExprListener)

Refer to ANTLR4 Java Runtime API Documentation for more information about them.

1. ExprEvalApp: Main Class

This class contains the main function. A flow of this program is following:

1. receives an input file from command arguments. If it is missing, it gets an user input from console.
2. If the input is "interactive", it receives input from the console repeatedly, until the input is "exit". Otherwise, it reads an input from the file.
3. Evaluates the input string. Go back to step 2 on interactive mode.

eval function corresponds to step 3 above. It is separated from main for unit test.

testFile and testString are unit test functions, but not used in this version.

2. ExprEvalListener: Listener Class

This class is a child class of ExprBaseListener. It listens to the trail of ParseTreeWalker through ParseTree, and does some tasks corresponds to current step.

In detail, this class executes shunting-yard algorithm and produces post fixed expression. At the end of each statement, it finally evaluates the formula. If it is an expr, the result is printed to the console. Otherwise, (i.e. if it is an assn,) the value is assigned to the indicated variable.

3. ExprValue: Value Wrapping Class

ExprValue encapsulates ID and NUM, which have actual values. Since the value extraction process is different, it need to be abstracted with val().

Additionally, this class manages object-pool to get the same ExprValue from the ID nodes have the same texts.

4. ExprOperator: Operator Wrapping Class

ExprOperator is an enumerator contains a character that corresponds to the operator, actual operation, precedence, and evaluation/operator stack management policy.

In this class, private interfaces IBinaryOperator and IPrecedence is declared, and operating and stack managing logic is delegated to them, respectively.

5. ExprUtil: Utility Class

This is an non-instantiable class that contains some useful static methods. Those methods are about handling objects defined in ANTLR4 and adapting them to the system of this calculator.