teaching machines

CS 330 Lecture 9 – Calculator + functions

February 10, 2012 by Chris Johnson. Filed under
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cs330, lectures, spring 2012.

Agenda

• evaluating expressions
• persistence of memory
• adding in functions
• avoiding reparsing with abstract syntax trees (ASTs)

Code

Flop.g

grammar Flop;

@header {
import java.util.HashMap;
}

@members {
  HashMap<String, Integer> memory;

  public void setMemory(HashMap<String, Integer> memory) {
    this.memory = memory;
  }
}

stat
  : ID EQUALS expr {memory.put($ID.text, $expr.value);}
  | expr {System.out.println($expr.value);}
  ;

expr returns [int value]
  : e1=precedence10
    {$value = $e1.value;}
    (PLUS e2=precedence10 {$value += $e2.value;})*
  ;

precedence10 returns [int value]
  : e1=precedence20
    {$value = $e1.value;}
    (TIMES e2=precedence20 {$value *= $e2.value;})*
  ;

precedence20 returns [int value]
  : NUMBER {$value = Integer.parseInt($NUMBER.text);}
  | ID {$value = memory.get($ID.text);}
  | LEFTPAREN expr RIGHTPAREN {$value = $expr.value;}
  ;

LEFTPAREN
  : '('
  ;

RIGHTPAREN
  : ')'
  ;

ID
  : '$' ('A'..'Z'|'a'..'z') ('A'..'Z'|'a'..'z'|'0'..'9')*
  ;

EQUALS
  : '='
  ;

PLUS
  : '+'
  ;

TIMES
  : '*'
  ;

NUMBER
  : '-'? '0'..'9'+
  ;

WHITESPACE
  : (' '|'\t'|'\r'|'\n')+ {skip();}
  ;

FlopInterpreter.java

import java.io.IOException;
import org.antlr.runtime.*;
import java.util.Scanner;
import java.util.HashMap;

public class FlopInterpreter {
  public static void main(String[] args) throws IOException, RecognitionException {

    Scanner lineReader = new Scanner(System.in);
    HashMap<String, Integer> memory = new HashMap<String, Integer>();

    System.out.print("> ");
    while (lineReader.hasNextLine()) {
      ANTLRStringStream in = new ANTLRStringStream(lineReader.nextLine());
      FlopLexer lexer = new FlopLexer(in);
      CommonTokenStream tokens = new CommonTokenStream(lexer);
      FlopParser parser = new FlopParser(tokens);
      parser.setMemory(memory);
      parser.stat();
      System.out.print("> ");
    }
  }
}

Haiku