Where Can I Find a Simple ANTLR Example for Building a Four-Operation Calculator?

Can I Find a Basic ANTLR Sample Code?

ANTLR can be a complex tool to understand, especially for beginners. For a more simplified approach, here's an example that demonstrates the process of creating a four-operations calculator using ANTLR. This example covers the entire process from grammar definition to generating Java code.

Grammar Definition

The grammar file, Exp.g, defines the grammar for the calculator.

grammar Exp;

// Entry point of the parser
eval returns [double value]
    :    additionExp
    ;

// Addition and subtraction have lower precedence
additionExp returns [double value]
    :    multiplyExp 
         ( '+' multiplyExp 
         | '-' multiplyExp
         )* 
    ;

// Multiplication and division have higher precedence
multiplyExp returns [double value]
    :    atomExp       {$value =  $a1.value;}
         ( '*' atomExp {$value *= $a2.value;} 
         | '/' atomExp {$value /= $a2.value;}
         )* 
    ;

// Expression atom: a number or parenthesis-enclosed expression
atomExp returns [double value]
    :    Number                {$value = Double.parseDouble($n.text);}
    |    '(' exp=additionExp ')' {$value = $exp.value;}
    ;

// Number: an integer or decimal value
Number
    :    ('0'..'9')+ ('.' ('0'..'9')+)?
    ;

// Ignore white space
WS
    :   (' ' | '\t' | '\r'| '\n') {$channel=HIDDEN;}
    ;

Java Code Integration

To add Java code to the grammar, use the { } block with plain Java inside. However, double return values require a dollar sign ($) before them.

// Entry point of the parser
eval returns [double value]
    :    exp=additionExp {$value = $exp.value;}
    ;

ANTLR Code Generation

Use the ANTLR jar to generate the lexer and parser from the grammar.

java -cp antlr-3.2.jar org.antlr.Tool Exp.g

Test Class

Create a test class to execute the calculator.

import org.antlr.runtime.*;

public class ANTLRDemo {
    public static void main(String[] args) throws Exception {
        ANTLRStringStream in = new ANTLRStringStream("12*(5-6)");
        ExpLexer lexer = new ExpLexer(in);
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        ExpParser parser = new ExpParser(tokens);
        System.out.println(parser.eval().value); // Print the result
    }
}

Running the Example

Compile the classes and run the test class. The result of the expression 12*(5-6) will be printed.

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