Computer Science Design Patterns/Interpreter

Examples

The following Reverse Polish notation example illustrates the interpreter pattern. The grammar

expression ::= plus | minus | variable | number
plus ::= expression expression '+'
minus ::= expression expression '-'
variable  ::= 'a' | 'b' | 'c' | ... | 'z'
digit = '0' | '1' | ... '9'
number ::= digit | digit number

defines a language which contains reverse Polish expressions like:

a b +
a b c + -
a b + c a - -

Following the interpreter pattern there is a class for each grammar rule.

Java

import java.util.Map;
 
interface Expression {
    public int interpret(Map<String, Expression> variables);
}

import java.util.Map;
 
class Number implements Expression {
    private int number;
 
    public Number(int number) {
        this.number = number;
    }
 
    public int interpret(Map<String, Expression> variables) {
        return number;
    }
}

import java.util.Map;
 
class Plus implements Expression {
    Expression leftOperand;
 
    Expression rightOperand;
 
    public Plus(Expression left, Expression right) {
        leftOperand = left;
        rightOperand = right;
    }
 
    public int interpret(Map<String, Expression> variables) {
        return leftOperand.interpret(variables) + rightOperand.interpret(variables);
    }
}

import java.util.Map;
 
class Minus implements Expression {
    Expression leftOperand;
 
    Expression rightOperand;
 
    public Minus(Expression left, Expression right) {
        leftOperand = left;
        rightOperand = right;
    }
 
    public int interpret(Map<String, Expression> variables)  {
        return leftOperand.interpret(variables) - rightOperand.interpret(variables);
    }
}

import java.util.Map;
 
class Variable implements Expression {
    private String name;
 
    public Variable(String name) {
        this.name = name;
    }
 
    public int interpret(Map<String, Expression> variables) {
        if (variables.get(name) == null) {
            // Either return new Number(0).
            return 0;
        } else {
            return variables.get(name).interpret(variables);
        }
    }
}

While the interpreter pattern does not address parsing a parser is provided for completeness.

import java.util.Map;
import java.util.Stack;
 
class Evaluator implements Expression {
    private Expression syntaxTree;
 
    public Evaluator(String expression) {
        Stack<Expression> expressionStack = new Stack<Expression>();
        for (String token : expression.split(" ")) {
            if  (token.equals("+")) {
                Expression subExpression = new Plus(expressionStack.pop(), expressionStack.pop());
                expressionStack.push( subExpression );
            }
            else if (token.equals("-")) {
                // it's necessary remove first the right operand from the stack
                Expression right = expressionStack.pop();
                // ..and after the left one
                Expression left = expressionStack.pop();
                Expression subExpression = new Minus(left, right);
                expressionStack.push( subExpression );
            }
            else                        
                expressionStack.push( new Variable(token) );
        }
        syntaxTree = expressionStack.pop();
    }
 
    public int interpret(Map<String,Expression> context) {
        return syntaxTree.interpret(context);
    }
}

Finally evaluating the expression "w x z - +" with w = 5, x = 10, and z = 42.

import java.util.Map;
import java.util.HashMap;
 
public class InterpreterExample {
    public static void main(String[] args) {
        String expression = "w x z - +";
        Evaluator sentence = new Evaluator(expression);
        Map<String,Expression> variables = new HashMap<String,Expression>();
        variables.put("w", new Number(5));
        variables.put("x", new Number(10));
        variables.put("z", new Number(42));
        int result = sentence.interpret(variables);
        System.out.println(result);
    }
}

C#

using System;
using System.Collections.Generic;
 
namespace Interpreter
{
    class Program
    {
        interface IExpression
        {
            int Interpret(Dictionary<string, int> variables);
        }
 
        class Number : IExpression
        {
            public int number;
            public Number(int number) { this.number = number; }
            public int Interpret(Dictionary<string, int> variables) { return number; }
        }
 
        abstract class BasicOperation : IExpression
        {
            IExpression leftOperator, rightOperator;
 
            public BasicOperation(IExpression left, IExpression right)
            {
                leftOperator = left;
                rightOperator = right;
            }
 
            public int Interpret(Dictionary<string, int> variables)
            {
                return Execute(leftOperator.Interpret(variables), rightOperator.Interpret(variables));
            }
 
            abstract protected int Execute(int left, int right);
        }
 
        class Plus : BasicOperation
        {
            public Plus(IExpression left, IExpression right) : base(left, right) { }
 
            protected override int Execute(int left, int right)
            {
                return left + right;
            }
        }
 
        class Minus : BasicOperation
        {
            public Minus(IExpression left, IExpression right) : base(left, right) { }
 
            protected override int Execute(int left, int right)
            {
                return left - right;
            }
        }
 
        class Variable : IExpression
        {
            private string name;
 
            public Variable(string name) { this.name = name; }
 
            public int Interpret(Dictionary<string, int> variables)
            {
                return variables[name];
            }
        }
 
        class Evaluator
        {
            private IExpression syntaxTree;
 
            public Evaluator(string expression)
            {
                Stack<IExpression> stack = new Stack<IExpression>();
                foreach (string token in expression.Split(' '))
                {
                    if (token.Equals("+"))
                        stack.Push(new Plus(stack.Pop(), stack.Pop()));
                    else if (token.Equals("-")){
                        IExpression right = stack.Pop();
                        IExpression left = stack.Pop();
                        stack.Push(new Minus(left, right));
                    }else
                        stack.Push(new Variable(token));
                }
                syntaxTree = stack.Pop();
            }
 
            public int Evaluate(Dictionary<string, int> context)
            {
                return syntaxTree.Interpret(context);
            }
        }
 
        static void Main(string[] args)
        {
            Evaluator evaluator = new Evaluator("w x z - +");
            Dictionary<string, int> values = new Dictionary<string,int>();
            values.Add("w", 5);
            values.Add("x", 10);
            values.Add("z", 42);
            Console.WriteLine(evaluator.Evaluate(values));
        }
    }
}

PHP

In a file we have the classes and the interface, defining the logic of the program (and applying the Interpreter pattern).

Now the expr.php file:

<?php
 
    interface expression{
        public function interpret (array $variables);
        public function __toString();
    }
 
    class number implements expression{
        private $number;
        public function __construct($number){
            $this->number = intval($number);
        }
        public function interpret(array $variables){
            return $this->number;
        }
        public function __toString(){
            return (string) $this->number;
        }
    }
 
    class plus implements expression{
        private $left_op;
        private $right_op;
        public function __construct(expression $left, expression $right){
            $this->left_op = $left;
            $this->right_op = $right;
        }
        public function interpret(array $variables){
            return ($this->left_op->interpret($variables) + $this->right_op->interpret($variables));
        }
        public function __toString(){
            return (string) ("Left op: {$this->left_op} + Right op: {$this->right_op}\n");
        }
    }
 
    class minus implements expression{
        private $left_op;
        private $right_op;
        public function __construct(expression $left, expression $right){
            $this->left_op = $left;
            $this->right_op = $right;
        }
        public function interpret(array $variables){
            return ($this->left_op->interpret($variables) - $this->right_op->interpret($variables));
        }
        public function __toString(){
            return (string) ("Left op: {$this->left_op} - Right op: {$this->right_op}\n");
        }
    }
 
    class variable implements expression{
        private $name;
        public function __construct($name){
            $this->name = $name;
        }
        public function interpret(array $variables){
            if(!isset($variables[$this->name]))
                return 0;
            return $variables[$this->name]->interpret($variables);
        }
        public function __toString(){
            return (string) $this->name;
        }
    }
 
?>

And the evaluate.php:

<?php
 
    require_once('expr.php');
 
    class evaluator implements expression{
        private $syntaxTree;
 
        public function __construct($expression){
            $stack = array();
            $tokens = explode(" ", $expression);
            foreach($tokens as $token){
                if($token == "+"){
                    $right = array_pop($stack);
                    $left = array_pop($stack);
                    array_push($stack, new plus($left, $right));
                }
                else if($token == "-"){
                    $right = array_pop($stack);
                    $left = array_pop($stack);
                    array_push($stack, new minus($left, $right));
                }else if(is_numeric($token)){
                    array_push($stack, new number($token));
                }else{
                    array_push($stack, new variable($token));
                }
            }
            $this->syntaxTree = array_pop($stack);
        }
 
        public function interpret(array $context){
            return $this->syntaxTree->interpret($context);
        }
 
        public function __toString(){
            return "";
        }
 
    }
 
    // main code
 
    // works for it:
    $expression = "5 10 42 - +";
    // or for it:
    //$expression = "w x z - +";
    $variables = array();
    $variables['w'] = new number("5");
    $variables['x'] = new number("10");
    $variables['z'] = new number("42");
    print ("Evaluating expression {$expression}\n");
    $sentence = new evaluator($expression);
    $result = $sentence->interpret($variables);
    print $result . "\n";
 
?>

And you can run on a terminal by typing php5 -f evaluator.php (or putting it on a web application).