ExpressionParser.java

package appl_alg_interpreter;

import java.io.File;

public class ExpressionParser {

	static ProgramScanner psc;

	static String NAME = "[a-zA-Z_][a-zA-Z_0-9]*";
	static String NUMBER = "(^|\\s+)((\\-?[0-9]+(\\.[0-9]*)?)|(-?\\.[0-9]+))($|\\s+)";
	static String OPERATOR = "[\\+\\-\\*/]";

	static class Node {
		public String value;
		public Node left;
		public Node right;

		public Node(String value, Node left, Node right) {
			this.value = value;
			this.left = left;
			this.right = right;
		}

		public static Node expr(ProgramScanner psc) throws Exception {
			String token = psc.nextTokenOrNull(NUMBER);
			if (token != null) {
				return new Node(token, null, null);
			}
			token = psc.nextTokenOrNull("\\(");
			Node left = Node.expr(psc);
			token = psc.nextToken(OPERATOR);
			String value = token;
			Node right = Node.expr(psc);
			token = psc.nextTokenOrNull("\\)");

			return new Node(value, left, right);
		}

		public Double evaluate() {
			Double result = 0.0;
			if (left == null)
				return Double.parseDouble(value);
			Double l = left.evaluate();
			Double r = right.evaluate();
			if (value.equals("+"))
				result = l + r;
			if (value.equals("-"))
				result = l - r;
			if (value.equals("*"))
				result = l * r;
			if (value.equals("/"))
				result = l / r;

			return result;
		}
	}

	public static void main(String[] args) {

		try {
			psc = new ProgramScanner(new File("appls/appl_algs/Test.apl"));
			Node result = Node.expr(psc);

			System.out.println("Success");
			System.out.println("Result = " + result.evaluate());
		} catch (Exception e) {
			System.out.println(e.getMessage());
		}
	}

}