InfixToPostfix

[ak13qa]

how to put the precedence class to this coding.


Infix2Postfix.java

import java.util.Scanner;
import java.util.Stack;

public class Infix2Postfix {

	private Stack<String> stack;
	private String infixExp;
	private String postfixExp = "";

	public Infix2Postfix() {}

	public void convertToPostfix(String exp){

		String str = "";
		infixExp = exp;
		stack = new Stack<String>();

		for (int i=0;i<infixExp.length();i++){

			str = infixExp.substring(i,i+1);
			if(str.matches("[a-zA-Z]|\\d"))
				postfixExp += str;
			else if (isOperator(str)){
				if (stack.isEmpty()){
					stack.push(str);
				}
				else{
					String stackTop = stack.peek();
					while (precedence(stackTop,str).equals(stackTop)&& !(stack.isEmpty())){
						postfixExp += stack.pop();
						if (!(stack.isEmpty()))
							stackTop = stack.peek();
					}
					stack.push(str);
				}
			}
		}

		System.out.println("The postfix form of the expression you entered is: " + postfixExp);
	}

	private boolean isOperator(String ch){

		String operators = "*/%+-";
		if (operators.indexOf(ch) != -1)
			return true;
		else
			return false;
	}

	private String precedence(String op1, String op2){

		String multiplicativeOps = "*/%";
		String additiveOps = "+-";

		if ((multiplicativeOps.indexOf(op1) != -1) && (additiveOps.indexOf(op2) != -1))
			return op1;
		else if ((multiplicativeOps.indexOf(op2) != -1) && (additiveOps.indexOf(op1) != -1))
			return op2;
		else if((multiplicativeOps.indexOf(op1) != -1) && (multiplicativeOps.indexOf(op2) != -1))
			return op1;
		else
			return op1;
	}

	public void getInfix(String infixExp)
	{
		this.infixExp = infixExp;
	}

	public String showInfix()
	{
		return infixExp;
	}

	public String showPostfix()
	{
		return postfixExp;
	}

}

TestProgInfix2Postfix.java

import java.util.Scanner;
import java.util.Stack;

public class TestProgInfix2Postfix

{

	public static void main(String[] args)
	{

		int count = 0;
		while(count < 5)
		{
			System.out.println("Enter an expression in the Infix form:");
			Scanner scanner = new Scanner(System.in);

			String expression = scanner.nextLine();
			Infix2Postfix x = new Infix2Postfix();

			x.convertToPostfix(expression);

			x.getInfix(expression);
			System.out.println();

			System.out.println("Trying to view infix: "+x.showInfix());
			System.out.println("Trying to view postfix: "+x.showPostfix());

			System.out.println();
			count++;
		}
	}
}

[Norm]

how to put the precedence class to this coding.

Can you explain?
What is the "precedence class"?

[ak13qa]

precedence: precedence between two operators. If the first operator is of higher or equal precedence than the second operator, its returns the value true, otherwise its return value false.

[JosAH]

The essential part of a stack based algorithm for infix -> posfix expressions is the priority/precedence of the operator on the stack top, compared to the priority/precedence of the operator to be entered on the stack. There's a nice explanation in Sahni and Horowitz "Data Structures" but I'm sure there's an answer on Google as well.

kind regards,

Jos