trab.java

/* GEC2010N-05 / JAVA III
 * Prof. Tomás de Aquino Tinoco Botelho
 */

package trabjava31;

/**
 * @author EQUIPE ROCKET w/ much help of Deitel and internet!
 */

import java.util.*;

public class NotationConverter {

    public String convertToPostfix(String inFix)
    {
        String outFix = "";
        Stack<String> memStack = new Stack<String>();
        String ch = "";

        for(int pos = 0;pos != inFix.length();pos++)
        {
            ch = inFix.substring(pos, pos + 1);

            if(ch.matches("[a-zA-Z]|\\d"))
            {
                outFix += ch;
            }
            else if(isOperator(ch))
            {
                if(memStack.isEmpty())
                {
                    memStack.push(ch);
                }
                else
                {
                    String stackTop = memStack.peek();
                    while (getPrecedence(ch,stackTop) && !(memStack.isEmpty()))
                    {
                        outFix += memStack.pop();
                        if(!(memStack.isEmpty()))
                            stackTop = memStack.peek();
                    }
                    memStack.push(ch);
                }

            }

        }

        while(!(memStack.isEmpty()))
            outFix += memStack.pop();

        return outFix;
    }
    
    private boolean isOperator(String ch)
    {
        if(ch.equals("+") || ch.equals("-") || ch.equals("*") ||
                ch.equals("/") || ch.equals("^"))
        {
            return true;
        }
        return false;
    }

    private boolean getPrecedence(String newOperator, String oldOperator)
    {
           if (newOperator.equals("^"))
                return true;
           else if (oldOperator.equals("^") )
                return false;
           else if (newOperator.equals("*") || newOperator.equals("/"))
                return true;
           else if (newOperator.equals("+") || newOperator.equals("-"))
           {
               if (oldOperator.equals("*") || oldOperator.equals("/"))
                    return false;
               else
                    return true;
            }
            return false;
    }
}

/* end of homework */