Recognizing negative values when converting infix to postfix

Question

This is my class:

import java.io.*;
import java.util.*;
import java.lang.*;
import java.util.Scanner;
import java.util.List;
import java.util.Stack;

/**
 *
 * @author rtibbetts268
 */
public class InfixToPostfix
{
        /**
         * Operators in reverse order of precedence.
         */
    private static final String operators = "_-+/*";
    private static final String operands = "0123456789x";

    public String xToValue(String postfixExpr, String x)
    {
        char[] chars = postfixExpr.toCharArray();
        StringBuilder newPostfixExpr = new StringBuilder();

        for (char c : chars)
        {
            if (c == 'x')
            {
                newPostfixExpr.append(x);
            }
            else
            {
                newPostfixExpr.append(c);
            }
        }
        return newPostfixExpr.toString();
    }

    public String convert2Postfix(String infixExpr)
    {
        char[] chars = infixExpr.toCharArray();
        StringBuilder in = new StringBuilder(infixExpr.length());

        for (int i : chars)
        {
            if (infixExpr.charAt(i) == '-')
            {
                if (isOperand(infixExpr.charAt(i+1)))
                {
                    if (i != infixExpr.length())
                    {
                        if (isOperator(infixExpr.charAt(i-1)))
                            in.append('_');
                    }
                    else
                    {
                        in.append(infixExpr.charAt(i));
                    }
                }
                else
                {
                   in.append(infixExpr.charAt(i));
                }
            }
            else
            {
                in.append(infixExpr.charAt(i));
            }
        }

        chars = in.toString().toCharArray();
        Stack<Character> stack = new Stack<Character>();
        StringBuilder out = new StringBuilder(in.toString().length());

        for (char c : chars)
        {
            if (isOperator(c))
            {
                while (!stack.isEmpty() && stack.peek() != '(')
                {
                    if (operatorGreaterOrEqual(stack.peek(), c))
                    {
                        out.append(stack.pop());
                    }
                    else
                    {
                        break;
                    }
                }
                stack.push(c);
            }
            else if (c == '(')
            {
                stack.push(c);
            }
            else if (c == ')')
            {
                while (!stack.isEmpty() && stack.peek() != '(')
                {
                    out.append(stack.pop());
                }
                if (!stack.isEmpty())
                {
                    stack.pop();
                }
            }
            else if (isOperand(c))
            {
                out.append(c);
            }
        }
        while (!stack.empty())
        {
            out.append(stack.pop());
        }
        return out.toString();
    }

    public int evaluatePostfix(String postfixExpr)
    {
        char[] chars = postfixExpr.toCharArray();
        Stack<Integer> stack = new Stack<Integer>();
        for (char c : chars)
        {
            if (isOperand(c))
            {
                stack.push(c - '0'); // convert char to int val
            }
            else if (isOperator(c))
            {
                int op1 = stack.pop();
                int op2 = stack.pop();
                int result;
                switch (c) {
                    case '*':
                        result = op1 * op2;
                        stack.push(result);
                        break;
                    case '/':
                        result = op2 / op1;
                        stack.push(result);
                        break;
                    case '+':
                        result = op1 + op2;
                        stack.push(result);
                        break;
                    case '-':
                        result = op2 - op1;
                        stack.push(result);
                        break;
                }
            }
        }
        return stack.pop();
    }

    private int getPrecedence(char operator)
    {
        int ret = 0;
        if (operator == '_')
        {
            ret = 0;
        }
        if (operator == '-' || operator == '+')
        {
            ret = 1;
        }
        else if (operator == '*' || operator == '/')
        {
            ret = 2;
        }
        return ret;
    }

    private boolean operatorGreaterOrEqual(char op1, char op2)
    {
        return getPrecedence(op1) >= getPrecedence(op2);
    }

    private boolean isOperator(char val)
    {
        return operators.indexOf(val) >= 0;
    }

    private boolean isOperand(char val)
    {
        return operands.indexOf(val) >= 0;
    }

}

In it I change infix expressions to postfix expressions with the method convert2Postfix()

There is a small section in it at the very beginning where I am rewriting the string input with all negative numbers having a '_' infront of them instead of a "-". It doesn't work.

ex: change -4 to _4

What do I need to do to make this work?

Answer 1

You have a number of errors here, here is the first one:

 for (int i : chars)

chars is converted to the corresponding int. For example if chars contains one single character 'A':

  char [] chars = new char[1];
    chars[0] = 'A';
    for(int i: chars){
        System.out.println(i); //You will have a '65' here
    }

What you really mean is probably:

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

This is also wrong:

  if (isOperator(infixExpr.charAt(i-1)))

It should be :

  if (isOperator(infixExpr.charAt(i)))

Then there is a problem in the way you put and remove elements in the stack. After all this changes you should also do this:

 return out.reverse().toString();

instead of :

 return out.toString();

Here is what I ended up with: http://pastebin.com/2TLqPUsH (Change the name of the classes of course)