Post Expression help

Question

does anyone know any sites that can give me the source code for PostFix expression?

Answer 1

Ok, here is one that I had to do for school. It may have different data structure files because I had to use the library that was from my text book. Hope this helps.
import java.lang.Math;
import javax.swing.*;
import java.io.*;
import ds.util.ALStack;
import java.util.StringTokenizer;
import java.text.DecimalFormat;
public class PostFix
      * This program will read in a string from prompt the user for a file
      * that they would like to read into the program.  If the file is
      * not there than an error will be displayed and the user will
      * be allowed to try again.  If correct, the program will use the
      * file and evaluate the postfix expressions.  It will give error
      * feedback or the correct answer.  It will be displayed by creating
      * a separate text file with the correct answers and error feedback.
     public static void main(String[] args) throws IOException, Exception
          // used to keep track of the file name and equations
          String fileName, outputFileName= "", tempEquation;
          // used to make sure the file is valid in the folder
          boolean goodFile = false;
          // loops until good file is found
          while(!goodFile)
               //prompts the user for file name
               fileName = JOptionPane.showInputDialog("Enter the file name: ");
               try{
                    // reads in the file name and lines
                    FileReader freader = new FileReader(fileName);
                    // Checks for the file name and makes the results consistent
                    if(fileName.toUpperCase().equals("POSTFIX1.TXT"))
                         outputFileName = "Result1.txt";
                    if(fileName.toUpperCase().equals("POSTFIX2.TXT"))
                         outputFileName = "Result2.txt";               
                    if(fileName.toUpperCase().equals("POSTFIX3.TXT"))
                         outputFileName = "Result3.txt";
                    FileWriter fwriter = new FileWriter(outputFileName);
                    PrintWriter outputFile = new PrintWriter(fwriter);
                    BufferedReader inputFile = new BufferedReader(freader);
                    tempEquation = inputFile.readLine();
                    goodFile = true;
                    // loops through each file line (equation)
                    while(tempEquation != null)
                         System.out.println(Calculation(tempEquation));
                         outputFile.println(Calculation(tempEquation));
                         tempEquation = inputFile.readLine();
                    outputFile.close();
               // catches an invalid file name
               catch(IOException e)
                    JOptionPane.showMessageDialog(null, "ERROR");
      * This method will take a string (s) and tokenize it using the delimiters
      * ((space) * / ^).  If the operator is found than the method will
      * pop() two numbers and evaluate the sub expression then push() it
      * back into the stack.  If the methods finds a number it will simply
      * just push() it onto the stack.  After the loop the method pops the
      * last remaining item in the stack and returns the answer.  If the stack
      * recognizes an operator and the stack size is less than two at any time
      * then it will report error that there is not enough operands.  If the
      * stack does not recognize and operators and cannot parse the token as
      * a double than it will report that there is an illegal operand/operator.   
     public static String Calculation(String s)
          // create the stack
          ALStack <Double> theStack = new ALStack <Double>();
          // create the variables for calculation
          double operand1, operand2, answer = 0.0;
          // create token to be read
          String token = "";
          // create tokenizer and delimiters
          StringTokenizer strTokenizer = new StringTokenizer(s, " */^", true);
          // decimal format for proper format.  (Three decimal places)
          DecimalFormat formatter = new DecimalFormat("#,##0.000");
          // loops through until there is no more tokens
          while(strTokenizer.hasMoreTokens())
                   token = strTokenizer.nextToken();
                    if(token.equals("+"))
                         if(theStack.size() < 2)
                              return s + " = error: missing operand";
                         operand1 = theStack.pop();
                         operand2 = theStack.pop();
                         answer = operand2 + operand1;
                         theStack.push(answer);
                    else if(token.equals("-"))
                         if(theStack.size() < 2)
                              return s + " = error: missing operand";
                         operand1 = theStack.pop();
                         operand2 = theStack.pop();
                         answer = operand2 - operand1;
                         theStack.push(answer);
                    else if(token.equals("*"))
                         if(theStack.size() < 2)
                              return s + " = error: missing operand";
                         operand1 = theStack.pop();
                         operand2 = theStack.pop();
                         answer = operand2 * operand1;
                         theStack.push(answer);
                    else if(token.equals("/"))
                         if(theStack.size() < 2)
                              return s + " = error: missing operand";
                         operand1 = theStack.pop();
                         operand2 = theStack.pop();
                         if(operand1 == 0)
                              return s + " = error: division by zero";
                         answer = operand2 / operand1;
                         theStack.push(answer);
                    else if(token.equals("^"))
                         if(theStack.size() < 2)
                              return s + " = error: missing operand";
                         operand1 = theStack.pop();
                         operand2 = theStack.pop();
                         answer = Math.pow(operand2, operand1);
                         theStack.push(answer);
                    // if white space than nothing happens
                    else if(token.equals(" "))
                    else
                         try
                              theStack.push(Double.parseDouble(token));
                         catch(Exception e)
                              return s + " = error: illegal operand";
          // returns error because only the answer should be left
          if(theStack.size() > 1)
               return s + " = error: too many operands";
          // returns a correct answer following the original equation
          return s + " = " + formatter.format(theStack.pop());
}