Infix to postfix conversion in java

Question

I have a java class that converts infix expression to postfix. I have managed to make the class run without errors but it is not giving me the right output. The output is suppose to be in form :

InToPost: converting expressions from infix to postfix...
[A, +, B]
[A, +, B, *, C]
[A, +, B, *, C, +, D]
[(, A, +, B, ), *, C]
[(, A, +, B, ), /, (, C, -, D, )]
[(, (, A, +, B, ), *, C, -, (, D, -, E, ), )]
InToPost: emitting postfix expressions...
A B +
A B C * +
A B C * + D +
A B + C *
A B + C D - /
A B + C * D E - -

But my output is:

InToPost: converting expressions from infix to postfix... [A, +, B]
[A, +, B, *, C]
[A, +, B, *, C, +, D]
[(, A, +, B, ), *, C]
[(, A, +, B, ), /, (, C, -, D, )]
[(, (, A, +, B, ), , C, -, (, D, -, E, ), )]
InToPost: emitting postfix expressions... A+B
A+BC
A+B*C+D
(A+B)*C
(A+B)/(C-D)
((A+B)*C-(D-E))

The convert method is as follows:

private List<String> convert(List<String> tokens) {
        // YOUR IMPLEMENTATION HERE

        StackNode opstack = new StackNode();

        //List<Object> postfix = new ArrayList<Object>();

        //int sizeOfList=tokens.size();      
        String postfixString = " ";

        for (int index = 0; index < tokens.size(); ++index) {
            String chValue = tokens.get(index);
            if (chValue == "(") {
                opstack.push("(");
            } else if (chValue == ")") {
                String oper = String.valueOf(opstack.top());
                while (!(oper.equals("(")) && !(opstack.empty())) {
                    postfixString += oper;
                    opstack.pop();
                    oper = String.valueOf(opstack.top());
                }
                opstack.pop();
            } else if (chValue == "+" || chValue == "-") {
                //Stack is empty
                if (opstack.empty()) {
                    opstack.push(chValue);
                    //current Stack is not empty
                } else {
                    String oper = String.valueOf(opstack.top());
                    while (!(opstack.empty() || oper.equals(new String("(")) || oper.equals(new String(")")))) {
                        opstack.pop();
                        postfixString += oper;
                    }
                    opstack.push(chValue);
                }
            } else if (chValue == "*" || chValue == "/") {
                if (opstack.empty()) {
                    opstack.push(chValue);
                } else {
                    String oper = String.valueOf(opstack.top());
                    while (!oper.equals(new String("+")) && !oper.equals(new String("-")) && !opstack.empty()) {
                        opstack.pop();
                        postfixString += oper;
                    }
                    opstack.push(chValue);
                }
            } else {
                postfixString += chValue;
            }
        }
        while (!opstack.empty()) {
            String oper = String.valueOf(opstack.top());
            if (!oper.equals(new String("("))) {
                opstack.pop();
                postfixString += oper;
            }
        }

String[] strValues = postfixString.split(",");        
        ArrayList<String> postfix = new ArrayList<String>(Arrays.asList(strValues));
       return postfix;
    }

The whole class code is:

package sr.stu;
import sr.cs.Stack;
import sr.stu.StackNode;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Scanner;

/**
 * A program that converts an infix expression to postfix.<br>
 * <br>
 *  Usage: java InToPost filename<br>
 * <br>
 * For example, prog1.in, containing infix expressions (one per line):<br>
 * <br>
 *  A + B<br>
 *  A + B * C<br>
 *  A + B * C + D<br>
 *  ( A + B ) * C<br>
 *  ( A + B ) / ( C - D )<br>
 *  ( ( A + B ) * C - ( D - E ) )<br>
 * <br>
 * When run: java InToPost prog1.in:<br>
 * <br>
 *  InToPost: converting expressions from infix to postfix...<br>
 *  [A, +, B]<br>
 *  [A, +, B, *, C]<br>
 *  [A, +, B, *, C, +, D]<br>
 *  [(, A, +, B, ), *, C]<br>
 *  [(, A, +, B, ), /, (, C, -, D, )]<br>
 *  [(, (, A, +, B, ), *, C, -, (, D, -, E, ), )]<br>
 *  InToPost: emitting postfix expressions...<br>
 *  A B +<br>
 *  A B C * +<br>
 *  A B C * + D +<br>
 *  A B + C *<br>
 *  A B + C D - /<br>
 *  A B + C * D E - -<br>
 * <br>

 */
public class InToPost{
    /** The add operator */
    public final static String ADD = "+";
    /** The subtract operator */
    public final static String SUBTRACT = "-";
    /** The multiply operator */
    public final static String MULTIPLY = "*";
    /** The divide operator */
    public final static String DIVIDE = "/";
    /** The open parentheses operator */
    public final static String OPEN_PAREN = "(";
    /** The close parentheses operator */
    public final static String CLOSE_PAREN = ")";

    /** The list of converted postfix expressions */
    private List<List<String>> expressions;
    /** The name of the infix source file */
    private String srcFile;
    /** A dictionary that maps operators (strings) to their precedence level
     * (integer) from highest (3) to lowest (1):
     *      *, /: 3
     *      +, -: 2
     *      (: 1
     */
    private Map<String, Integer> precedence;

    /**
     * Create a new IntoPost object.
     * @param filename The name of the infix source file
     */
    public InToPost(String filename) {
        this.expressions = new LinkedList<>();
        this.srcFile = filename;

        /** populate the precedence map */
        this.precedence = new HashMap<>();
        this.precedence.put(MULTIPLY, 3);
        this.precedence.put(DIVIDE, 3);
        this.precedence.put(ADD, 2);
        this.precedence.put(SUBTRACT, 2);
        this.precedence.put(OPEN_PAREN, 1);
    }

    /**
     * A private utility method that can be used by the private convert
     * method when dealing with the precedence of math operators on
     * the stack.  Indicates whether the top token on stack has greater
     * than or equal precedence than the current token.  For example:
     *  greaterEqualPrecedence("+", "*") -> false
     *  greaterEqualPrecedence("*", "+") -> true
     *  greaterEqualPrecedence("+", "-") -> true
     *  greaterEqualPrecedence("(", "/") -> false
     *
     * @param token the current math token, e.g. "+"
     * @param top the top token to compare to, e.g. "*"
     * @return true if top has greater than or equal precedence than
     *  token, false otherwise
     */
    private boolean greaterEqualPrecedence(String top, String token) {
        return this.precedence.get(top) >= this.precedence.get(token);
    }

    /**
     * A private helper conversion function that takes a list of tokens
     * (strings) in infix form and converts them to a list of tokens
     * (strings) in postfix form.
     *
     *  For example:
     *      tokens = ["A", "+", "B"]
     *      postfix = ["A", "B", "+"]
     *
     *  Note, this method must use either StackList<T> or StackNode<T>
     *  to represent the stack structure in the conversion algorithm.
     *
     * @param tokens the list of tokens (strings) in infix form
     * @return a new list of tokens (strings) in postfix form
     */
    private List<String> convert(List<String> tokens) {
        // YOUR IMPLEMENTATION HERE

        StackNode opstack = new StackNode();

        //List<Object> postfix = new ArrayList<Object>();

        //int sizeOfList=tokens.size();      
        String postfixString = " ";

        for (int index = 0; index < tokens.size(); ++index) {
            String chValue = tokens.get(index);
            if (chValue == "(") {
                opstack.push("(");
            } else if (chValue == ")") {
                String oper = String.valueOf(opstack.top());
                while (!(oper.equals("(")) && !(opstack.empty())) {
                    postfixString += oper;
                    opstack.pop();
                    oper = String.valueOf(opstack.top());
                }
                opstack.pop();
            } else if (chValue == "+" || chValue == "-") {
                //Stack is empty
                if (opstack.empty()) {
                    opstack.push(chValue);
                    //current Stack is not empty
                } else {
                    String oper = String.valueOf(opstack.top());
                    while (!(opstack.empty() || oper.equals(new String("(")) || oper.equals(new String(")")))) {
                        opstack.pop();
                        postfixString += oper;
                    }
                    opstack.push(chValue);
                }
            } else if (chValue == "*" || chValue == "/") {
                if (opstack.empty()) {
                    opstack.push(chValue);
                } else {
                    String oper = String.valueOf(opstack.top());
                    while (!oper.equals(new String("+")) && !oper.equals(new String("-")) && !opstack.empty()) {
                        opstack.pop();
                        postfixString += oper;
                    }
                    opstack.push(chValue);
                }
            } else {
                postfixString += chValue;
            }
        }
        while (!opstack.empty()) {
            String oper = String.valueOf(opstack.top());
            if (!oper.equals(new String("("))) {
                opstack.pop();
                postfixString += oper;
            }
        }

String[] strValues = postfixString.split(",");        
        ArrayList<String> postfix = new ArrayList<String>(Arrays.asList(strValues));
       return postfix;
    }

    /**
     * The public conversion function that converts all infix expressions
     * in the source file (one per line), into postfix expressions.
     * @throws FileNotFoundException if the file is not found
     */
    public void convert() throws FileNotFoundException {
        Scanner in = new Scanner(new File(this.srcFile));
        while (in.hasNext()) {
            // convert the line into a list of strings, e.g.
            //      line = "A B +"
            //      tokens = ["A", "B", "+"]
            List<String> tokens = Arrays.asList(in.nextLine().split(" "));
            // if the line isn't empty, convert it to postfix and add it to
            // the list of expressions
            if (tokens.size() > 0) {
                System.out.println(tokens);
                this.expressions.add(convert(tokens));
            }
        }
    }

    /**
     * Display the converted postfix expressions.
     */
    public void emit() {
        for (List<String> expression : this.expressions) {
            for (String token : expression) {
                System.out.print(token + " ");
            }
            System.out.println();
        }
    }

    /**
     * The main program takes the source file of infix expressions, converts
     * them to postfix, and displays them.
     * @param args command line arguments
     * @throws FileNotFoundException if the source file is not found
     */
    public static void main(String[] args) throws FileNotFoundException {
        if (args.length != 1) {
            System.out.println("Usage: java InToPost.py source-file.in");
            return;
        }

        InToPost inToPost = new InToPost(args[0]);
        System.out.println("InToPost: converting expressions from infix to postfix...");
        inToPost.convert();
        System.out.println("InToPost: emitting postfix expressions...");
        inToPost.emit();
    }
}

Answer 1

Here is a possibility. You could analyze this code and find the error(s) in yours.

public class InToPost {
   private Stack theStack;
   private String input;
   private String output = "";
   public InToPost(String in) {
      input = in;
      int stackSize = input.length();
      theStack = new Stack(stackSize);
   }
   public String doTrans() {
      for (int j = 0; j < input.length(); j++) {
         char ch = input.charAt(j);
         switch (ch) {
            case '+': 
            case '-':
            gotOper(ch, 1); 
            break; 
            case '*': 
            case '/':
            gotOper(ch, 2); 
            break; 
            case '(': 
            theStack.push(ch);
            break;
            case ')': 
            gotParen(ch); 
            break;
            default: 
            output = output + ch; 
            break;
         }
      }
      while (!theStack.isEmpty()) {
         output = output + theStack.pop();
      }
      System.out.println(output);
      return output; 
   }
   public void gotOper(char opThis, int prec1) {
      while (!theStack.isEmpty()) {
         char opTop = theStack.pop();
         if (opTop == '(') {
            theStack.push(opTop);
            break;
         }
         else {
            int prec2;
            if (opTop == '+' || opTop == '-')
            prec2 = 1;
            else
            prec2 = 2;
            if (prec2 < prec1) { 
               theStack.push(opTop);
               break;
            }
            else
            output = output + opTop;
         }
      }
      theStack.push(opThis);
   }
   public void gotParen(char ch){ 
      while (!theStack.isEmpty()) {
         char chx = theStack.pop();
         if (chx == '(') 
         break; 
         else
         output = output + chx; 
      }
   }
   public static void main(String[] args) 
   throws IOException {
      String input = "1+2*4/5-7+3/6";
      String output;
      InToPost theTrans = new InToPost(input);
      output = theTrans.doTrans(); 
      System.out.println("Postfix is " + output + '\n');
   }
   class Stack {
      private int maxSize;
      private char[] stackArray;
      private int top;
      public Stack(int max) {
         maxSize = max;
         stackArray = new char[maxSize];
         top = -1;
      }
      public void push(char j) {
         stackArray[++top] = j;
      }
      public char pop() {
         return stackArray[top--];
      }
      public char peek() {
         return stackArray[top];
      }
      public boolean isEmpty() {
         return (top == -1);
     }
   }
}

Source: http://www.tutorialspoint.com/javaexamples/data_intopost.htm

Answer 2

you can checkout my code. Here, infix to postfix conversion is performed by following all rules of Precedence and Associativity of operators.

Some rules for conversion are:

1. Print operands as they arrive.
2. If stack is empty or contains a left parenthesis on top, push the incoming operator on stack.
3. If incoming symbol is '(' push it onto the stack.
4. If the incoming symbol is ')' pop the stack and print the operators unit left '(' is encountered; pop '(' as well but do not add it to result.
5. If the incoming symbol has higher precedence than the top of stack, push it onto the stack.
6. If the incoming symbol has lower precedence than the top of the stack, pop and print the top.Then test the incoming operator against the new top of stack.
7. If the incoming symbol has equal precedence with the top of the stack, use Associativity rule.
8. Associativity rule, If associativity is L-R, then pop and print the top of stack and push the incoming operator.
9. Associativity rule, If associativity is R-L, then push the incoming operator on to stack.
10. At the end of expression, pop and print all operator of stack.

For more info refer: Coding-Problems

package com.stack.problems;

import java.util.Arrays;
import java.util.List;
import java.util.Stack;

public class InfixToPostfixConversionUsingStack {

    static int precedence(Character ch) {

        switch (ch) {
            case '+':
            case '-':
                return 1;
            case '*':
            case '/':
                return 2;
            case '^':
                return 3;

        }
        return -1;
    }

    static boolean associativityForEqualPrecedence(char ch1, char ch2) {

        /*  Here, true represent L-R associativity and vice-versa.
         * */
        List<Character> charList = Arrays.asList('+', '-', '*', '/');
        return charList.contains(ch1) || charList.contains(ch2);    //Rule 8,9
    }

    private static String infixToPostfix(String exp) {

        if (exp == null || exp.isEmpty())
            return exp;
        Stack<Character> stack = new Stack<>();
        StringBuilder result = new StringBuilder();

        for (int index = 0; index < exp.length(); index++) {
            char ch = exp.charAt(index);
            if (Character.isLetterOrDigit(ch)) {
                result.append(ch);      //Rule 1
            } else if (ch == '(') {     //Rule 3
                stack.push(ch);
            } else if (ch == ')') {     //Rule 4
                while (!stack.isEmpty() && stack.peek() != '(') {
                    result.append(stack.pop());
                }
                if (!stack.isEmpty())
                    stack.pop();
                else { //For invalid expression...
                    System.out.println("Mismatched parenthesis....");
                    return null;
                }
            } else if (!stack.isEmpty()) {
                if (precedence(ch) > precedence(stack.peek())) {        //Rule 5
                    stack.push(ch);
                } else if (precedence(ch) < precedence(stack.peek())) {     //Rule 6

                    while (!stack.isEmpty() && precedence(ch) <= precedence(stack.peek())) {
                        result.append(stack.pop());
                    }
                    stack.push(ch);
                } else {        //Rule 7
                    //Here operators are having equal precedence therefore associativity rules are applied...
                    if (associativityForEqualPrecedence(ch, stack.peek())) {
                        result.append(stack.pop());
                    }
                    stack.push(ch);
                }
            } else if (stack.isEmpty() || stack.peek() == '(') {        //Rule 2
                stack.push(ch);
            }
        }
        //For handling invalid expression...
        if (!stack.isEmpty() && stack.search('(') != -1) {
            System.out.println("Mismatched parenthesis...");
            return null;
        }
        if (!stack.isEmpty()) {     //Rule 10
            while (!stack.isEmpty()) {
                result.append(stack.pop());
            }
        }
        return result.toString();
    }

    public static void main(String[] args) {

        String[] exprArray = {
                "K+L-M*N+(O^P)*W/U/V*T+Q^J^A",
                "a+b*(c^d-e)^(f+g*h)-i",
                "((a+b-c)*d^e^f)/g",
                "1+2+3*5+(6+7)+8",
                "((a-b)/c)",
                "a*b-a*b+a*b",
                "(a*b"};

        for (int index = 0; index < exprArray.length; index++) {
            System.out.println("Infix Expression: " + exprArray[index]);
            System.out.println("Postfix Expression: " + infixToPostfix(exprArray[index]));
        }
    }
}