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Oct 30th, 2009

Infix to Postfix (HELP NEEDED)

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Hello,

I am having some difficulty in trying to figure out where I am going wrong in my program. I am trying to convert infix to postfix and from there I am evaluating the postfix. However, it is not compiling correctly. I've tried several things but nothing seems to work correctly for me.

Java Syntax (Toggle Plain Text)
import java.util.*;
import java.io.*;
 
public class calculator {
 
	private String infix;
	private String postfix = " ";
 
//	public calculator(String in) {
	//	input = in;
	//	int SIZE = input.length();
 
//	}
 
	public String toPostfix(String infix) {
 
		String expression;
		String postfix = " ";
 
		Stack operatorStack = new Stack();
 
		StringTokenizer s = new StringTokenizer(infix);
 
		while (s.hasMoreTokens()) {
 
			expression = s.nextToken();
			if (Character.isDigit(expression.charAt(0)))
				postfix = postfix + " " + (Integer.parseInt(expression));
			else if (expression.equals("("))
 
			{
				Character operator = new Character('(');
				operatorStack.push(operator);
			} else if (expression.equals(")"))
 
			{
				while (((Character) operatorStack.peek()).charValue() != '(') {
					postfix = postfix + " " + operatorStack.pop();
				}
				operatorStack.pop();
			} else
 
			{
				while (!operatorStack.isEmpty()
						&& !(operatorStack.peek()).equals("(")
						&& prec(expression.charAt(0)) <= prec(((Character) operatorStack
								.peek()).charValue()))
					postfix = postfix + " " + operatorStack.pop();
				Character operator = new Character(expression.charAt(0));
				operatorStack.push(operator);
			}
		}
		while (!operatorStack.isEmpty())
			postfix = postfix + " " + operatorStack.pop();
		return postfix;
	}
 
	private static int prec(char x) {
		if (x == '+' || x == '-')
			return 1;
		if (x == '*' || x == '/' || x == '%')
			return 2;
		return 0;
	}
 
	public  int evaluate(String postfix) {
		int a;
		int b;
		int result = 0;
 
		Stack<Integer> myStack = new Stack<Integer>();
 
		for (int i = 0; i < postfix.length(); i++) {
			char ch = postfix.charAt(i);
			if (ch >= '0' && ch <= '9') {
				myStack.push((int) (ch - '0'));
 
			} else {
				switch (ch) {
				case '+':
					a = myStack.pop();
					b = myStack.pop();
					result = a + b;
					myStack.push(result);
					break;
 
				case '-':
					a = myStack.pop();
					b = myStack.pop();
					result = a - b;
					myStack.push(result);
					break;
 
				case '*':
					a = myStack.pop();
					b = myStack.pop();
					result = a * b;
					myStack.push(result);
					break;
				case '/':
					a = myStack.pop();
					b = myStack.pop();
					result = a / b;
					myStack.push(result);
					break;
 
				}
				myStack.push(result);
 
			}
		}
		myStack.push(result);
		return result;
	}
}
import java.util.*;
import java.io.*;

public class calculator {

	private String infix;
	private String postfix = " ";

//	public calculator(String in) {
	//	input = in;
	//	int SIZE = input.length();

//	}

	public String toPostfix(String infix) {

		String expression;
		String postfix = " ";

		Stack operatorStack = new Stack();

		StringTokenizer s = new StringTokenizer(infix);

		while (s.hasMoreTokens()) {

			expression = s.nextToken();
			if (Character.isDigit(expression.charAt(0)))
				postfix = postfix + " " + (Integer.parseInt(expression));
			else if (expression.equals("("))

			{
				Character operator = new Character('(');
				operatorStack.push(operator);
			} else if (expression.equals(")"))

			{
				while (((Character) operatorStack.peek()).charValue() != '(') {
					postfix = postfix + " " + operatorStack.pop();
				}
				operatorStack.pop();
			} else

			{
				while (!operatorStack.isEmpty()
						&& !(operatorStack.peek()).equals("(")
						&& prec(expression.charAt(0)) <= prec(((Character) operatorStack
								.peek()).charValue()))
					postfix = postfix + " " + operatorStack.pop();
				Character operator = new Character(expression.charAt(0));
				operatorStack.push(operator);
			}
		}
		while (!operatorStack.isEmpty())
			postfix = postfix + " " + operatorStack.pop();
		return postfix;
	}

	private static int prec(char x) {
		if (x == '+' || x == '-')
			return 1;
		if (x == '*' || x == '/' || x == '%')
			return 2;
		return 0;
	}

	public  int evaluate(String postfix) {
		int a;
		int b;
		int result = 0;

		Stack<Integer> myStack = new Stack<Integer>();

		for (int i = 0; i < postfix.length(); i++) {
			char ch = postfix.charAt(i);
			if (ch >= '0' && ch <= '9') {
				myStack.push((int) (ch - '0'));

			} else {
				switch (ch) {
				case '+':
					a = myStack.pop();
					b = myStack.pop();
					result = a + b;
					myStack.push(result);
					break;

				case '-':
					a = myStack.pop();
					b = myStack.pop();
					result = a - b;
					myStack.push(result);
					break;

				case '*':
					a = myStack.pop();
					b = myStack.pop();
					result = a * b;
					myStack.push(result);
					break;
				case '/':
					a = myStack.pop();
					b = myStack.pop();
					result = a / b;
					myStack.push(result);
					break;

				}
				myStack.push(result);

			}
		}
		myStack.push(result);
		return result;
	}
}

annunfanwen

Reputation Points: 10

Solved Threads: 0

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since Oct 2009

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Oct 30th, 2009

Re: Infix to Postfix (HELP NEEDED)

properly write constructor
reply , what does mean parameter "String in" in contructor?
What information is transmitted to a class?

quuba

Reputation Points: 123

Solved Threads: 106

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since Nov 2008

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Oct 30th, 2009

Re: Infix to Postfix (HELP NEEDED)

Try searching the forum, this question gets posted all the time. In fact my first post ever was regarding converting infix to postfix using stack. Also, I know there is a code snppet for converting infix to postfix in java. Give it a try.

Grn Xtrm

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Solved Threads: 48

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Nov 3rd, 2009

Re: Infix to Postfix (HELP NEEDED)

Quote ...

I know there is a code snppet for converting infix to postfix in java

here is! , and annunfanwen is an author

method

Quote ...

public String toPostfix(String infix)

adds usefull spaces as separator L28,38,48,54

method

Quote ...

public int evaluate(String postfix)

don't know about it . Also can't collect digits together to form number

In addition little stack tracer

Java Syntax (Toggle Plain Text)
        Stack<Integer> myStack = new Stack<Integer>() {
 
            public Integer push(Integer i) {
                Integer j = super.push(i);
                System.out.println("push " + i);
                return j;
            }
 
            public Integer pop() {
                Integer i = super.pop();
                System.out.println("pop " + i);
                return i;
            }
        };
        Stack<Integer> myStack = new Stack<Integer>() {

            public Integer push(Integer i) {
                Integer j = super.push(i);
                System.out.println("push " + i);
                return j;
            }

            public Integer pop() {
                Integer i = super.pop();
                System.out.println("pop " + i);
                return i;
            }
        };

Last edited by quuba; Nov 3rd, 2009 at 6:22 am.

quuba

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Solved Threads: 106

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