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Thread: Infix to Postfix Conversion
 01022013, 09:32 PM #1Member
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Infix to Postfix Conversion
I'm trying to convert a given infix expression into postfix. Right now I'm trying to get a simple test case working. So far it just accepts integer inputs, then converts the expression into postfix notation and stores it in an ArrayList for simplifying later. I have the simplification working but my conversion does something weird. If I take the expression 4*(2+3), it should come out "4 2 3 + *" but my converter is throwing the left (opening) parenthesis into the equation so it looks like "4 2 3 + ( *" and I can't figure out why. Here's the code I'm using:
Java Code:import java.util.ArrayList; import java.util.Scanner; import java.util.Stack; public class Converter { public static void main(String[ ] args) { Scanner in = new Scanner(System.in); System.out.println("Enter an infix expression"); String input = in.nextLine(); ArrayList<String> postfixEquation = new ArrayList<String>(); Stack<String> stack = new Stack<String>(); String curNum = ""; String curToken = ""; int curTokenPrecedence = 0; for(int i = 0; i < input.length(); i++) { curToken = input.substring(i, i+1); if(isInteger(curToken)) //If the current token is a number curNum += curToken; //Add it to the curNumber string else //Not a number { if(curNum != "") //If there is a number in curNum { postfixEquation.add(curNum); //Add the current number to the postfix string curNum = ""; //Reset curNum } curTokenPrecedence = getPrecedence(curToken); //Get the current token's precedence //If there is no number on the stack or the token's operator is greater than the first operator on the stack if(stack.empty()  curTokenPrecedence > getPrecedence(stack.peek())  curToken.equals("(")) stack.push(curToken); else //Current token's precedence less than token on the stack { //Until there is no operator on the stack or an operator with lower precedence while(!stack.empty() && curTokenPrecedence <= getPrecedence(stack.peek())) postfixEquation.add(stack.pop()); //Pop operators off the stack stack.push(curToken); } } if(curToken.equals(")")) { stack.pop(); //Discard the right parenthesis while(!stack.empty()) { if(stack.peek().equals("(")) { stack.pop(); break; } postfixEquation.add(stack.pop()); } } } if(curNum != "") postfixEquation.add(curNum); //Pop remaining operators off the stack while(!stack.empty()) postfixEquation.add(stack.pop()); for(int i = 0; i < postfixEquation.size(); i++) System.out.println(postfixEquation.get(i)); in.close(); } //Returns whether the given string is an integer or an operator public static boolean isInteger(String num) { try { Integer.parseInt(num); return true; //Number } catch(NumberFormatException e) { return false; //Operator } } //Returns precedence for the given operator public static int getPrecedence(String operator) { int precedence = 0; switch(operator) { case "!": precedence = 4; case "^": precedence = 3; break; case "*": case "/": precedence = 2; break; case "+": case "": precedence = 1; break; case "(": precedence = 0; break; } return precedence; } }
 01022013, 09:39 PM #2
Re: Infix to Postfix Conversion
Have you stepped through this with a debugger, or at least added some print statements, to figure out what's going on? Debugging your code is an important lesson that every programmer has to learn.
Hint: Don't use == or != with Strings. Do a search on this forum or on google for an explanation of what to use instead, and why.How to Ask Questions the Smart Way
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 01032013, 12:09 AM #3Member
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Re: Infix to Postfix Conversion
I had been running through this section of code:
Java Code:if(curToken.equals(")")) { stack.pop(); //Discard the right parenthesis while(!stack.empty()) { if(stack.peek().equals("(")) { stack.pop(); break; } postfixEquation.add(stack.pop()); } }
Java Code:while(!stack.empty() && curTokenPrecedence <= getPrecedence(stack.peek())) postfixEquation.add(stack.pop()); //Pop operators off the stack
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