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Thread: Java:Evaluation of Mathematical Expression only from left to right only.

  1. #1
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    Default Java:Evaluation of Mathematical Expression only from left to right only.

    I have written a Java program which evaluates a mathematical expression from left to right only.however im stuck up with the output.Im not getting the desired output.
    Please Note: There is no precedence in operators(evaluation should be from LEFT->RIGHT.)

    /*
     * Created on Jul 10, 2011
     *
     * To change the template for this generated file go to
     * Window>Preferences>Java>Code Generation>Code and Comments
     */
    package com.comcast;
    import java.util.*;
    public class Evaluation
    {
        private static final char[] validOperators = {'/','*','+','-'};
     
    	private Evaluation()
    	{
    		 /* Using a private contructor to prevent instantiation
    			 Using class as a simple static utility class
    		 */
    	}
     
    	private static int evaluate(String leftSide, char oper, String rightSide)
    	 throws IllegalArgumentException
    	{
    		System.out.println("Evaluating: " + leftSide +  " (" + oper + ") " + rightSide);
    		int total = 0;
    		int leftResult = 0;
    		int rightResult = 0;
    		String originalString =leftSide;
    		int operatorLoc  = findOperatorLocation(leftSide);
    		leftSide = leftSide.substring(0,operatorLoc);
    		rightSide = originalString.substring(operatorLoc+1,operatorLoc+2);
    		String remainingString = originalString.substring(operatorLoc+2,originalString.length());
     
     
    		System.out.println("leftSide -->"+leftSide);
    		System.out.println("rightSide -->"+rightSide);
    		System.out.println("remainingString --->"+remainingString);
     
    	 try{
    			 leftResult = Integer.parseInt(leftSide);
    		}catch(Exception e){
    				throw new IllegalArgumentException("Invalid value found in portion of equation: "
    				  + leftSide);
    	    }
     
     
    	 try{
    			 rightResult = Integer.parseInt(rightSide);
    	   }catch(Exception e){
    				throw new IllegalArgumentException("Invalid value found in portion of equation: "
    				  + rightSide);
    	   }
     
     
    		System.out.println("Getting result of: " + leftResult + " " + oper + " " + rightResult);
    		switch(oper)
    		{
    			case '/':
    			 total = leftResult / rightResult; break;
    			case '*':
    			 total = leftResult * rightResult; break;
    			case '+':
    			 total = leftResult + rightResult; break;
    			case '-':
    			 total = leftResult - rightResult; break;
    			default:
    			 throw new IllegalArgumentException("Unknown operator.");
    		}
    		System.out.println("Returning a result of: " + total);
    		String totally = String.valueOf(total)+remainingString;
    		return evaluate(totally,findCharacter(totally),remainingString);
    	}
     
    	private static int findOperatorLocation(String string)
    		{
    			int index = -1;			
    				index = string.indexOf(string.substring(1,2));			    
    				if(index >= 0){				
    				 return index;
    				}
    			return index;
    		}
     
    	private static char findCharacter(String string){
     
    		char c='\u0000';  
    		int index = -1;			
    						index = string.indexOf(string.substring(1,2));
    						if(index >= 0){				
    						 c = string.charAt(index);
    						 return c;
    						} 		 			 	
    		 return c;	
    	}
     
    	public static int processEquation(String equation)
    	  throws IllegalArgumentException
    	{
    		  return evaluate(equation,'+',"0");
    	}
     
    	public static void main(String[] args)
    	{
    		//String usage = "Usage: java MathParser equation\nWhere equation is a series"
    		// + " of integers separated by valid operators (+,-,/,*)";
     
    		//if(args.length < 1 || args[0].length() == 0)
    		// System.out.println(usage);
    		Scanner input = new Scanner(System.in); 
    		System.out.print("Enter the equation to be evaluated ");
     
    		String equation = (String)input.next();
    			int result = Evaluation.processEquation(equation);
    		 System.out.println("The result of your equation ("
    			   + equation + ") is: " + result);
     
    		//catch(IllegalArgumentException iae)
    		//{
    		//	System.out.println(iae.getMessage() + "\n" + usage);
    		//}
    	}
    }


    Expression Input:3+5*2-5 =>3+5*2-5 =>8*2-5 =>16-5 =>Expected Output :11

    im getting as below output Enter the equation to be evaluated 3+5*2-5
    Evaluating: 3+5*2-5 (+) 0
    leftSide -->3
    rightSide -->5
    remainingString --->*2-5
    Getting result of: 3 + 5
    Returning a result of: 8
    Evaluating: 8*2-5 ( * ) *2-5
    leftSide -->8
    rightSide -->2
    remainingString --->-5
    Getting result of: 8 * 2
    Returning a result of: 16
    Evaluating: 16-5 (6) -5
    leftSide -->1
    rightSide -->-
    remainingString --->5
    Exception in thread "main" java.lang.IllegalArgumentException: Invalid value found in portion of equation: -
    at Evaluation.evaluate(Evaluation.java:49)
    at Evaluation.evaluate(Evaluation.java:70)
    at Evaluation.evaluate(Evaluation.java:70)
    at Evaluation.processEquation(Evaluation.java:98)
    at Evaluation.main(Evaluation.java:112)

    Im unable to make my program generic for any equation entered.Help provided will be appreciated.Please note this is not a homework question.
    Need help !!!!!Code fix required for the above program


  2. #2
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    Default Re: Java:Evaluation of Mathematical Expression only from left to right only.

    You must have a logic error or a bad expression.
    Have you tried more debugging aids by adding more printlns to show how the variables change?

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    Unhappy Re: Java:Evaluation of Mathematical Expression only from left to right only.

    The problem is in the method operatorLoc where im hardcoding the subString(1,2) to find location of operator,when i encounter 16-5,it returns a wrong location for operator.which is (6) instead of (-).

    Need help in resolving this ? Please provide the bug fix for the same.

  4. #4
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    Default Re: Java:Evaluation of Mathematical Expression only from left to right only.

    operatorLoc where im hardcoding the subString(1,2) to find location of operator,when i encounter 16-5,it returns a wrong location for operator.which is (6) instead of (-).
    Can you explain what this code is supposed to do:
    index = string.indexOf(string.substring(1,2));
    Also what different values will it return?

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    Default Re: Java:Evaluation of Mathematical Expression only from left to right only.

    Quote Originally Posted by deepakl_2000 View Post
    The problem is in the method operatorLoc where im hardcoding the subString(1,2)
    So don't do that. You can either split your String up so that 2 or more digit numbers stay as a whole number or parse your equation by reading until you reach an operator. Everything upto that point must be the operand.
    Improving the world one idiot at a time!

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    Default Re: Java:Evaluation of Mathematical Expression only from left to right only.

    Hi Junky,Can you provide the code fix for my code.??

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    Default Re: Java:Evaluation of Mathematical Expression only from left to right only.

    No. It is your job to write it not mine.
    Improving the world one idiot at a time!

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    Default Re: Java:Evaluation of Mathematical Expression only from left to right only.

    Look at post#4 again. Can you describe what that code is supposed to do?
    The way it is coded, it will always set index to 1. Unless the string is too short.

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