Question

I have written a Java program which evaluates a mathematical expression from left to right (no precedence, just left to right). However, I’m not getting the desired output.

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);
        //}
    }
}

Here is the input I’m trying to use, and what I expect:

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

But I’m getting this 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)

I’m unable to make my program generic for any equation entered.
I appreciate any help you can provide.
Please note this is not a homework question.

Answer 1

Your findOperatorLoc is incorrect.

It automatically assumes that the operator is the second character.

index = string.indexOf(string.substring(1,2));

EDIT — A cleaner implementation of this might be like so. split your equation based on operators and then based on operands. You will be left with 2 arrays , one with all the operators and one with all operands.

            String[] aa = op.split("[*+-]");
            for ( String s : aa )
                    System.out.println(s);
            String[] bb = op.split("[0-9]");
            for ( String s : bb )
                    System.out.println(s);

            // Now loop through the operand array and apply the necessary oeprator in order
            for ( int i = 0 ; i < aa.length ; i++ ) {
                      int val = applyOperator(Integer.parseInt(aa[i]), Integer.parseInt(aa[i+1]), bb[i]);
            }

This is pseudocode only .. I will leave you to implement the applyOperator method