How do I parse a formula? I have a string containing a mathematical formula formula like "144/12". How do I calculate its value?

David Cole

[Jo Desmet has created a forumla parser package called JMEP (or com.neemsoft.jmep). Write him at Jo_Desmet@yahoo.com to learn more. Maybe with enough input, he'll release it as open source... In the meantime, you can build on David Cole's simple calculator below.

- Alex Chaffee]

Heres a little sample program that should get you going. It only handles the basic operators '-','+','/','*' and doesn't handle parens (). Hope you find it useful.

public class MathParser
{
 private static final char[] validOperators = {'/','*','+','-'};

    private MathParser()
    {
         /* 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;

        int operatorLoc = findOperatorLocation(leftSide);
        if( operatorLoc > 0 && operatorLoc < leftSide.length()-1 )
        {
     leftResult = evaluate(leftSide.substring(0,operatorLoc),
            leftSide.charAt(operatorLoc),
                leftSide.substring(operatorLoc+1,leftSide.length()));
        }
  else
    {
     try
       {
             leftResult = Integer.parseInt(leftSide);
            }
            catch(Exception e)
            {
                throw new IllegalArgumentException("Invalid value found in portion of equation: "
                  + leftSide);
            }
        }

        operatorLoc = findOperatorLocation(rightSide);
        if( operatorLoc > 0 && operatorLoc < rightSide.length()-1 )
        {
            rightResult = evaluate(rightSide.substring(0,operatorLoc),
                    rightSide.charAt(operatorLoc),
                          rightSide.substring(operatorLoc+1,rightSide.length()));
        }
  else
    {
     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);
        return total;
    }

    private static int findOperatorLocation(String string)
    {
        int index = -1;
        for(int i = validOperators.length-1; i >= 0; i--)
        {
            index = string.indexOf(validOperators[i]);
            if(index >= 0)
             return index;
        }
        return index;
    }


    public static int processEquation(String equation)
      throws IllegalArgumentException
    {
          return evaluate(equation,'+',"0");
    }

    public static void main(String[] args)
    {
        String usage = "Usage: java MathParser equation
Where equation is a series"
         + " of integers separated by valid operators (+,-,/,*)";

        if(args.length < 1 || args[0].length() == 0)
         System.out.println(usage);

        String equation = args[0];
        try
        {
            int result = MathParser.processEquation(equation);
         System.out.println("The result of your equation ("
               + equation + ") is: " + result);
        }
        catch(IllegalArgumentException iae)
        {
            System.out.println(iae.getMessage() + "
" + usage);
        }
    }
}

Good Luck!