# 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);
}

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!