public static double Evaluate(string expression, double[] variables)
{
// split expression to separate tokens, which represent functions ans variables
string[] tokens = expression.Trim().Split(' ');
// arguments stack
Stack<double> arguments = new Stack<double>();
// walk through all tokens
foreach (string token in tokens)
{
// check for token type
if (char.IsDigit(token[0]))
{
// the token in numeric argument
arguments.Push(double.Parse(token));
}
else if (token[0] == '$')
{
// the token is variable
arguments.Push(variables[int.Parse(token.Substring(1))]);
}
else
{
// each function has at least one argument, so let's get the top one
// argument from stack
double v = (double)arguments.Pop();
// check for function
switch (token)
{
case "+": // addition
arguments.Push((double)arguments.Pop() + v);
break;
case "-": // subtraction
arguments.Push((double)arguments.Pop() - v);
break;
case "*": // multiplication
arguments.Push((double)arguments.Pop() * v);
break;
case "/": // division
arguments.Push((double)arguments.Pop() / v);
break;
case "sin": // sine
arguments.Push(System.Math.Sin(v));
break;
case "cos": // cosine
arguments.Push(System.Math.Cos(v));
break;
case "ln": // natural logarithm
arguments.Push(System.Math.Log(v));
break;
case "exp": // exponent
arguments.Push(System.Math.Exp(v));
break;
case "sqrt": // square root
arguments.Push(System.Math.Sqrt(v));
break;
default:
// throw exception informing about undefined function
throw new ArgumentException("Unsupported function: " + token);
}
}
}
// check stack size
if (arguments.Count != 1)
{
throw new ArgumentException("Incorrect expression.");
}
// return the only value from stack
return (double)arguments.Pop();
}
}
