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( Math.Sin( v ) );
break;
case "cos": // cosine
arguments.Push( Math.Cos( v ) );
break;
case "ln": // natural logarithm
arguments.Push( Math.Log( v ) );
break;
case "exp": // exponent
arguments.Push( Math.Exp( v ) );
break;
case "sqrt": // square root
arguments.Push( 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( );
}
}
