Turn user input string into mathematical function

Question

For the purposes of what I want to do I need to take a user input as a string and convert it into an unevaluated function. For example, if the user input was \"x^2*sin(x)\"

Answer 1

Essentially, you need to implement your own syntax parser and convert input string into an Expression Tree.

Answer 2

I ended up using NCalc. I passed in the user's string expression, replaced the variables with the values I am evaluating at, then using the Evaluate method and parsing to a double.

private double Function(double t, double y)
    {
        NCalc.Expression expression = new NCalc.Expression(this.Expression);
        expression.Parameters["t"] = t;
        expression.Parameters["y"] = y;

        double value;

        double.TryParse(expression.Evaluate().ToString(), out value);

        return value;        
    }

For example, given the inputs t = .5 and y = 1 and the expression "4*y + Tan(2*t)", we would evaluate the string "4*1 + Tan(2*.5)" using NCalc.

It is not perfect, NCalc throws an exception if it cannot parse the user's string or it the datatypes of functions are different. I am working on polishing it.