evaluate an arithmetic expression stored in a string (C#)

Question

I'm working on a application in C# in which I want to calculate an arithmetic expression that is given as a string. So like I got a string:

string myExpr="4*(80+(5/2))+2";

And I want to calculate the outcome of the arithmetic expression. While in a language such as Javascript, PHP etc. you could just use Eval to do the trick this doesnt seem to be an option in C#.

I suppose it is possible to write a code to devide it into countless simple expressions, calculate them and add them together but this would take quite some time and I'm likely to have lots of troubles in my attempt to do so.

So... my question, Is there any 'simple' way to do this?

Answer 1

You could just call the JScript.NET eval function. Any .NET language can call into any other.

Answer 2

There's a javascript library you can reference, then just do something like:

var engine = VsaEngine.CreateEngine();
Eval.JScriptEvaluate(mySum, engine);

Edit; Library is Microsoft.JScript

Answer 3

Have you seen http://ncalc.codeplex.com ?

It's extensible, fast (e.g. has its own cache) enables you to provide custom functions and varaibles at run time by handling EvaluateFunction/EvaluateParameter events. Example expressions it can parse:

Expression e = new Expression("Round(Pow(Pi, 2) + Pow([Pi2], 2) + X, 2)");

  e.Parameters["Pi2"] = new Expression("Pi * Pi");
  e.Parameters["X"] = 10;

  e.EvaluateParameter += delegate(string name, ParameterArgs args)
    {
      if (name == "Pi")
      args.Result = 3.14;
    };

  Debug.Assert(117.07 == e.Evaluate());

It also handles unicode & many data type natively. It comes with an antler file if you want to change the grammer. There is also a fork which supports MEF to load new functions.

It also supports logical operators, date/time's strings and if statements.

Answer 4

I've used NCalc with great success. It's extremely flexible and allows for variables in your formulas. The formula you listed in your question could be evaluated this easily:

string myExpr = "4*(80+(5/2))+2";
decimal result = Convert.ToDecimal(new Expression(myExpr).Evaluate());

Answer 5

You need to implement an expression evaluator. It's fairly straightforward if you have the background, but it's not "simple". Eval in interpreted environments actually re-runs the language parser over the string; you need to emulate that operation, for the bits you care about, in your C# code.

Search for "expression evaluators" and "recursive descent parser" to get started.

Answer 6

If you have Bjarne Stroustrup's The C++ Programming Language, in Chapter 6 he explains step by step (in C++) how to do exactly what Chris Tavares suggests.

It's straightforward but a little heady if you're not familiar with the procedure.

Answer 7

I needed to do something similar for an undergrad projectand I found this

Reverse Polish Notation In C#

Tutorial and code to be extremely valuable.

It's pretty much just an implementation of converting the string to Reverse Polish Notation then evaluating it. It's extremely easy to use, understand and add new functions. Source code is included.

Answer 8

Try something like this:

int mySum = 4*(80+(5/2))+2;

var myStringSum = mySum.toString();