VB, C# how to evaluate a mathematical expression as a string written in VB or C# style

Replies to This Discussion

Permalink Reply by Thomas on August 4, 2014 at 12:36pm

You can write your own math parser (complex) or use an existing one (e.g. NCalc).

Another possibility is to compile the expression at runtime (Python, C#/VB). Keep in mind that the user can enter any kind of code.

Greetings!

Permalink Reply by Damon on August 4, 2014 at 1:06pm

May I ask a question? Why do you need to write your own math parser when there is already one in GH? I'm not asking to be difficult, but because I am curious.

Permalink Reply by Thomas on August 4, 2014 at 1:46pm

Because 2^2^3 is NOT 64

You don't have to write your own parser and of course you still can use the parser provided by Grasshopper:

var parser = new Grasshopper.Kernel.Expressions.GH_ExpressionParser();
parser.AddVariable("x", x);
parser.AddVariable("y", y);
parser.AddVariable("z", z);
result = parser.Evaluate(f);

Just keep in mind that the power function has this little bug.

Cheers!

Attachments:

• evalCSharp.gh, 5 KB

Permalink Reply by Pirouz Nourian on August 4, 2014 at 2:18pm

Thanks for your prompt complete response!!! 

Did it with a bit of optimization. In case anybody needs in future:

Dim vals(P.count-1) As Double
Dim CorrectExpression = Grasshopper.Kernel.Expressions.GH_ExpressionSyntaxWriter.RewriteAll(F)
Dim Expr As New Grasshopper.Kernel.Expressions.GH_ExpressionParser()
Expr.CacheSymbols(CorrectExpression)
Dim k As Integer
For Each v As point3d In P
Expr.ClearVariables
Expr.AddVariable("x", V.x)
Expr.AddVariable("y", V.y)
Expr.AddVariable("z", V.z)
vals(k) = Expr.Evaluate(Expr.CachedSymbols())._Double
k += 1
Next
E = Vals

Cheers, Pirouz