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(INTEGRATE) Convert a string to a Function

I am trying to integrate a math function 'func' which will be entered(typed) by the user, but I cannot figure out how to convert the string to a function which would be called by the integtal function.

double integral(double xLeft, double xRight, double func(double), int nDivs)
{
int i;
double area = 0.0;

double deltaX = (xRight - xLeft)/nDivs;

double midpointX;

midpointX = xLeft + deltaX/2.0;

for (i = 0; i < nDivs; i++)
{
area += func(midpointX) * deltaX;

midpointX += deltaX;

}

return area;
}

Thanks for any help

Electronics and Computer Hobbyist

arpan_1234

do u mean, how to convert string to double?

Rahul Saxena

Hi. what you need is something similar to callback functions? That can be done using delegates.

delegate double Func(double str);

public double realFunc(double val)
{
...
}

double integral(double xLeft, double xRight, Func func, int nDivs)
{
...
area += func(midpointX) * deltaX;
...
}

void Test()
{
integral(..., ..., new Func(realFunc), ...);
}

Bye.

daniel testa

No, he means he's got a string like "x+1", "x*x" or "sin(x)" and he wants to programmatically evaluate this for any given x.

I don't know a good way to do this. For simple maths syntax, this may be possible using Microsoft.CSharp.CSharpCodeProvider to parse the expression and generate a function.

Rup

Thanks ,

Yes I wanted to enter a string like : "x+1", "x*x" or "sin(x)".

I am trying the hint by "Rup" , but i want some more help regarding this.

Electronics and Computer Hobbyist

arpan_1234

I can't help you with that, sorry, I've never used that class myself -

but an alternative is to write an expression parser and calculator yourself. There's a few on Google: one here in VB

http://www.codeproject.com/vb/net/math_expression_evaluator.asp

which probably won't be too hard to integrate with your C# code (or port over to C#) or, if you're a bit more adventerous, there's a number on sourceforge.net written in Java or C you could try instead. (I searched for "expression".) Given you're going to be computing the function for very many values to approximate the integral you'd want an approach that parsed the text expression into a tree of operations or similar so that you could easily recompute it without having to parse the string each time.

Rup

Fallback to XPath:

class Program
{
static void Main(string[] args)
{
XPathExpression expr = XPathExpression.Compile("2+4+6");
XmlDocument xmlDoc = new XmlDocument();
XPathNavigator xPathNavigator = xmlDoc.CreateNavigator();
Console.WriteLine(xPathNavigator.Evaluate(expr));
}
}

xPathNavigator.Evaluate will give you the result of the string. You can change the variables either via a stringreplace before calling Compile or if you want to be more sophisticated you can implement your own xpath resolver that will get the values from somewhere else during execution.

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=365098&SiteID=1

John.Doe

Working with in memory compilation is also a nice idea, e.g.:

class Program
{
static Type PrepareCalcClass(string calcString)
{
// namespace CalcNS
CodeNamespace codeNamespace = new CodeNamespace("CalcNamespace");
// class Calc
CodeTypeDeclaration calcType = new CodeTypeDeclaration("CalcClass");
codeNamespace.Types.Add(calcType);
// public double Calc(double x) { return <calcString>; }
CodeMemberMethod method = new CodeMemberMethod();
method.Attributes = MemberAttributes.Public | MemberAttributes.Static;
method.Name = "Calc";
method.Parameters.Add(new CodeParameterDeclarationExpression(typeof(double), "x"));
method.ReturnType = new CodeTypeReference(typeof(double));
method.Statements.Add(new CodeMethodReturnStatement(new CodeSnippetExpression(calcString)));
calcType.Members.Add(method);

// compile
CodeCompileUnit compileUnit = new CodeCompileUnit();
compileUnit.Namespaces.Add(codeNamespace);
CodeDomProvider provider = CodeDomProvider.CreateProvider("CSharp");
CompilerParameters parameters = new CompilerParameters();
parameters.GenerateInMemory = true;
CompilerResults compilerResults = provider.CompileAssemblyFromDom(parameters, compileUnit);

// errors?
if ((compilerResults.Errors != null) &&
(compilerResults.Errors.Count > 0))
{
string errors = String.Empty;
foreach (string error in compilerResults.Errors)
errors += error + "\n";
throw new Exception("compilation failed\n" + errors);
}
Type type = compilerResults.CompiledAssembly.GetType("CalcNamespace.CalcClass");
return type;
}

static void Main(string[] args)
{
Type type = PrepareCalcClass("x*x");
MethodInfo calcMethodInfo = type.GetMethod("Calc", BindingFlags.Static|BindingFlags.Public);
for (int x = 0; x < 10; x++)
{
double result = (double)calcMethodInfo.Invoke(null, new object[] { (double)x });
Console.WriteLine(result);
}
}
}