[math] Throw exception when RootFinder is constructed with invalid algo.

When MathMore cannot be loaded, the tutorial exampleFunction.py used to
crash. Here, an exception is added thrown in the constructor, which can
be caught in the Python tutorial.

Author: hageboeck

math/mathcore/src/RootFinder.cxx

@@ -41,11 +41,15 @@ RootFinder::RootFinder(RootFinder::EType type) :
    fSolver(nullptr)
 {
    // constructor passing type (default is kBRENT)
-   SetMethod(type);
+   const bool success = SetMethod(type);
+   if (!success)
+      throw std::runtime_error("ROOT::Math::RootFinder: ROOT was built without the requested algorithm");
 }
 
 bool RootFinder::SetMethod(RootFinder::EType type)
 {
+   fSolver = nullptr;
+
    // set method - Use the plug-in manager if method is implemented in MathMore
    if ( type == RootFinder::kBRENT )
    {
@@ -125,16 +129,16 @@ bool RootFinder::SetMethod(RootFinder::EType type)
          return false;
       }
 
-      fSolver = reinterpret_cast<ROOT::Math::IRootFinderMethod *>( h->ExecPlugin(0) );
-      assert(fSolver != nullptr);
+      fSolver = reinterpret_cast<ROOT::Math::IRootFinderMethod *>(h->ExecPlugin(0));
    }
-   else {
+
+#endif
+
+   if (!fSolver) {
       MATH_ERROR_MSG("RootFinder::SetMethod","Error loading RootFinderMethod");
       return false;
    }
 
-#endif
-
    return true;
 }
 

tutorials/math/exampleFunction.py

@@ -69,25 +69,19 @@ def RosenbrockFunction(xx):
 
 def g(x): return 2 * x
 
-gradFunc = ROOT.Math.GradFunctor1D(f, g)
-
-#check if ROOT has mathmore
-prevLevel = ROOT.gErrorIgnoreLevel
-ROOT.gErrorIgnoreLevel=ROOT.kFatal
-ret = ROOT.gSystem.Load("libMathMore") 
-ROOT.gErrorIgnoreLevel=prevLevel
-if (ret < 0) :
-   print("ROOT has not Mathmore")
-   print("derivative value at x = 1", gradFunc.Derivative(1) )
-
-else :
+# GSL_Newton is part of ROOT's MathMore library, which is not always active.
+# Let's therefore run this in a try block:
+try:
+   gradFunc = ROOT.Math.GradFunctor1D(f, g)
    rf = ROOT.Math.RootFinder(ROOT.Math.RootFinder.kGSL_NEWTON)
    rf.SetFunction(gradFunc, 3)
    rf.Solve()
    value = rf.Root()
    print("Found root value x0 : f(x0) = 0  :  ", value)
    if (value != 1):
       print("Error finding a ROOT of function f(x)=x^2-1")
+except Exception as e:
+   print(e)
 
 
 print("\n\nUse GradFunctor for making a function object implementing f(x,y) and df(x,y)/dx and df(x,y)/dy")