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problem with using external functions

  • To: mathgroup at
  • Subject: [mg91547] problem with using external functions
  • From: Valery Yundin <yundin at>
  • Date: Wed, 27 Aug 2008 06:40:46 -0400 (EDT)

Dear MathGroup,

I've a problem with using external functions defined in MathLink app.
I guess the problem is in the evaluation order. I'm giving more 
details below.

This is the function template:
    :Function: my_xfxM
    :Pattern: xfxM[nset_Integer, x_Real, Q_]
    :Arguments: {nset, x, Q}
    :ArgumentTypes: {Integer, Real64, Real64}
    :ReturnType: Manual

The function body (the return value is list of doubles):
    void my_xfxM(const int nset, const double x, const double Q) {
      vector<double> result = LHAPDF::xfxM(nset, x, Q);
      double* c_result = &(*result.begin());
      MLPutReal64List(stdlink, c_result, result.size());

I'm defining another function in Mathematica:
(it should multiply list of values returned by fucntion call by reversed 
list of another function call and drop first 7 elements.
eg. if xfxM(1, x, 100.) returns {x1, ..., x14} the result should be:
    myintegrand =
      Function[{x, y},
        (Plus @@ (Drop[xfxM[1, x, 100.] Reverse[xfxM[1, y, 100.]], 7]))]

Direct evaluation works well
    myintegrand[0.5, 0.3]

But smth like this does not work:
    myintegrand[x, y] /. {x->0.5, y->0.3}

And also when I try to integrate this function:
    NIntegrate[myintegrand[x, y], {x, 0.001, 1}, {y, 0.001, 1},
      Method -> {"LocalAdaptive"}]

The NIntegrate gives error message:
    Drop::drop: Cannot drop positions 1 through 7 in xfxM[1,x,100.] \
      xfxM[100.,y,1]. >>
    NIntegrate::inum: "Integrand (7+xfxM[1,x,100.]\xfxM[100.,y,1])/(14000\
      \x\y) is not numerical at {x,y} = {0.50048828125`,0.50048828125`}"

How should I define my function to make this work? I think the problem is 
that Mathematica tries to evaluate function body with formal parameters 
(of "Symbol" type) not with just numerical values.

With best regards, Valery Yundin.

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