Re: problem with using external functions

• To: mathgroup at smc.vnet.net
• Subject: [mg91587] Re: problem with using external functions
• From: David Bailey <dave at Remove_Thisdbailey.co.uk>
• Date: Thu, 28 Aug 2008 03:18:16 -0400 (EDT)
• References: <g93av9\$k3e\$1@smc.vnet.net>

```Valery Yundin wrote:
> 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:
>     :Begin:
>     :Function: my_xfxM
>     :Pattern: xfxM[nset_Integer, x_Real, Q_]
>     :Arguments: {nset, x, Q}
>     :ArgumentTypes: {Integer, Real64, Real64}
>     :ReturnType: Manual
>     :End:
>
> 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());
>     }
>
> 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:
> Plus@@{x8*y14,x9*y13,...,x14*y8})
>     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},
>
> 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.
>
Rather than defining myintegrand using a pure function, define it with
pattern arguments:

myintegrand[x_Real,y_Real]:=expression;

or, if you think think the arguments might be integers, rationals, etc:

myintegrand[x_?NumericQ,y_?NumericQ]:=expression

This will prevent any evaluation unless the arguments are numbers.

David Bailey
http://www.dbaileyconsultancy.co.uk

```

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