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Re: How do you control evaluation when using apply?

• To: mathgroup at smc.vnet.net
• Subject: [mg130803] Re: How do you control evaluation when using apply?
• From: Christoph Lhotka <christoph.lhotka at univie.ac.at>
• Date: Fri, 17 May 2013 04:33:46 -0400 (EDT)
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• References: <20130516072800.B38966A00@smc.vnet.net>

Hello,

  both of your versions work fine in M9.

Here is even a shorter one:

In[]:= jacFun[func_, vars_] :=
  Module[{f}, f = Function @@ {{x, y}, D[func, {vars}]};
   f @@ # &]

In[]:=jacFun[{Sin[x y], Cos[x + y]}, {x, y}][{10, 2}]


Out[]:={{2 Cos[20], 10 Cos[20]}, {-Sin[12], -Sin[12]}}


BR,

   Christoph


On 05/16/2013 09:28 AM, Brentt wrote:
> Why does this work
>
> In[0]: = jacobianFunction[func_, vars_List] := Module[{f},
>     f = Function[Evaluate[vars], Evaluate[D[func, {vars}]]];
>    f
>     ];
> jacobianFunction[{Sin[x y], Cos[x + y]}, {x, y}] @@ {10, 2}
>
> out[0]:= {{2 Cos[20], 10 Cos[20]}, {-Sin[12], -Sin[12]}}
>
> But this does not (the goal is to make the function take a point as an
> argument)
>
>
> In[0]: = jacobianFunction[func_, vars_List] := Module[{f},
>     f = Evaluate[Function[Evaluate[vars], Evaluate[D[func, {vars}]]]];
>    f@@ # &
>     ];
> jacobianFunction[{Sin[x y], Cos[x + y]}, {x, y}][{10, 2}]
>
>
>
> I can't get f@@ # & to evaluate properly (I've tried wrapping it in
> ReleaseHold and evaluate statements, nothing seems to get it to evaluate.
>
> I know I can just rewrite the function to take the point but I'm just
> curious why it won't work.
>
>

• References:
  • How do you control evaluation when using apply?
    • From: Brentt <brenttnewman@gmail.com>