Re: How do you control evaluation when using apply?

[Christoph Lhotka]

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.
>
>