       Re: Functions with data hidden in them

• To: mathgroup at smc.vnet.net
• Subject: [mg81863] Re: Functions with data hidden in them
• From: Hannes Kessler <HannesKessler at hushmail.com>
• Date: Fri, 5 Oct 2007 04:50:44 -0400 (EDT)
• References: <fe28v5\$muq\$1@smc.vnet.net>

```Hello Neil,

here is a possible approach:

As an example, define a function of the single argument x. The
function depends on a given matrix m as parameter.
We can write this MyFunc[m_?MatrixQ][x_].
In the considered example, the function MyFunc[m] multiplies x with
the largest eigenvalue of the matrix m.
The function is defined as follows:

In:=
MyFunc[m_?MatrixQ] := MyFunc[m] = Module[{eigenMax},
eigenMax = Max  @  N  @  Eigenvalues  @  m;
Unprotect  @  Function;
Format[Times[eigenMax, #] &] :=
MyFuncData[Short[m, 1], eigenMax];
Protect  @  Function;
Times[eigenMax, #] &
];

Repeated calculations of the eigenvalues of m are avoided by the
assignment in the assignment
MyFunc[m_?MatrixQ] := MyFunc[m] = Module[{eigenMax}, ...];
Obviously, the maximum eigenvalue has to be calculated only during the
first function call.
Furthermore, Format[Times[eigenMax, #] &] := ... achieves the wanted
formatting of the virtual function returned from the Module.
The virtual function can be applied then to different arguments x over
and over again.

Here are 2 matrices m1 and m2.

In:=
m1 = {{10, 30, 40, 50, 60}, {30, 20, 50, 60, 70}, {40, 50, 30, 70,
80}, {50, 60, 70, 40, 90}, {60, 70, 80, 90, 50}};
m2 = 20 m1;

Here we get the formatted virtual functions for m1 and m2:

In:=
f1 = MyFunc[m1]
f2 = MyFunc[m2]

Out:=
MyFuncData[{{10,30,40,50,60},{30,20,50,60,70},{<<1>>},{50,60,70,40,90},
{60,70,80,90,50}},280.327]

Out:=
MyFuncData[<<1>>,5606.53]

And here, the functions are mapped on the elements of a vector

In:=
f1  /@  {1, 2}

Out:=
{280.327,560.653}

In:=
f2  /@  {1, 2}

Out:=
{5606.53,11213.1}

You may also have a look into the Splines package
\Splines.m

Best regards,
Hannes

On 4 Okt., 10:41, Neil Stewart <neil.stew... at warwick.ac.uk> wrote:
> The Interpolation[] function somehow "hides" the data passed to it in the
> InterpolationFunction object that it returns. In the example below, when
> f is evaluated it is using information from the list data, but does not
> have the list data passed to it as parameter.
>
> In:= data = {{1, 1}, {2, 2}, {3, 3}}
>
> In:=  f = Interpolation[data]
>
> In:= f
> Out= 1
>
> How can I write my own function that stores data inside itself in the same
> way that Interpolation does? I'm aiming to write a functions that takes a
> number as a parameter and consults a large data set to return a number. I'm
> not sure where to start - any ideas very welcome!
>
> Thanks,
> Neil.

```

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