Re: NotebookGetExpression

["Udo und Susanne Krause" <su.krause@XXXXXXX.ch>]

Hallo Robert,

es kommt darauf an, welche Art Funktionen benötigt werden:

(i) symbolische Funktionen, dann sagt die Hilfe:

Although := is probably used more often than = in defining functions,  
there is one important case in which you must use = to define a function.  
If you do a calculation, and get an answer in terms of a symbolic  
parameter x, you often want to go on and find results for various specific  
values of x. One way to do this is to use the /. operator to apply  
appropriate rules for x in each case. It is usually more convenient,  
however, to use = to define a function whose argument is x.

(ii) numerische Funktionen, dann kann man Funktionen verwenden, die sich  
ihrer Werte entsinnen:

fib[1] = fib[2] = 1;
fib[n_] := fib[n] = fib[n - 1] + fib[n - 2]

(iii) könnte man ein Package schreiben, das die Funktion myNowak[x_, ...,  
manQ_:False] enthält. Falls manQ nicht spezifiziert wird, müsste die  
Lösung bereits im Package drinstehen und von daher reproduziert werden.  
Falls myNowak[x, ..., True] gerufen wird, darf das package selber rechnen.

(iv) A cell that cannot be evaluated:

CellPrint[ExpressionCell[Defer[1 + 1], "Input", Evaluatable -> False]]

Man könnte die bewusste Zelle taggen, sie mit NotebookLocate[] anhand des  
Tags wiederfinden und dann allenfalls mit FrontEndExecute[] die Option  
Evaluatable der Zelle auf False setzen.

Warum kann es nicht die build-in Methode (ii) sein?
Habe ich die Aufgabe überhaupt verstanden?

Gruss
Udo.



On Tue, 17 May 2011 12:09:28 +0200, Nowak Robert <Robert.Nowak@XXXXXXX.at>  
wrote:

> Liebe MMA user vielleicht fällt euch hierzu etwas ein.
>
>
>
> Dear all !
>
> I am looking for a way how to revert to an expensive (processing time)
> evaluated expression.
>
> I would like to have a notebook (or package) containing the expression
> in question which should be reevaluated only on a manual request and
> otherwise the once preevaluated expression should be used.
>
> Any hints on how this could be achieved simpler ?
> I found the following solution:
>
> (*---------------------------------------------------------------------------------------------------
> *)
>
> (* get the expression of the preceding cell *)
> NotebookGetExpression[] := Module[{nb, e},
>        nb = SelectedNotebook[];
>        SelectionMove[nb, Before, EvaluationCell];
>        SelectionMove[nb, Previous, CellContents];
>        e = ToExpression@NotebookRead[nb];
>        SelectionMove[nb, After, EvaluationCell];
>        e
>  ]
>
> (* this stands exemplary for an expression expensive to evaluate *)
> (* after once evaluated the cell will be marked by hand as non
> evaluatable *)
> (* would be nice if the non evaluatable marking could be automated *)
>
> Expand[(x + y)^2]
>
> Out[ ]= x^2 + 2 x y + y^2
>
> (* her we get the preceding expression and define a function for it *)
> Ex[x_, y_] = NotebookGetExpression[];
>
> (* test the defined function *)
> Ex[a, b]
>
> Out[ ]= a^2 + 2 a b + b^2
>
> (*---------------------------------------------------------------------------------------------------
> *)
>
> (* CellExpression starting here : *)
> {
> Cell[BoxData[
>  RowBox[{
>  RowBox[{"(*", " ",
>   RowBox[{
>   "gets", " ", "the", " ", "expression", " ", "of", " ", "the", " ",
>    "preceding", " ", "cell"}], " ", "*)"}], "\n",
>  RowBox[{
>   RowBox[{"NotebookGetExpression", "[", "]"}], ":=",
>   RowBox[{"Module", "[",
>    RowBox[{
>     RowBox[{"{",
>      RowBox[{"nb", ",", "e"}], "}"}], ",", "\n", "\t",
>     RowBox[{
>      RowBox[{"nb", "=",
>       RowBox[{"SelectedNotebook", "[", "]"}]}], ";", "\n", "\t",
>      RowBox[{"SelectionMove", "[",
>       RowBox[{"nb", ",", "Before", ",", "EvaluationCell"}], "]"}],
>      ";", "\n", "\t",
>      RowBox[{"SelectionMove", "[",
>       RowBox[{"nb", ",", "Previous", ",", "CellContents"}], "]"}],
>      ";", "\n", "\t",
>      RowBox[{"e", "=",
>       RowBox[{"ToExpression", "@",
>        RowBox[{"NotebookRead", "[", "nb", "]"}]}]}], ";", "\n",
> "\t",
>      RowBox[{"SelectionMove", "[",
>       RowBox[{"nb", ",", "After", ",", "EvaluationCell"}], "]"}],
>      ";", "\n", "\t", "e"}]}], "\n", "]"}]}]}]], "Input",
>  CellChangeTimes->{{3.514611168095347*^9, 3.514611328704722*^9}, {
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>   3.514613427110442*^9, 3.5146134734076815`*^9},
>   3.514613562174443*^9, {3.5146136409098253`*^9,
>   3.514613720004588*^9}}],
>
> Cell[CellGroupData[{
>
> Cell[BoxData[
>  RowBox[{
>  RowBox[{"(*", " ",
>   RowBox[{
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>  RowBox[{"(*", " ",
>   RowBox[{
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>  RowBox[{"(*", " ",
>   RowBox[{
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>  RowBox[{"Expand", "[",
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>    RowBox[{"(",
>     RowBox[{"x", "+", "y"}], ")"}], "2"], "]"}]}]], "Input",
>  Evaluatable->False,
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>
> Cell[BoxData[
>  RowBox[{
>  SuperscriptBox["x", "2"], "+",
>  RowBox[{"2", " ", "x", " ", "y"}], "+",
>  SuperscriptBox["y", "2"]}]], "Output",
>  CellChangeTimes->{3.514611341923472*^9}]
> }, Open  ]],
>
> Cell[BoxData[
>  RowBox[{
>  RowBox[{"(*", " ",
>   RowBox[{
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>    " ", "a", " ", "function", " ", "for", " ", "it"}], " ", "*)"}],
>  "\n",
>  RowBox[{
>   RowBox[{
>    RowBox[{"Ex", "[",
>     RowBox[{"x_", ",", "y_"}], "]"}], "=",
>    RowBox[{"NotebookGetExpression", "[", "]"}]}], ";"}]}]], "Input",
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> Cell[CellGroupData[{
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> Cell[BoxData[
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>  RowBox[{"Ex", "[",
>   RowBox[{"a", ",", "b"}], "]"}]}]], "Input",
>  CellChangeTimes->{{3.5146114477219257`*^9, 3.5146114497063894`*^9}, {
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>  CellChangeTimes->{3.514614846234047*^9}]
> }, Open  ]]
> }