| Albert 2007-11-25, 4:46 am |
| Joe Sneed wrote:
> I run V6.01 under Windows XP
>
> I want to construct a button that reads a string input and writes
> something determined by the input to a notebook. When I click on "close"
> of "X" in the input dialog box, I want nothing to appear in the notebook.
>
> I have two questions: one dealing with evaluation method; another
> dealing with the use of If[ _, _, _] in the button function.
>
> As a test, I try to read the input and write it to a selected notebook using
>
> Button[Style["LABEL", Black, 10, FontFamily -> "Arial Black"],
> tag=InputString[];
> nb=SelectedNotebook[];
> If[tag ? $Canceled,
> NotebookWrite[nb,Cell[tag,"Text"]]]]
>
> to construct the button.
>
> This yields a button whose underlying expression is
>
> Cell[BoxData[
>
> ButtonBox[
> StyleBox["\<\"LABEL\"\>",
> StripOnInput->False,
> FrontFaceColor->GrayLevel[0],
> BackFaceColor->GrayLevel[0],
> GraphicsColor->GrayLevel[0],
> FontFamily->"Arial Black",
> FontSize->10,
> FontColor->GrayLevel[0]],
> Appearance->Automatic,
> ButtonFrame->"DialogBox",
> ButtonFunction:>($CellContext`tag =
> InputString[]; $CellContext`nb =
> SelectedNotebook[];
> If[$CellContext`tag != $Canceled,
> NotebookWrite[$CellContext`nb,
> Cell[$CellContext`tag, "Text"]]]),
> Evaluator->Automatic,
> Method->"Preemptive"]], "Output",
> CellChangeTimes->{3.40483095753125*^9},
> CellLabel->"Out[1]="]
>
> This produces an input dialog box that times out without accepting
> input. if I change the default
>
> Method->"Preemptive" -- to---> Method->"Queued"
>
> My first question is , "Why must I change the method?".
Because Preemptive means that the kernel is interrupted in doing what it
does for a quick calculation so that these buttons are reactive even
when the kernel is busy doing something else. To make sense these
actions are limited to a default of 5 or 6 seconds. Using Queued, the
evaluation is done in the usual way like when evaluating cells, that is
it is queued, and of course then the need for a timeout does not apply.
To learn more about this, read the tutorials for Dynamic.
> Having made the method change, I can enter input and without the If[ ]
> write the input to the selected notebook.
> But the conditional does not work. If I enter input and click OK,
> nothing is entered into the selected notebook.
>
> The purpose of the conditional If[ ] is to avoid the appearance of
> "$Canceled" in the selected notebook when one clicks on "Cancel" or "X"
> in the input dialog box.
>
> My second question has two parts: 1) "Why doesn't the If[ ] work; 2)
> what will work to accomplish my purpose?".
This is a very common pitfall when using == which is in mathematica used
for mathematical equivalence and usualy is returned unevaluated when
either the right or lefthand side contain a symbol, which you can check
by evaluating this:
"input string" == $Canceled
Usually if you are not handling mathematical expression SameQ (===) is a
better choice, like:
"input string" === $Canceled
which will be evaluated to False. So the following should cure your
problems:
Button[
Style["LABEL", Black, 10, FontFamily -> "Arial Black"],
tag=InputString[];
nb=SelectedNotebook[];
If[tag === $Canceled, NotebookWrite[nb,Cell[tag,"Text"]]],
Method->"Queued"
]
hth,
albert
|