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Re: Grabbing a value from a Dynamic Object

• To: mathgroup at smc.vnet.net
• Subject: [mg86858] Re: Grabbing a value from a Dynamic Object
• From: Szabolcs Horvát <szhorvat at gmail.com>
• Date: Sun, 23 Mar 2008 03:29:07 -0500 (EST)
• Organization: University of Bergen
• References: <fs4rrq$p93$1@smc.vnet.net>

Todd Allen wrote:
> Hi Folks,
> 
>    I am trying to use a series of control objects,
> specifically PopupMenus, so that the input to one
> PopupMenu will be used dynamically to provide the
> range of values for the subsequent PopupMenu.
> 
>    My area is not computer science, but I believe I
> get the idea that Dynamic is not evaluated in the
> kernal, but rather simply formatted in the front end
> of Mathematica.  To extract a Dynamic value I have
> been attempting to use the command CurrentValue with
> no sucess.
> 
> Here is my basic code:
> 
> x = Null; y = Null;
> totgrades = PopupMenu[Dynamic[x], Range[12]]
> temp1 = CurrentValue[totgrades]
> gradesnow = PopupMenu[Dynamic[y], Range[temp1]]
> 
> 
> I keep getting the error message:
> 
> "Range::range: Range specification in
> Range[FrontEnd`CurrentValue[BoxData[3]]] does not have
> appropriate bounds. >>"
> 
> I understand that the Head of temp1 is NOT a number,
> but rather is a FrontEnd`CurrentValue.......so, how do
> I get a number that the second PopupMenu can actually
> use to produce a range of potential inputs from the
> user?
> 
> I certainly appreciate any advice you might have! 
> Mathematica is a wonderful piece of software, but
> there are times when it is truly frustating for a
> beginner.

Dynamic[x] is usually used for *displaying* (or sometimes setting) the 
value of x.  x must not be wrapped in Dynamic when its value is used.

Try this:

PopupMenu[Dynamic[x], Range[12]]
Dynamic@PopupMenu[Dynamic[y], Range[x]]