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Re: Variables in Iterator limits?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg109630] Re: Variables in Iterator limits?
  • From: "christopher arthur" <aarthur at tx.rr.com>
  • Date: Sun, 9 May 2010 07:49:13 -0400 (EDT)

----- Original Message ----- 
From: "AES" <siegman at stanford.edu>
To: <mathgroup at smc.vnet.net>
Sent: Saturday, May 08, 2010 6:05 AM
Subject: [mg109630] [mg109608] Variables in Iterator limits?


> If I execute the following lines, I get the graphic that I want:
>
>   x; Remove["Global`*"];
>
>    lines := Graphics[ Table[ Line[{{k 10/kmax,-1},
>                                     {k  10/kmax,1}}], {k,1,kmax}]]
>
>   km = 30; Show[lines]
>

but kmax is undefined?  probably you don't mean 'km' but 'kmax' in last 
line.  Then it works fine.

> If I execute the following lines, I get exactly the same graphic, all OK,
>
>   x; Remove["Global`*"];
>
>   lines := Graphics[ Table[ Line[{{k 10/kmax,-1},
>                                     {k  10/kmax,1}}], {k,1,kmax}]]
>
>   testValues = {kmax->30};  Show[lines] /. testValues
>
> -- except that I also get a beep, and an error msg in the Messages
> window:
>
>     "Iterator {k,1,kmax} does not have appropriate bounds"
>

The problem is the order of operations.  The replacement rule is applied
after Show[] executes (or attempts).  This means that 'kmax' does not yet
have a value.   Perhaps the best way to think about it is to consider that 
the replacement rule won't change a length of a list in general but can change 
its contents.


Christopher Arthur


> (but no little red square in the graphics output cell).
>
> Same thing happens in the /. case if I do the two tests in reverse order.
>
> Seems like another annoying little Mathematica "gotcha" to me . . . ?
>
> 




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