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MathGroup Archive 2007

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Questions about InputField

  • To: mathgroup at smc.vnet.net
  • Subject: [mg80293] Questions about InputField
  • From: Fred Simons <f.h.simons at tue.nl>
  • Date: Thu, 16 Aug 2007 07:25:00 -0400 (EDT)

The following contains a highly simplified version of a problem I ran 
into using Inputfields.

Here is a construction for displaying the updated values of two numbers 
n and m, and to enter values for m:

Column[{Dynamic[n], InputField[Dynamic[m]]}]

I want to use this column for controlling the following (infinite) loop:

n = 100; m = 0;
While[n > 0,
 n = n - m; m = 0]

Having started this loop, the column shows the numbers 100 and 0. Now 
enter 8 in the input field. Since the pre-emptive link will be used, the 
evaluation of the loop will be interrupted and the assignment m=8 is 
done. Then the loop is resumed, so n gets the value 92 and m is reset to 0.

Having done this, the column indeed shows the value 92 for n, but for m 
the value 8 is displayed. Aborting the loop and inspecting the value of 
m gives 0, so here we have a Dynamic[m] that does NOT show the updated 
value for m.

It is very strange that we can force the updating of m by explicitly 
entering the default value for the second argument of Dynamic:

Column[{Dynamic[n], InputField[Dynamic[m, (m = #) &]]}]

Restart the loop and repeatedly enter not too large positive values for 
m. Then, on my Windows Visa system, sometimes the number will be 
subtracted, sometimes not.

Though this looks buggish, it probably is not. We have to take into 
consideration that we do not know at which point the execution of the 
loop is interrupted.  That it does make a difference can be seen from 
the following two examples. In the next two loops it is practically sure 
that the interrupt will happen during the Pause statement. So the first 
loop can be perfectly controlled and in the second loop nothing seems to 
happen:

n = 100; m = 0;
While[n > 0,
 Pause[0.1]; n = n - m; m = 0]
n = 100; m = 0;

While[n > 0,
 n = n - m; Pause[0.1]; m = 0]

So I suppose that in the situations that no subtraction happened, the 
execution of the loop was interrupted just before the assignment m=0.

Can someone explain why it is necessary to use the default value for the 
second argument in Dynamic, and find a better and safer way of 
controlling the loop, not using a Pause statement?

Fred Simons
Eindhoven University of Technology


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