Date: Mar 23, 2013 3:24 AM
Author: Simons, F.H.
Subject: Manipulate and Initialization

I want to create a rather complicated Manipulate, in which I do not know 
the limits for the parameter beforehand; they depend on the result of an
initialization. Here is a highly simplfied example:

Clear[nn];
Manipulate[n, {n,1,nn,1},
Initialization:>(nn=RandomInteger[{5,15}];Print[nn])]

This works fine. The slider can be used in the range from 1 to the
printed value.

However, when nn already has a value, the behaviour is different. When
we execute

nn=23;
Manipulate[n, {n,1,nn,1},
Initialization:>(nn=RandomInteger[{5,15}];Print[nn])]

a value between 5 and 15 for nn is printed, but that value is not used
in the result; the slider now runs from 1 to 23.

So, whether nn has a value or not, just before the box structure is
printed, the frontend asks the kernel to evaluate the initialization.
But the returned value of nn is used in Manipulate only when nn did not
have a value before. That embarasses me. Any explanation is highly
apprectiated.

Fred Simons
Eindhoven University of Technology