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Re: Using a variable set elsewhere within Manipulate

  • To: mathgroup at smc.vnet.net
  • Subject: [mg98159] Re: Using a variable set elsewhere within Manipulate
  • From: Daan Gerla <daan at gerla.nl>
  • Date: Wed, 1 Apr 2009 05:54:33 -0500 (EST)
  • Reply-to: daan at gerla.nl

Hello Mathematica users,

This message reports back on the answers I got on my previous message 
"[mg98019] Using a variable set elsewhere within a function that has 
attribute HoldAll", but where "HoldAll" got cut off from the subject line.

The problem was that I wanted to use a variable assigned to a 
differential equation within Manipulate to dynamically solve it as a 
parameter in the equation is tweaked. I want to the write the equation 
outside of Manipulate and use it both within and outside of Manipulate, 
without having to copy-paste it.

This does not work:

ode = x'[t] == r x[t] - x[t] ^2;
Manipulate[
 sol = NDSolve[
   {ode, x[0] == 0.1},
   x,
   {t, 0, 10}
   ];
 Plot[x[t] /. sol, {t, 0, 10}, PlotRange -> {0, 1}],
 {r, 0.1, 1}]

First I thought it did not work because of the HoldAll attribute of 
Manipulate, because I can get some similar bits of code working by 
wrapping the first argument of Manipulate in Evaluate. For instance

x = y;
Manipulate[x, {y, 0, 1}]

does not work, but then

Manipulate[Evaluate[x], {y, 0, 1}]

does.

An easier way of understanding that the the dynamic ode solving above 
does not work is by seeing that r in the assignment of ode is not the 
same r as in the last argument of Manipulate. Two approaches were 
suggested to me.

David (besides giving a nice example of how to use Dynamic instead of 
Manipulate) made ode a function of r:

ode =.; sol =.
ode[r_] := x'[t] == r x[t] - x[t] ^2;
Manipulate[
 sol = NDSolve[
   {ode[s], x[0] == 0.1},
   x,
   {t, 0, 10}
   ];
 Plot[x[t] /. sol, {t, 0, 10}, PlotRange -> {0, 1}],
 {s, 0.1, 1}]

Jens used replacement:

ode =.; sol =.
ode = x'[t] == r x[t] - x[t] ^2;
Manipulate[
 sol = NDSolve[
   {ode /. r -> s, x[0] == 0.1},
   x,
   {t, 0, 10}
   ];
 Plot[x[t] /. sol, {t, 0, 10}],
 {s, 0.1, 1}]

Both work. However, the actual ODE's I want to solve involve many 
parameters, all of which I want to manipulate. Writing ode1, ode2 etc. 
as functions of the parameters doesn't look nice, so that is why I 
prefer Jens' solution with replacement.

I am still looking for a nice way to get a list of rules set defined 
elsewhere to work within Manipulate! ;-)

Thank you, David, Jens and Raffy.

Regards,

Daan



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