Re: Using a variable set elsewhere within a function that has attribute

*To*: mathgroup at smc.vnet.net*Subject*: [mg98062] Re: Using a variable set elsewhere within a function that has attribute*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Sun, 29 Mar 2009 02:45:01 -0500 (EST)*References*: <gqkuru$488$1@smc.vnet.net>

Hi, ode = x'[t] == r x[t] - x[t] 2; Manipulate[ode /. r -> \[ScriptR], {\[ScriptR], 0.1, 1}] Regards Jens Daan Gerla wrote: > Hello Mathematica users, > > I am trying to use a variable defined elsewhere within Manipulate. I > have assigned an expression to the variable that involves another > variable. That second variable should not be assigned to anything > outside of manipulate, however, within Manipulate, I want that second > variable to change as I drag a slider and anything that uses the first > variable to change accordingly. I am not quite sure I am saying this > right, but this example should make clear what I mean: > > ode = x'[t] == r x[t] - x[t]^2; > Manipulate[ > ode, > {r, 0.1, 1} > ] > > I do not get any warnings or errors, but the result is not as you might > expect: in the output the equation is displayed without a value for r > and the slider does nothing. > > Manipulate[ > Evaluate[ode], > {r, 0.1, 1} > ] > > does what you expect. The output contains a value for r which changes as > the slider is dragged. Now I want to use ode for something else. > > Manipulate[ > Plot[x[t] /. NDSolve[{Evaluate[ode], x[0] == 0.1}, x, {t, 0, 10}], {t, > 0, 10}], > {r, 0, 1} > ] > > Here I get errors: > > NDSolve::dsvar: 0.0002042857142857143` cannot be used as a variable. >> > ReplaceAll::reps: {NDSolve[{(x^\[Prime])[0.000204286]==r > x[0.000204286]-x[<<1>>]^2,x[0]==0.1},x,<<1>>]} is neither a list of > replacement rules nor a valid dispatch table, and so cannot be used for > replacing. >> > [...] > > It looks like it takes a number for t when it should not, and not a > number for r when it should. > > What I want is of course a plot of x vs. t that changes as I change r by > dragging the slider. I've tried many things, including changing the > attributes of Manipulate, but no good results. I am not even sure what > the problem exactly is... > > How do I solve this problem? What is the best way to do what I want to do? > > Regards, > > Daan >