Using a variable set elsewhere within a function that has attribute

*To*: mathgroup at smc.vnet.net*Subject*: [mg98019] Using a variable set elsewhere within a function that has attribute*From*: Daan Gerla <daan at gerla.nl>*Date*: Sat, 28 Mar 2009 05:41:37 -0500 (EST)*Reply-to*: daan at gerla.nl

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