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Topic: simple harmonic oscillator using ode45 function
Replies: 9   Last Post: May 28, 2013 9:04 AM

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Shane

Posts: 5
Registered: 12/14/12
Re: simple harmonic oscillator using ode45 function
Posted: Jan 24, 2013 10:03 AM
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"Steven_Lord" <slord@mathworks.com> wrote in message <kdrhc6$o8r$1@newscl01ah.mathworks.com>...
>
>
> "Shane " <shane_t@hotmail.co.uk> wrote in message
> news:kdrf88$fp6$1@newscl01ah.mathworks.com...

> > "Steven_Lord" <slord@mathworks.com> wrote in message
> > <kafm1a$c49$1@newscl01ah.mathworks.com>...

>
> *snip*
>

> > Hi Steve back after the winter break seen your help this is the code you
> > asked for:

> >>> dbtype 1:3 shm0.m
> >
> > 1 tspan=[0 50];
> > 2 init=[-2 0];
> > 3 [t,x]= ode45(shm0,tspan,init)

>
> There are three problems here.
>
> The first is that ODE45 expects its first input argument to be a function
> handle[*] and you are calling it with whatever the shm0 function returns. If
> shm0 returned a function handle, that would be okay. But this ties into the
> second and third problems.
>
> The second problem would be that before MATLAB can call ODE45 it needs to
> call shm0 to determine what to pass into ODE45 as the first input. Doing
> this requires shm0 to call shm0 to determine what to pass into ODE45 as the
> first input. Doing this requires shm0 to call shm0 to determine what to pass
> into ODE45 as the first input. ... We're going around in circles. You should
> not call the ODE solvers and ask them to use the function in which you're
> calling them as the ODE function first input. But actually we don't get this
> far due to the third problem.
>
> The third problem is that shm0 is a script not a function. That means that
> neither this:
>
> [t, x] = ode45(shm0, tspan, init) % Which would require shm0 to return a
> value to be used as ODE45's first input, but scripts don't return values.
>
> nor this:
>
> [t, x] = ode45(@shm0, tspan, init) % Which would require shm0 to accept
> input arguments and return an output argument when ODE45 calls it, neither
> of which scripts do.
>
> will work. If you want to keep shm0 a script, modify your ODE45 call to use
> a function handle to a SEPARATE function to evaluate the right-hand side of
> your system of ODEs. Personally, so that I could use a subfunction for that
> purpose and not have to create a separate file, I'd turn shm0 into a
> function.
>
>
> % begin shm0.m
> function [t, x] = shm0
> tspan = [0 50];
> init = [-2 0];
> [t, x] = ode45(@myODEfun, tspan, init);
>
> function yprime = myODEfun(t, y)
> % Evaluate the RHS of your ODE system here
>
> % end shm0.m
>
>
> --
> Steve Lord
> slord@mathworks.com
> To contact Technical Support use the Contact Us link on
> http://www.mathworks.com
>
> [*] Yes, I know ODE45 _can_ accept things other than function handles as its
> first input, but I recommend that you don't call the ODE solvers with
> anything but some flavor ("named" or anonymous) of function handle.


-----------------------------------------------------------------------------------------
I think i understand Steve the function is calling itself with no value, so if i use the text you suggested then follow it with:
> > a=1;
> > b=1;
> > xprime= zeros(2,1);
> > xprime(1)=x(2);
> > xprime(2)= -a*x(2) - b*x(1)


then it should run fine if I just typed in shm0 as this would call the function with the values given?
Thanks again Shane



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