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Topic: diff eq
Replies: 8   Last Post: Apr 11, 2013 2:27 AM

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Torsten

Posts: 1,472
Registered: 11/8/10
Re: diff eq
Posted: Apr 10, 2013 10:17 AM
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"richard " <richard7893@embarqmail.com> wrote in message <kk3r1m$1c2$1@newscl01ah.mathworks.com>...
> Hi again
>
> So I used the link you posted along with some other info on the site and my code ran this time, and I think its right. This is what my code looks like so far:
>
> function question1_a
> [t,a] = ode45(@eq1, [0:.1:10], [1;0;0;0])
> end
> function da = eq1(t,a)
> da = zeros(4,1);
> da(1) = -2.4712 * a(1) + 0.005 * a(2) + 0.56 * a(3) + 0.1 * a(4);
> da(2) = 0.385 * a(1) - 0.1002 * a(2);
> da(3) = 0.002 * a(2) - 0.6462 * a(3);
> da(4) = 0.007 * a(2) - 0.2042 * a(4);
> end
>
> What comes out is
>
> 1) column vector for t from 0 -->10 days in steps of 0.1 days
>
> 2) and what comes out for 'a' is a 4 column matrix with the activities ('a') corresponding to the times provided, ie a(1) at t=0, a(2) at t=0, a(3) at t=0, and a(4) at t=0 and so on for the times up to 10 days. There are many rows in this matrix but only 4 columns
>
> Now I am trying to plot the results of the "a's" up to the 10 days (a vs t) and this is what I wrote out
>
> plot(t,a(:,1),'-',t,a(:,2),'-.',t,a(:,3),'.',t,a(:,4),'o')
>
> and this error message comes up:
> ??? Undefined function or variable "a".
>
> Error in ==> question1_a>eq1 at 6
> da(1) = -2.4712 * a(1) + 0.005 * a(2) + 0.56 * a(3) + 0.1 * a(4);
>
> Error in ==> odearguments at 110
> f0 = feval(ode,t0,y0,args{:}); % ODE15I sets args{1} to yp0.
>
> Error in ==> ode45 at 173
> [neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...
>
> Error in ==> question1_a at 2
> [t,A] = ode45(@eq1, [0:.1:10], [1;0;0;0])
>
> The line I write for the plot is basically following the example 1 from the link you provided. However when I delete the plot line from my code, the program runs. Can you see anything wrong with what I wrote for the plot line?
>
> Also what is the purpose of the 'da = zeros(4,1)' line on my code? What does it do because if I leave it out I get error messages.
>
> Thanks again
> "Torsten" wrote in message <kk3eo9$msg$1@newscl01ah.mathworks.com>...

> > "richard " <richard7893@embarqmail.com> wrote in message <kk3e55$lbv$1@newscl01ah.mathworks.com>...
> > > Hello
> > >
> > > I am doing as you posted and calling ode45 from a different mfile. In my code I am really using a's instead of x's, does that cause a problem?
> > >
> > > So when I run the first file which is this:
> > > function da = system_ex(t,a)
> > > da(1) = -2.4712*a(1) + 0.005*a(2) + 0.56*a(3)+ 0.1*a(4);
> > > da(2) = 0.385*a(1) - 0.1002*a(2)
> > > da(3) = 0.002*a(2) - 0.6462*a(3)
> > > da(4) = 0.007*a(2) - 0.2042*a(4)
> > > It is still saying input argument 'a' is undefined.
> > >
> > > but the a(t) is what I am trying to find. In a way I think I'm defining 'a' with this file, what's wrong with this part?
> > >
> > > and for my second mfile, when I am calling ode45 it wont run probably due to 'a' function being undefined
> > > "Bruno Luong" <b.luong@fogale.findmycountry> wrote in message <kjtnkm$ro7$1@newscl01ah.mathworks.com>...

> > > > "richard " <richard7893@embarqmail.com> wrote in message <kjrsih$mgb$1@newscl01ah.mathworks.com>...
> > > >

> > > > >
> > > > > this is what I am typing into MATLAB:
> > > > > function dx = system_ex(t,x)
> > > > > dx = zeros(4,1);
> > > > > dx(1) = -2.4712*x(1) + 0.005*x(2) + 0.56*x(3)+ 0.1*x(4);
> > > > > dx(2) = 0.385*x(1) - 0.1002*x(2)
> > > > > dx(3) = 0.002*x(2) - 0.6462*x(3)
> > > > > dx(4) = 0.007*a(2) - 0.2042*x(4)
> > > > > [T,X] = ode45(@systems_ex,[0 10],[1 0 0 0])
> > > > > plot(t,x(:,1))
> > > > >

> > > >
> > > > I hope you save system_ex in an mfile and call ode45 on another mfile. You seem to put everything in the same mfile.
> > > >
> > > > Bruno

> >
> > Start from example 1 under
> > http://www.mathworks.de/de/help/matlab/ref/ode23.html
> > and see whether it works.
> > Then replace the relevant parts step by step.
> >
> > Best wishes
> > Torsten.


According to the error message, you use
[t,A] = ode45(@eq1, [0:.1:10], [1;0;0;0])
as the call to ode45, but you want to plot with
plot(t,a(:,1),'-',t,a(:,2),'-.',t,a(:,3),'.',t,a(:,4),'o')
Because MATLAB differs between small and capital letters, the error message
??? Undefined function or variable "a"
makes sense.

By the way:
You should call ODE45 with a row vector of inital values, i.e.
[t,a] = ode45(@eq1, [0:.1:10], [1 0 0 0])

Best wishes
Torsten.


Date Subject Author
4/7/13
Read diff eq
Guest
4/8/13
Read Re: diff eq
Bruno Luong
4/10/13
Read Re: diff eq
Guest
4/10/13
Read Re: diff eq
Torsten
4/10/13
Read Re: diff eq
Guest
4/10/13
Read Re: diff eq
Torsten
4/10/13
Read Re: diff eq
Bart
4/11/13
Read Re: diff eq
Torsten
4/10/13
Read Re: diff eq
richard

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