```Date: Apr 10, 2013 10:17 AM
Author: Torsten
Subject: Re: diff eq

"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 wishesTorsten.
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