Torsten
Posts:
1,627
Registered:
11/8/10


Re: diff eq
Posted:
Apr 10, 2013 10:17 AM


"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.

