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Matlab Help

  1. Jan 31, 2008 #1
    Dear all. I am trying to solve an ODE in MATLAB using ODE45 solver. But the programe keep saying

    ??? Input argument "y" is undefined.

    Error in ==> lvv at 4
    yprime=[a*y(1)-b*y(1)*y(2);-r*y(2)+c*y(1)*y(2)];

    Can anybody be of help? Please.

    The complete code is this:
    function yprime=lvv(t,y)
    %LV: Contains Lotka-Volterra equations
    a=.5471;b=.0281;c=.0266;r=.8439;
    yprime=[a*y(1)-b*y(1)*y(2);-r*y(2)+c*y(1)*y(2)];
    [t,y]=ode45(@lvv,[0 20],[30;4])
    plot(t,y(:,1))
    Thanks
     
  2. jcsd
  3. Jan 31, 2008 #2
    make on m-file called whatever you want an place:

    [t,y]=ode45(@lvv,[0 20],[30;4])
    plot(t,y(:,1))

    this in that. Then make another m-file called lvv and place

    function yprime=lvv(t,y)
    %LV: Contains Lotka-Volterra equations
    a=.5471;b=.0281;c=.0266;r=.8439;
    yprime=[a*y(1)-b*y(1)*y(2);-r*y(2)+c*y(1)*y(2)];

    this in that, then it works, have tested it. You should always place functions in their own m-file, there are ways to put them in the same, but if you place it in another, then you never get problems.
     
  4. Feb 2, 2008 #3
    a way I sometimes use, is to trick matlab to think of the lvv function as a nested function by making your m-file into a function that doesn't do anything then it works, that is writing

    function idontdoanything=LaLaLa

    [t,y]=ode45(@lvv,[0 20],[30;4])
    plot(t,y(:,1))


    function yprime=lvv(t,y)
    %LV: Contains Lotka-Volterra equations
    a=.5471;b=.0281;c=.0266;r=.8439;
    yprime=[a*y(1)-b*y(1)*y(2);-r*y(2)+c*y(1)*y(2)];


    that is start you m-file with a dummy function.
     
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