How could I plot this function ( I =A* sum( ( (dt.^2)*(B.^2) )/(C.^2) ) with time please? I am getting just a constant line while I expect to get a curve because it is a function of time.

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Avan Al-Saffar 2014-9-9

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https://ww2.mathworks.cn/matlabcentral/answers/154146-how-could-i-plot-this-function-i-a-sum-dt-2-b-2-c-2-with-time-please-i-am-gett

评论： Avan Al-Saffar 2014-11-5

I am working with this code but it does not give me the result that I expect :

function RunFisherfixedb
  x0=[12 5];
  dt = 0.055;
  t=(0:dt:1);
  n=length(t);
  Iall=zeros(1,n);
  for i = 1:n
    I = Fisherfixedb(t,x0,A,dt);
    Iall(i)=I;
  end
  figure(01)
  plot(t,Iall,'k')
  xlabel('Time')
  ylabel('Fisher Information')
  title('Average Fisher information in Two-species predator-prey model')
  1;

% function I = Fisherfixedb(t,x,A,dt)

% a1 = 15*x(1) - 3*x(1)*x(2);

% b1 =-5*x(2) + 0.5*x(1)*x(2);

% C= (a1.^2) + (b1.^2);

% B = ( (a1.*(15-3*x(2))+b1.*(.5*x(2))).*a1 ) + ( (a1.*(-3*x(1))+ b1.*(-5+.5* x(1))).*b1);

% I = A*sum( (dt.^2)*((B.^2)/(C.^2)));

end

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dpb 2014-9-9

编辑：dpb 2014-9-9

在 MATLAB Online 中打开

There's no variable A defined in the function prior to calling Fisherfixedb altho w/o the source code for that function it's not possible to know for sure what that would do it's a fair assumption that that is a problem...

Also, in the plot statement you've use the intermediary variable I returned by the function which after the loop will only be the last value returned. I'd presume since you're plotting against (and the question title refers to) t that you intended that to be

plot(t,Iall,'k')

instead. Presuming, of course, take care of the arguments to the function so it works as intended.

Also, it's not at all clear what part B and C are to play in the overall scheme of things but since they're functions of two x values, neither of which is x0, nothing's going to happen with them in the code as given. That would be a surface plot and meshgrid is often most useful for such. See the example in

doc meshgrid

Avan Al-Saffar 2014-11-4

在 MATLAB Online 中打开

This code what I am doing,, My problem is in plot(4),The result is not what I want it is completely different ,,so Is there anyone could help me to use another routines to get the result for this code please?

function Runsystem

A=1;

b=3;

x0=[12 5];

dt=0.01;

tspan=0:dt:1;

[t,x]=ode45(@system,tspan,x0,[],b);

figure(1)

plot(t,x)

figure(2)

plot(x(:,1),x(:,2),'-')

grid

dt= 0.095;

a1 = ( 15*x(1) - b*x(1).*x(2) );

b1 = ( -5*x(2) + 0.5*x(1).*x(2) );

I = A.*(dt.^2).*( ( ( ( (a1.*(15-b*x(2))+b1.*(.5*x(2))).*a1 ) + ( (a1.*(-b*x(1))+ b1.*(-5+.5*x(1))).*b1) ).^2)./(( (a1.^2) + (b1.^2) ).^2) )

ball=2:1:10 ;

n = length(ball) ;

Iall = zeros(1,n) ;

for i = 1 : n

    [t,x]=ode45(@system,(0:0.0001:1),x0,[],ball(i)) ;
    figure(3)
    plot(x(:,1),x(:,2))
    hold on    
    dt= 0.095;
    a1 =  ( 15*x(1) - ball(i).*x(1).*x(2) );
    b1 = ( -5*x(2) + 0.5*x(1).*x(2) );
    I = A.*(dt.^2).*( ( ( ( (a1.*(15-ball(i).*x(2))+b1.*(.5*x(2))).*a1 ) + ( (a1.*(-ball(i).*x(1))+ b1.*(-5+.5*x(1))).*b1) ).^2)./(( (a1.^2) + (b1.^2) ).^2) ) ;
    Iall(i) = I

end

figure(4)

plot(ball,Iall,'r-')

1;

% function dxdt = system(t,x,b)

% dxdt=zeros(2,1);

% dxdt(1) = 15 * x(1) - b * x(1).* x(2);

% dxdt(2) = -5 * x(2) + .5 * x(1).* x(2);

% end

Guillaume 2014-11-4

What is the result you see, and the result you expect?

Unfortunately, we can't test your function, since you haven't shown the code for the (badly named) system function.

Note that you may be better off starting a new question as most people won't be looking at a question that has been marked as answered. If you do that, pleas use the {} code button to format your code.

Avan Al-Saffar 2014-11-5