Fixing code to plot an ODE

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frozentangled
frozentangled on 24 Dec 2020
Commented: Mischa Kim on 26 Dec 2020
Hi guys,
I was just coding to plot an ODE, but ran into some problems dispalying the result
The code are as follows
function dxdt = ODE(t, x, alpha, beta)
dxdt = diag(alpha - beta*[x(1);x(2);x(3)])*[x(1);x(2);x(3)];
for n=1:200
p=.01*n;
alpha=[.1; .5; .3];
beta=[.2 .3 .4; p 1.5 .6; .2 .5 .8];
time = [0 100];
initial=[0.5; 0.2; 0.3];
[t,y] = ode45(@(t,y) ODE(t,y,alpha,beta), time, initial);
F1=y(100,1);
F2=y(100,2);
F3=y(100,3);
end
figure(1)
plot(p,F1);
hold on
plot(p,F2);
plot(p,F3);
hold off
Could you tell me what's off?
Many thanks in advance

Accepted Answer

Mischa Kim
Mischa Kim on 24 Dec 2020
Hi, you are almost there. This should get you started:
hold on
for n=1:200
p=.01*n;
alpha=[.1; .5; .3];
beta=[.2 .3 .4; p 1.5 .6; .2 .5 .8];
time = [0 100];
initial=[0.5; 0.2; 0.3];
[t,y] = ode45(@(t,y) ODE(t,y,alpha,beta), time, initial);
F1=y(end,1); % assuming you are trying to access last elements
F2=y(end,2);
F3=y(end,3);
plot(p,F1,'ro'); % a cleaner way would be to save the values...
hold on % ...for Fi in a matrix and then plot outside...
plot(p,F2,'ko'); % ...of the for loop
plot(p,F3,'bo');
end
hold off
function dxdt = ODE(t, x, alpha, beta)
dxdt = diag(alpha - beta*[x(1);x(2);x(3)])*[x(1);x(2);x(3)];
end
  2 Comments
Mischa Kim
Mischa Kim on 26 Dec 2020
Have a joyful holiday season, as well.

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