# Fixing code to plot an ODE

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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

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 CommentsShowHide 1 older comment
Mischa Kim on 26 Dec 2020
Have a joyful holiday season, as well.