Conditional variables with running ODE45

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Brilliant Purnawan
Brilliant Purnawan 2020-11-19
回答： Alan Stevens 2020-11-19
Hi all, is it possible to call variables within an ODE45 and use if statements? So the general idea can be see in the code below, what I'm trying to do is that if the value of y(:,2) is greater than 0.3, than A will be 0.1 else, if its lower it will just run with a value of 0.1. Is this possible?
clear all, close all, clc
if y(:,2)>0.3
A = 0.2;
else
A = 0.1;
end
B = 0.1;
tspan = [0:4:200];
y0 = [0.95 0.05 0];
[t,y] = ode45(@(t,y)odefcn(y,A,B),tspan,y0);
figure(1)
hold on
plot(t,y(:,1),'-o')
plot(t,y(:,2),'-*')
plot(t,y(:,3),'-d')
function dydt = odefcn(y,A,B)
dydt = zeros(3,1);
dydt(1) = -A*y(2)*y(1);
dydt(2) = A*y(2)*y(1)-B*y(2);
dydt(3) = B*y(2);
end

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采纳的回答

Alan Stevens
Alan Stevens 2020-11-19
Yes, but rearrange the coding as follows:
tspan = [0:4:200];
y0 = [0.95 0.05 0];
[t,y] = ode45(@(t,y)odefcn(t,y),tspan,y0);
figure(1)
hold on
plot(t,y(:,1),'-o')
plot(t,y(:,2),'-*')
plot(t,y(:,3),'-d')
function dydt = odefcn(~,y)
if y(2)>0.3
A = 0.2;
else
A = 0.1;
end
B = 0.1;
dydt = zeros(3,1);
dydt(1) = -A*y(2)*y(1);
dydt(2) = A*y(2)*y(1)-B*y(2);
dydt(3) = B*y(2);
end

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