Matlab ode solver - putting condition in function

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kkh1897 2020-12-6

评论： Ameer Hamza 2020-12-6

I use ode23s solver

and the results show that variables oscillate around zero plus and minus.

I want to fix the variables to zero when they first go under zero.

ode23s solver can't use odeset 'NonNegative' option,

so we should have to put some condition in the function.

Please give me help.

Ameer Hamza 2020-12-6

编辑：Ameer Hamza 2020-12-6

The easiest option might be to do this after getting the solution from ode23s. For example

[t, y] = ode23s(..)
idx = find(y<0, 1);
y(idx:end) = 0

This assumes that your ODE is first-order and y is a column vector.

kkh1897 2020-12-6

I sent you a email. Thank you

Ameer Hamza 2020-12-6

This is the code with ode45(). However, due to some reason, the result is different as compared to dsolve(). I am not sure why.

ode = @(t, x) [x(2);
               -sign(x(1)+x(2))];
IC = [1; 0];
tspan = [0 5];
ode(ode, tspan, IC)

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