how can I condition my ode solver

I use an ode solver here

[t, R] = ode78(@odefun, tspan, r0);

but I don't want it to run until the final t, I want it to run until R is at a certain value, and I have no idea how to go about doing that.

Answer 1

[Torsten]

Use the Event Facility of the ODE integrators:

https://uk.mathworks.com/help/matlab/math/ode-event-location.html

[Noya Linder]

And what happens if the "position" is not exactly zero? How can I specify tolerance to the value which will result in a termination?

[Torsten]

ode78 checks where the expression (R - certain value) changes sign. This is the position where R reaches the specified value. There is no need to specify a tolerance.