Error using griddedInterpolant. The grid vectors are not strictly monotonic increasing.

I am trying to solve a System of differential equations for a torsional pendulum with euler forward finite difference method. (lines 37 to 51) After every step I need to find an specific value of time (tn) by interpolation with the angular position y(1,:) and the time t (lines 59 to 65). It keeps saying that the values are not strictly monotonic increasing but I am making sure they are by setting a constraint that angular speed y(2,:) is never negative or equal to zero. (lines 53 to 57)

If I run the code for a very small time It works but if I want to do it for a longer time it keeps giving me that error. Is there anything I can do?

Answer 1

[Walter Roberson]

You initialize y to all 0. You loop over the column number, extending the non-zero portion one item at a time via

y(:,i+1) = y(:,i) + ydot*h;

You have a test

if y(1,i+1)<2*pi/N

and if that test is true then you do not do the interp1(). It so happens that the first time the test is false is at i = 536. At that point you will have filled in up to y(:,537) leaving y(:,538:end) as 0. But your interp1() call is

t_tn = interp1(y(1,:),t,PHIt_tn);

which uses all if y(1,:) including the part that is still 0. As the previous values were increasing and greater than 0, the sudden transition to 0 is non-monotonic.

[Rosana Reyes]

Thanks so much for your help!

I just changed my code to: t_tn=interp1(y(1,1:i),t(:,1:i),PHIt_tn);

So it will just evaluate the interpolation in the previous points and not the part that is still 0. This seems to be working good!