#61 coco_plot_bd many points

[Marc]

Assume you have saved many points (say 1000) during a continuation run.
It can take some time to plot the run using coco_plot_bd.

It seems that this function uses coco_bd_read to access the stored data, which in its turn calls coco_slot_data.
When coco_slot_data(fid, results) is called with fid=‘bd’, the returned varargout is a cell array containing the data needed for the (2D or 3D) plot, featuring one line per .mat file (sol1.mat etc.).

What would be the easiest way to let coco_plot_bd plot only each 10th or 100th point, so plotting is sped up? Tweaking the coco_slot_data function so it returns a varargout cell array with less lines?

[Harry Dankowicz]

Hi Marc,

Let me begin by asking for the reason for such a large number of points (I assume this is from continuation along a one-dimensional manifold). Is this necessary or a result of setting the step size maximum to a small value?

Best,

Harry

[Marc]

Hello
I am following e.g. SN points as function of two parameters, for a quite stiff system.
In some cases, even when setting a large maximum step size, and a minimum step size of 1E-16, the chosen step size gradually becomes smaller, so many points are calculated.

1. Sometimes, this works fine: you just have to wait long enough.
2. Sometimes, COCO gets stuck at the minimal step size of 1E-16, when the Newton algorithm is not converging (even having increased ItMX from 20 to say, 100). Stopping and restarting the continuation from the last points often helps: with a bigger (initial) step, COCO can continue without problem.

[Harry Dankowicz]

Hi Marc,

If you find yourself adjusting the minimum stepsize to 10^-16 in order to achieve convergence, then there is likely something amiss in your implementation (or the problem is highly unusual). Does convergence appear to be quadratic?

Generally speaking, I would recommend against adjusting continuation or nonlinear solver settings until you are sure that the need is motivated by unusual aspects of your problem. I am inclined to suspect an error in your Jacobians.

Harry

[Marc]

Hello Harry

Thanks for your comments.

In any case, if plotting would become too slow because too many data files have to be accessed, one can find in fact easy workarounds such as extracting the relevant data and storing them in another form (.mat/.fig/...) to speed up manipulation in Matlab.

This question can therefore be closed.

Best regards
Marc

[Harry Dankowicz]

Indeed, Marc, there are surely (as you point out) other ways to address the problem of too many data files. I am encouraging of users developing alternatives to, or modifications of, the routines that ship with the release.

I do want to stress, however, that the situation you describe is quite unusual, and very possibly a symptom of something amiss. I am assuming that you are tracking this down in your code.

Best,

Harry