Re: Should interp1.m function allow jumps in X-values?

[Ben Abbott]

On Aug 22, 2012, at 9:50 AM, Ben Abbott wrote:

> 
> On Aug 22, 2012, at 9:29 AM, Michael D Godfrey wrote:
> 
>> On 08/22/2012 08:33 AM, Ben Abbott wrote:
>>> The reason for the difference is subtle (and sorry for not pointing it out 
>>> earlier).  The sort() function does not change the order of the duplicate X 
>>> values.  Thus, the continuity condition is not switched from right to left. 
>>>  The flipup() function is used in interp1 to do that.  Below I used the 
>>> fliplr() to ensure the proper continuity.
>>> 
>>>     X = [ 2 1 3 2];
>>>     Y = [ 9 1 3 10]; 
>>>     % These are right-continuos (returns 10)
>>>     y1 = interp1 (X, Y, 2)
>>>     [~, n] = sort (X);
>>>     y2 = interp1 (X(n), Y(n), 2)
>>>     [~, n] = sort (X, "descend");
>>>     y3 = interp1 (X(n), Y(n), 2)
>>>     y4 = interp1 (X, Y, 2, "-right")
>>>     % These are left-continuous (returns 9)
>>>     [~, n] = sort (fliplr (X), "descend");
>>>     y5 = interp1 (X(n), fliplr(Y)(n), 2)
>>>     y6 = interp1 (X, Y, 2, "-left")
>>> 
>>> Ben
>>> 
>> Very good.  I could have thought of that!  So, that gets rid of an important
>> problem.  And this adds a bit in favor of your addition of -right and -left,
>> or at least another point to be made clear in the documentation.  This
>> is definitely a subtle point about sorting.  Time to look up what Knuth
>> has to say about this.
>> 
>> By the way, the current devel interp1.m does not do the flipr().  Did you
>> add that in your local patch?
>> 
>> Michael
> 
> I haven't pushed the patch that includes the flip part yet.
> 
> It has beed posted to the bug-tracker.
> 
>       https://savannah.gnu.org/bugs/?36732
> 
> See attachement #26413
> 
> Ben

I pushed a changeset.

        http://hg.savannah.gnu.org/hgweb/octave/rev/94d512d712e3

Please comment if there are any remaining concerns.

Ben