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

[Ben Abbott]

On Aug 21, 2012, at 8:01 PM, Michael D Godfrey wrote:

> On 08/21/2012 07:50 PM, Ben Abbott wrote:
>>      X = [ 2 1 3 2];
>>      Y = [ 2 1 3 10];
>>      [X, n] = sort (X, "descend")
>> 
> Did you really mean to only sort X?  It seems to
> me that both X and Y should be sorted using the same
> option.  Does this work?  I do not see how it could different
> than your option for -right or -left.

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