I was pretty sure I'd done something with splines years back, but
CTRL-E I "spline" only in my user
folder revealed two relevant scripts. One was my edited crib from
~addons/demos/wdplot/plexam.ijs,
the other a copy of what Patrick Harrington posted to the J Forum way
back in 2004.
The latter starts with triPGA, "Solve tridiagonal matrix iteratively
using the Parallel Gauss
Algorithm"; it has a preamble very similar to that for tridgl near the
top of spline.ijs in the
link Scott gives, just below here. That link doesn't obviously
indicate authorship. Patrick might
wish to comment; I see he's still corresponding about matters J.
But Ruda has since commented that he considers that "the data as a whole
do not follow a
simple trend." It seems to me that to use mere (my term!) linear
interpolation is still to
impose some kind of mathematical model. Anyway, if the independent
values are "even quite
random", you might find that some values are repeated, in which case I
suppose you take
the average value at a repeated X value. Also, what should happen if
your data points are at,
say, X =: 0 5 6 8 12 26 ? (How) should Y(6) and Y(8) both be used?
Thanks,
Mike
On 07/05/2021 12:12, Scott Locklin wrote:
FWIIW there's a splines mini package here:
https://www.astro.umd.edu/~jph/J_page.html
-SL
On Fri, May 7, 2021 at 11:58 AM 'Rudolf Sykora' via Programming <
[email protected]> wrote:
Dear Raul,
thanks for your writing, and also for directing this from the general to
the programming forum, where I probably should have posted it.
On the topic. I am afraid you misunderstood what I meant. The data
as a whole do not follow a simple trend, like lying on a line. they can
be even quite random. The task is to go from
X: 0 5 6 26
Y: 10 _10 80 100
to, say:
U: 0 5 10 15 20 25 30 NB. Equidistant mesh.
V: 10 _10 84 90 94 99 104 NB. Linear interpolation and extrapolation
when needed.
NB. The numbers with U>5 are rounded here for
readability.
NB. If U=26 were, by chance, in the mesh,
NB. the value would be (exactly) 100.
(This is not what the data look like in reality, but as an example it
could suffice.)
Thanks once more!
Ruda
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