PP form for griddedInterpolant

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Mohammad Shojaei Arani
回答： Damon Landau 2022-5-2
Hello friends!
This morning I was going to use interp1 but noticed that matlab is going to remove it soon. Instead, they
recommend to use griddedInterpolant. No problem. But, then I realized that I cannot use many of the benifits
of interp1 easily. Especially and sadly, griddedInterpolant does not aprarently create a 'pp form' which is quite annoying
and non-aceptable (without pp form, I am not able to easily use 'fnder' and get the derivative). So, I am wondering wether
I should use interp1 (which will not work soon and I do not have time to correct my loads of codes in the future) or use
griddedInterpolant and accept the inconvenience of working with it (and hope that in the future they improve it). So, it seems to me
that matlab developers irresponsibly change the existing functions without transfering all the capabilities to the new commands
they like.
My question is not limitted to the case of 1D interpolation (oitherwise, I would not bother you). My question is "how to
find pp form with regards to griddedInterpolant command, and therefore being able to conveniently do whatever we cxould
do with interp1, interp2, etc"
Thanks in advance!
Babak
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Steven Lord
Steven Lord 2022-1-21
That Note appears only in one part of the Description section on the interp1 documentation page, the part for the syntax: "pp = interp1(x,v,method,'pp') returns the piecewise polynomial form of v(x) using the method algorithm." If it were a message about discouraging use of the interp1 function as a whole it would appear more like the hist function where the message discouraging its use comes at the top of the documentation page referring to all the syntax options for that function.
You can submit an enhancement request via Technical Support asking for a way to evaluate the derivative of a griddedInterpolant in those cases where it makes sense (what would the derivative be if the interpolation method was 'nearest', for example?)
Mohammad Shojaei Arani
Mohammad Shojaei Arani 2022-1-22
Thanks a lot for your kind responses!
Well, in my opinion the fact that interp1 can support a 'pp form' and from this we are able to use 'fnder' to get derivatives (of arbitrary order) of the interpolated approximation is the most important task interp1 can do. Many mathematical algorithms need this. Therefore, I can say that griddedInterpolant suffers from this.

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回答（1 个）

Damon Landau
Damon Landau 2022-5-2
The built-in csapi (or csape if you want to specify end conditions) outputs PP form of multi-dimensional splines. You can then use fnder to get derivatives and fnval to interpolate. This method also allows complex inputs for complex step as an alternative method to take partials.

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