Applying boundary conditions on a cubic spline interpolation

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Marc Abdel Nour
Marc Abdel Nour 2022-11-4
评论： Marc Abdel Nour 2022-11-10
Hello,
I'm trying to implement the function interp1 in my code, as follows:
S=interp1(x,y,xq,'spline')
As output, I'm getting the interpolated value at position xq, using cubic spline interpolation. But interp1 uses not-a-knot as boundary condition, is there any way i can get S using different boundary conditions?like clamped or periodic or ....?
I tried using csape, but the thing is that i couldn't get the interpolated value at the specific location xq.
Thank you in advance for your help

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Santosh Fatale
Santosh Fatale 2022-11-10
Hi Marc,
As you mentioned, you can use “csape” function for different end conditions. Note that “csape” returns interpolants in ppform. You can use “fnval” function to calculate value of interpolant in ppform at the specific location xq.
For more information about “csape” and “fnval” functions and ppform follow these links:

更多回答（1 个）

Bruno Luong
Bruno Luong 2022-11-10
You can use my function spline1d available here https://fr.mathworks.com/matlabcentral/fileexchange/24996-spline-derivative?s_tid=srchtitle
You can select periodic bc, natural bc [default]; not-a-knot, or clamping first or second derivative on boundary.
This FEX is written long ago and a little bit out-date, but it still working =.
x=cumsum(rand(1,6));
y=rand(size(x));
xi=linspace(min(x),max(x));
yi_notaknot=interp1(x,y,xi,'spline');
yi_natural=spline1d(x,y,xi,[]);
figure
plot(x,y,'or');
hold on
h1=plot(xi,yi_notaknot,'b');
h2=plot(xi,yi_natural,'r');
legend([h1 h2],'not-a-knot','natural')

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