polynomial interpolation with lagrange form!!!

2022 1 25
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更新时间：2022 1 28
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I implemented a function of lagrange form and i want to use it to interpolate the function f(x) = sin(4x) for some datapoints!! i wrote the script for that but it always gives me error !!! can anyone help me with that !!!
this is my code
xval = 0:10;
pval = xval.^2;
xx = linspace(0,10);
yy = lagrange1(xx,xval,pval);
plot(xval,pval,'o',xx,yy,'.')

4 个评论
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Torsten
Torsten 2022-1-25
Since we don't see lagrange1, we can't reproduce the error.
Anas Gharsa
Anas Gharsa 2022-1-25
this is my lagrange1
function [Pval,Lval] = lagrange1(x,xval,f)
n = length(x); ntab=length(xval);
if nargout ==2 % in this case also the tabulation of the basis Lagrange polynomials is desired
Lval=ones(n,ntab);
for i=1:n
for j=[1:i-1,i+1:n]
Lval(i,:) = Lval(i,:).*(xval-x(j))/(x(i)-x(j));
end
end
f = f(:);
Pval = Lval'*f;
Pval=Pval'; % row vector in output
else % in this case only the tabulation of the interpolating polynomial is desired
Pval = zeros(1,ntab);
for i=1:n
Lval = ones(1,ntab);
for j=[1:i-1,i+1:n]
Lval = Lval.*(xval-x(j))/(x(i)-x(j));
end
Pval = Pval + Lval*f(i);
end
end
end

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 采纳的回答

Alagu Sankar Esakkiappan
编辑：Alagu Sankar Esakkiappan 2022-1-28

0 个投票

Hello!
I see that you're trying to interpolate a polynomial but running into errors. I checked your code along with your function lagrange1 from your comments. It seems that you are running into an indexing error in the function lagrange1 on the line "Pval = Pval + Lval*f(i)". From what I observed using Breakpoint, you are trying to access f(12) when infact f only contains 11 elements to begin with.
Here are the Workspace values before the exact occurence of error:
If you try to evaluate the expression "Pval = Pval + Lval*f(i)" at this point ( based on values from above image ), you access f(12), thereby running into the error. I suggest for easier debugging, Try creating a Breakpoint on the line where error is caused in MATLAB. It'll give a more comprehensive view of the Codeflow and to narrow down exact occurence of error.

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