About plotting Lagrange polynomial

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Bo Wang 2016-3-25

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https://ww2.mathworks.cn/matlabcentral/answers/275357-about-plotting-lagrange-polynomial

编辑： Star Strider 2016-3-25

I have a problem that let me plot Lagrange polynomial for the abscissas 1, 3, 4, 5, 8 in one plot. I've got the function for calculating Lagrange polynomial:

function y = mylagrange(xdp, j, x)
% input: xdp = abscissas of data points
% j = evaluate j-th lagrange polynomial
% x = points where polynomial or derivative is evaluated:
% scalar, vector, or matrix
% output: y = ell_j(x)
ndp = length(xdp);
y = 1; % or y = ones(size(x))
for k = 1:ndp
if k ~= j
y = y .* (x - xdp(k))/(xdp(j) - xdp(k));
end
end
end

I am a little confused about what x and j is. Here is my code:

xdp=[1 3 4 5 8];
n=length(xdp);
for  j=1:n
    for x=xdp(1:n);    
    mylagrange(xdp,j,x)
    plot(x,sum(mylagrange(xdp,j,x)))
    hold on
      end
  end

Is this correct?

Thank you!

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Bo Wang 2016-3-25

在 MATLAB Online 中打开

the function code:

if true
  % code
function y = mylagrange(xdp, j, x)
% input: xdp = abscissas of data points
% j = evaluate j-th lagrange polynomial
% x = points where polynomial or derivative is evaluated:
% scalar, vector, or matrix
% output: y = ell_j(x)
ndp = length(xdp);
y = 1; % or y = ones(size(x))
for k = 1:ndp
if k ~= j
y = y .* (x - xdp(k))/(xdp(j) - xdp(k));
end
end
end
end

Bo Wang 2016-3-25

在 MATLAB Online 中打开

my code:

if true
  % code
xdp=[1 3 4 5 8];
n=length(xdp);
for  j=1:n
  for x=xdp(1:n);    
  mylagrange(xdp,j,x)
  plot(x,sum(mylagrange(xdp,j,x)))
  hold on
  end
end
end