Newtons Form Trouble Shooting

So I am using Newton's form for polynomial interpolation and I am using code I found from "Applied Numerical Analysis Using MATLAB" by Fausett. Below is my what I have in my M-file with my nodes of interpolation (x and y).

x = [-81.091,-81.058,-80.818,-80.387,-79.789]'
y = [48,72,96,120,140]'
function a = NewtonCoef(x, y)
n = length(y)
%calculate coefficients of Newton interpolating polynomial
a(1) = y(1)
for k = 1 : n-1
  d(k,1) = (y(k+1)-y(k))/(x(k+1)-x(k));
end
for j = 2 : n-1
  for k = 1 : n-j
    d(k,j) = (d(k+1,j-1) - d(j,j-1))/(x(k+j) - x(k));
  end
end
d
for j = 2 : n
  a(j) = d(1,j-1);
end

My problem is when ever I try to run this I get the error...

 error: 'y' undefined near line 5 column 12
 error: evaluating argument list element number 1
 error: called from:
 error:   NewtonCoef at line 5, column 3

I don't get it, do I need to enter my y and x in a different way? As far as I can tell the vector should work. I am a novice user of MATLAB but have done well in the class until now.

Any help would be appreciated.

Thanks

Answer 1

[Geoff Hayes]

Carter - your definitions of x and y are fine, but these are initializations that must occur in the base workspace and not be put in the m file. From your above code, it looks like you pasted the

 x = [-81.091,-81.058,-80.818,-80.387,-79.789]'
 y = [48,72,96,120,140]'

into the file, and then tried to run it. (Though this might give a different error if you had done this.)

Make sure that the first line in your NewtonCoef.m file is the function signature

 function a = NewtonCoef(x, y)

and then do the following - in the Command Window, create the x and y as

 >> x = [-81.091,-81.058,-80.818,-80.387,-79.789]'
 >> y = [48,72,96,120,140]'

and then call your function with these two input parameters

 >> a = NewtonCoef(x,y);

When I run the above, I see that

 a =
           48       727.27            0      -1.9383            0

Note that you may want to add some semicolons at the end of your assignment lines (in NewtonCoef) to prevent the Command Window from being cluttered with extra information.