Can anyone help me with this matlab code?

Hello out there! I have a problem with following code. I have tre equations, which I have to find solutions to with help from newtons method in matlab. It's keep telling me, that I have a undefined variable.

function x = NEWT(x0,k)
  x=x0;
  for i=1:k
    b=F(x);
    s=DF(x)\b;
    x=x-s;
  end
end

function y = F(x)
  y=zeros(3,1); % 0-punkter i 3x1 matrix - it is a vector
  y(1)=2*x(1)^2-4*x(1)+x(2)^2+3*x(3)^2+6*x(3)+2;
  y(2)=x(1)^2+x(2)^2-2*x(2)+2*x(3)^2-5;
  y(3)=3*x(1)^2-12*x(1)+x(2)^2+3*x(3)^2+8;
end

function dy = DF(x)
  dy=zeros(3,3);
  dy(1,1)=4*x(1)-4;
  dy(1,2)=2*x(2);
  dy(1,3)=6*x(3)+6;
  dy(2,1)=2*x(1);
  dy(2,2)=2*x(2)-2;
  dy(2,3)=4*x(3);
  dy(3,1)=6*x(1)-12;
  dy(3,2)=2*x(2);
  dy(3,3)=6*x(3);
end

[Image Analyst]

How are you calling it? What values for x0 and k are you passing in?

[Walter Roberson]

Which variable is being reported as undefined, and at which line?

[Angelina]

LINE 7, I call my function NEWT. I tried with different values for x0 = 2 and else, but it keeps saying its undefined. And when I have to make my script work I just press on run. I'm very new in matlab.

[Image Analyst]

You can't do that. When it requires arguments you must first save your function then call it from the command line or another script or function and supply those arguments. You can't simply press the green run triangle button.

[Angelina]

Can you tell me how to do that... Step by step :(

[Image Analyst]

Edit and save your function by clicking on the disk icon. Then click in the command window panel. Type this (without the >> of course because that's just the MATLAB prompt):

>> NEWT(2, 3)

You can use different numbers than 2 and 3 - use whatever you think you should, I just used those as an example. It should report some errors. You fix those errors and repeat the above process until it runs to your satisfaction.

[Angelina]

So after fixing the errors, I would get what I want? Thanks a lot for the help!

[Image Analyst]

Of course. If you don't then there are still errors -- either errors in your logic and algorithm, or errors in the syntax.

Answer 1

[Image Analyst]

If you say NEWT(2,3) then x = 2, but in F(x) you say y(1)=2*x(1)^2-4*x(1)+x(2)^2+3*x(3)^2+6*x(3)+2 so then x must be an array, not a single number. What are your plans regarding that?

[Angelina]

This is my exercise: (Computer problem 2.7.4 in [1]): Use Newton's method to find the two

solutions of the system. \begin{equation*} 2u^2 - 4u + v^2 + 3w^2 + 6w + 2 = 0 \end{equation*}

\begin{equation*} u^2 + v^2 - 2v + 2w^2 - 5 = 0 \end{equation*}

\begin{equation*} 3u^2 - 12u + v^2 + 3w^2 + 8 = 0 \end{equation*}

And I didn't thought that I should put any values x0= something, but my lector told me that I had to find a point in R^3. Hmm I don't mean it is an matrix. I have three equations, which is my array, so I wouldn't call my x an array.