Can anyone help me with this matlab code?

2012 12 2
1 个回答
10 次查看（30 天）

0 个投票

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

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Image Analyst
Image Analyst 2012-12-2
How are you calling it? What values for x0 and k are you passing in?
Walter Roberson
Walter Roberson 2012-12-2
Which variable is being reported as undefined, and at which line?
Angelina
Angelina 2012-12-2
编辑：Image Analyst 2012-12-2
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
Image Analyst 2012-12-2
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
Angelina 2012-12-2
Can you tell me how to do that... Step by step :(
Image Analyst
Image Analyst 2012-12-2
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
Angelina 2012-12-2
So after fixing the errors, I would get what I want? Thanks a lot for the help!
Image Analyst
Image Analyst 2012-12-2
Of course. If you don't then there are still errors -- either errors in your logic and algorithm, or errors in the syntax.

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回答（1 个）

Image Analyst
Image Analyst 2012-12-2

0 个投票

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?

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Angelina
Angelina 2012-12-2
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.

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Angelina
Angelina
2012-12-2

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