i get a wrong answer for my code, can someone please help!

Hey everybody!

(see Attached file in the hyperlink: http://www.docdroid.net/ozso/exercise-matlab-1.pdf.html

I need to calculate an estimated value of the circle area. Giving some x coordinates and y coordinates. i Have written the following code, but there is something wrong with it. can test my code by the test script:

 circleAreaMC([-0.1, 0.7, 0.8, 0.5, -0.4], [0.3, -0.1, 0.9, 0.6, -0.3]) and it should give 3.2. But it gives 0.303 etc.

I really feel like this problem have a much easier solution, can somebody please help me? I am new to matlab. (started 5 days ago :-) )

function A = circleAreaMC(xvals,yvals)
%A function that estimates the area of a circle by Monte Carlo simulation.
N=5;
InC = zeros(size(xvals));
DTC = zeros(size(xvals));
%A loop that test wether or not the points are inside the circle:
for i=1:length(xvals)
  %Distance to center:
  DTC(i)=norm(xvals(i))+norm(yvals(i));
  %Inside the circle:
  InC(i)=DTC(i)<=1;
end
%the 2 vectors:
xvals = (xvals.*InC(i));
yvals = (yvals.*InC(i));
xvals(xvals==0)=[];
yvals(yvals==0)=[];
x = zeros(size(xvals));
y = zeros(size(xvals));
%for loop:
for n=2:length(xvals)
  x(n)=xvals(n)-xvals(1);
  y(n)=yvals(n)-yvals(1);
end    
x(x==0)=[];
y(y==0)=[];
area = 0;
for q = 1:(length(x)-1)
  v = [x(q);y(q);0];
  v1 = [x(q+1);y(q+1);0];
  cv = cross(v,v1);
  area = area+norm(cv*cv');
end
disp(area);

Answer 1

[Titus Edelhofer]

Hi,

I'm not sure what you are doing in the second half of the function, but shouldn't the line

DTC(i)=norm(xvals(i))+norm(yvals(i));

read more like

DTC(i) = norm([xvals(i) yvals(i)]);

or

DTC(i) = sqrt(xvals(i).^2 + yvals(i).^2);

Titus

[Ibrahim]

Thanks!

At first i had written: DTC(i) = sqrt(xvals(i).^2 + yvals(i).^2); but changeed it to norm.... after reading more about matlab (i am a new user) but yes there is a big difference between:

DTC(i)=norm(xvals(i))+norm(yvals(i)); and DTC(i) = norm([xvals(i) yvals(i)]);

that is corrected now! thanks for your time!

Answer 2

[Julia]

Hi,

I agree with Titus that the line

DTC(i)=norm(xvals(i))+norm(yvals(i));

looks strange. Did you mean abs() here?

And the lines below

%Inside the circle:
  InC(i)=DTC(i)<=1;

do not test if the points are inside the circle.

Do you mean something like that:

If DTC(i)<=1
   InC(i)=DTC(i)
end

?

[Titus Edelhofer]

Inc(i) = DTC(i)<=1; is a test for being inside. InC(i) is 1 if DTC(i)<=1, and zero else.

Titus

[Ibrahim]

Thanks Julia!

it Works now!! :-D

Answer 3

[Ibrahim]

mmm.. Guys?

now it writes the answer 3.2 in the command window, but not : ans = 3.2 in the workspace ? how come :/

[Julia]

A is your return value of the function, set

A=area;

and A should be stored in the workspace.

[Ibrahim]

aaah! thanks!