How can I calculate the number of data points within the area of a cirle

Basically I need to select a data point within a certain range (which I cant do) then calculate the number of data points within the area of a circle of specific radius (which i have absolutely no idea how to do)

if someone can help me out it would be much appreciated

[Star Strider]

You need to be a bit more specific. Assuming that your data are (x,y) pairs:

What is the nature of the data that makes it impossible for you to select data within a certain range? What is the range and what are the data?

Do you know the center of the circle as well as its radius?

Answer 1

[Image Analyst]

You can use the first code sample in the FAQ: http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F Just sum circlePixels to sum up the pixels inside the circle.

Answer 2

[Babak]

 data = [25 4 -10 -8 20 -5 -15 2 12 22 1000];
 range = [-10 10];

 % data-range(1)>0
 % data-range(2)<0
 sum((data-range(1)>0) & (data-range(2)<0))

 % gives you the number of elements in data that are in the range

for the case of a circle and the elements in a circle you can use the same strategy but in polar coordinates

Answer 3

[Matt]

I am using variable of type double and they are x,y pairs. I need to select data within a certain range. With this selected data I will pick a specific point, i then want to look at all of the data within the area of a circle around that point..

Answer 4

[Babak]

Here is how you do it for numbers in a circle:

 x = [6 -5 -2 3 -1 0 2 1 8 -12 7 6 -6 5 -7 41 3 ];
 y = [3 2 -4 -4 -1 77 8 -6 3 -3 3 -9 -12 8 -7 2 -9 ];
 center_of_circle=[3 -5];
 radius=6;

 x0 = x - center_of_circle(1);
 y0 = y - center_of_circle(2);

 sum(x0.^2+y0.^2-radius^2<0)  % gives you the number of elements in the circle

Answer 5

[Star Strider]

I'm not claiming this is the most elegant or most efficient way to do it, but here's my solution:

% Generate data ...
XY = (0.5-rand(45,2))*50;

% Select ranges ...
xrange = [-20 20];
yrange = [ -5 45];

% Find data within original ranges ...
xidx = find( (XY(:,1) >= xrange(1)) & (XY(:,1) <= xrange(2)) );
yidx = find( (XY(:,2) >= yrange(1)) & (XY(:,2) <= yrange(2)) );

didx = intersect(xidx,yidx);

% Select a point ...
sampleidx = randi([1 length(didx)]);

% Determine a center & radius ...
circtr = XY(didx(sampleidx),:);
cirrad = 15;

% Calculate radii of all points from center ...
radii = hypot( (XY(:,1)-circtr(1)), (XY(:,2)-circtr(2)) );

% Find the radii <= defined radius
incirc = find( radii <= cirrad);
excirc = setdiff([1:size(XY,1)],incirc);

% Generate table of data within radius ...
data = XY(incirc,:);

% Calculate circle for plot ...
cirx = [(circtr(1)-cirrad) : 0.1 : (circtr(1)+cirrad)];
cirplt = [circtr(1)+cirrad*cos([0:0.025:1]'*2*pi)  circtr(2)+cirrad*sin([0:0.025:1]'*2*pi)];

figure(1)
plot( cirplt(:,1), cirplt(:,2), '-b')
hold on
plot( XY(incirc,1), XY(incirc,2), '.r')
plot( XY(excirc,1), XY(excirc,2), '.g')
hold off
axis equal
grid

The number of data within the circle is length(incirc) if that information is also important.