How to determine which points in an array are with a ring? Stupid question.

Hi All,

I have an arrays of equal length consisting of x position, y position and energy. I wish to determine which of those entries fall within a ring without resorting to doing it iteratively.

Code (which doesn't work) is:

distances = sqrt(X_Position.^2 + Y_Position.^2);

if (distances > Inner_Radius) & (distances < Outer_Radius)

X_Position_on_Instrument = X_Position;

Y_Position_on_Instrument = Y_Position;

Energy_on_Instrument = Energy;

end

In other words the new arrays (on_Instrument) will only contain values which fall within the ring. I know the answer will be really simple but I can't see it at the mo'.

Thanks again.

Regards

Tim

Answer 1

[Cris LaPierre]

Use logical indexing.

distances = sqrt(X_Position.^2 + Y_Position.^2);
% create logical index to indicate points that meet condition
ind = distances>Inner_Radius & distances<Outer_Radius;

% Use that index to extract the corresponding data
X_Position_on_Instrument = X_Position(ind);
Y_Position_on_Instrument = Y_Position(ind);
Energy_on_Instrument = Energy(ind);

[Tim Fulcher]

Cheers Cris, worked a treat.