Sorting Points (2D) clockwise
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Hello is there a function or easy way to sort 2D Points clockwise?
For example:
Given:
x = (-1,-1)
y = (4,2)
z = (-1,4)
Answer: Sorted clockwise: (y,x,z)
Thank you
采纳的回答
Clockwise about what center point?
And what decides the starting coordinate?
If you want to sort 2D coordinates by their polar angle relative to (0,0) in the clockwise direction, convert the Cartesian coordinates to polar coordinates, wrap the radian angle values to [0:2*pi], then sort the results in descending order for the clockwise direction. The first value will be the coordinate closest to 2*pi radians.
data = [-1 -1; 4 2; -1 4]
figure()
plot(data(:,1), data(:,2), 'o')
text(data(:,1)+0.2,data(:,2),{'x','y','z'})
axis equal
axis padded
grid on
% Convert to polar coordinates
rad = cart2pol(data(:,1), data(:,2));
radWrapped = mod(rad,2*pi);
radWrapped(radWrapped==0 & rad>0) = 2*pi;
[~, sortIdx] = sort(radWrapped, 'descend');
text(data(:,1)-0.2, data(:,2),compose('%d',sortIdx),'HorizontalAlignment', 'right')
10 个评论
Adam Danz
2022-2-11
Daniela Würmseer's comment moved here from the answers section
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Thank you.
but if i try out the code with the two first points exchanged so data = [-1 0; 0 -1; -1 -1]; then sortIdx = 2 3 1. This makes no sense to me normally sortIdx should be 3 1 2 or ?
The sortIdx is the order of the input values. Your input values change order so the sortIdx is expected to change order, too.
This demo below shows that the sorted inputs are the same for both sets of values.
data = [0 -1; -1 0; -1 -1];
% Convert to polar coordinates
rad = cart2pol(data(:,1), data(:,2));
radWrapped = mod(rad,2*pi);
radWrapped(radWrapped==0 & rad>0) = 2*pi;
[~, sortIdx] = sort(radWrapped, 'descend');
sortedData = data(sortIdx,:)
data2 = [-1 0; 0 -1; -1 -1];
% Convert to polar coordinates
rad = cart2pol(data2(:,1), data2(:,2));
radWrapped = mod(rad,2*pi);
radWrapped(radWrapped==0 & rad>0) = 2*pi;
[~, sortIdx2] = sort(radWrapped, 'descend');
sortedData2 = data(sortIdx,:)
The variables sortIdx and sortIdx2 show the indices of the input rows.
Daniela Würmseer
2022-2-11
Adam Danz
2022-2-11
Happy to help.
Daniela Würmseer
2022-7-2
Adam Danz
2022-7-2
@Daniela Würmseer one of your data points lies on (0,0). This solution sorts in clockwise, descending order starting at angle 0 which is, by convention, at 3 o'clock (from (0,0) to (0,x) where x is >0). Any data point at 0 degrees will be sorted last since the sort is in descending order. If you'd like points at 0 deg to be considered at 2pi, you can change this part of my answer
radWrapped(radWrapped==0 & rad>0) = 2*pi;
to
radWrapped(radWrapped==0 & rad==0) = 2*pi;
Another solution would be to center your data using
dataCentered = data - mean(data,1); % Assuming data is nx2
and then perform the solution in my answer on the centered data.
Daniela Würmseer
2022-7-5
Adam Danz
2022-7-5
Are these [x,y] coordinates? If so, the values in row-2 of the sorted list (-0.0017, -8.569) doesn't exist in the initial list. It looks like you're sorting x and y independently but, if I understand your comments correctly, you should be sorting the (x,y) pairs.
Daniela Würmseer
2022-7-5
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