Index of matrix using colon

I have a matrix of points split into two by X and Y (both 2D arrays) and another matrix of specific points split into two by X and Y (again both 2D arrays). I wish to find the closest point in the main matrix to each point in my other matrix.

for i = 1:length(Xpoints)

[~,Index] = min(sqrt(abs(mainmatrixX(:)-Xpoints(i) + sqrt(abs(mainmatrixY(:)-Ypoints(i))))));

end

this only ever returns the index value of 1 i assume because it's not taking the position of the colon.

Any help would be much appreciated

Answer 1

[Image Analyst]

If you have the Statistics and Machine Learning Toolbox you can use pdist2 to get the distance from every point in set 1 to every point in set 2, then use min and find to find the indexes of the closest. Untested code (because you forgot to attach your data):

% Get all distances
xy1 = [x1(:), y1(:)];
xy2 = [x2(:), y2(:)];
distances = pdist2(xy1, xy2);
% Find closest (minimum) distance.
minDistance = min(distances)
% Find out where it occurs
[rowInSet1, rowInSet2] = find(distances == minDistance)
x1Closest = xy1(rowInSet1, 1);
y1Closest = xy1(rowInSet1, 2);
x2Closest = xy2(rowInSet2, 1);
y2Closest = xy2(rowInSet2, 2);

[Christopher Longhurst]

Thank you so much

Answer 2

[Karim]

you need to save the index value into a new array, otherwise it will overwrite the value and only keep the last one

mainmatrixX = rand(100,1);
mainmatrixY = rand(100,1);

Xpoints = rand(100,1);
Ypoints = rand(100,1);


ImClosest = zeros(length(Xpoints),1);
for i = 1:length(Xpoints)
    [~,currIndex] = min(sqrt( (mainmatrixX(:)-Xpoints(i)).^2 + (mainmatrixY(:)-Ypoints(i)).^2 ));
    ImClosest(i) = currIndex;
end

ImClosest

ImClosest = 100×1

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