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search multiple results for multiple conditions

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K VdB
K VdB 2017-8-4
关闭： MATLAB Answer Bot 2021-8-20
THE CONTEXT:
I have a set of points in a virtual 2D space. Every point has its own x and y coordinate in space.
P = [1,2 ; 2 5; 4 6; 7 8; ...] % first column are x coordinates and second column are y coordinates
If I want to find the point that is closest to the first one [1,2] I can find the distance between the first point an the other points:
dis = sqrt((P(:,1)-1).^2 + (P(:,2)-2).^2)
dis = [0 ; 3.1623 ; 5 ; 8.4853 ; ...]
if I then search for the second smallest value in 'dis' I know which point is closest to the first point
THE QUESTION:
To generate the closest point for every point I can use a for loop
for i=1:length(P)
dis = sqrt((P(:,1)-P(i,1).^2 + (P(:,2)-P(i,2).^2);
index = secondsmallest(dis) % find index of second smallest value somehow
P_indices(i) = index; % store index in an array'
end
Is there a way to find these indices without using a for loop?
.
Kind regards
Kjartan

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回答（3 个）

Star Strider
Star Strider 2017-8-4
I would use the pdist (link) function. You can use ordinary matrix indexing to find the indices of the various values. Also see the sort (link) function, as well as the functions linked in and at the end of the documentation pages.
dpb
dpb 2017-8-4
doc pdist
Jan
Jan 2017-8-4
编辑：Jan 2017-8-4
for i=1:length(P)
dis2 = (P(:,1) - P(i,1).^2 + (P(:,2) - P(i,2).^2; % Omit SQRT() to save time
index = secondsmallest(dis2); % find index of second smallest value somehow
P_indices(i) = index; % store index in an array'
end
function k = secondsmallest(x)
[~, k] = min(x);
x(k) = Inf;
[~, k] = min(x);
end
This is faster than sorting the complete vector.
Be sure that P_indices is pre-allocated before the loop.

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