MATLAB: I have 2 matrices, containing x,y data, how do i find the closest point and extract data about that point

nearest neighbour within 2 datasets

Hey matlabians!

A is a matrix with two columns, A= [X,Y], that give the position x and y.

B is a matrix with 3 columns,B=[X,Y,P], the position x and y, and P is simply a value assigned to that position in space.

I could use a nearest neighbour search to find the value of B closest to A, what i actually want is for every point A find the closest B and give A the value of P.

I have seen nearest neighbor analysis attempted elsewhere but that has always been for one dataset, is there a command for this? does anyone have any help, it would help me out no end. Many thanks Rich

Best Answer

Why don't you use knnsearch in MATLAB and the indices of the point that is closest in B that in A, and use the index to extract the P value.

IDX = knnsearch(B(:,1:2),A)
B(:,IDX)

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MATLAB: Find the closest point and taking the difference of y coordinates

I recommen using pdist2() to calculate a matrix of distances between all points in A and all points in B. See comments within the block of code below.

% produces fake data
A = randi(100,3,2);
B = randi(100,5,2);
distance  = pdist2(A,B); 
% distance(i,j) is the distance between A(i,:) and B(j,:)

To find the nearest point in B to A(m,:),

% for m = 2, the row in B closest to A(m,:) is "minIdx"
m = 2; 
[~, minIdx] = min(distance(m,:));

I didn't follow the second part of your quesiton but here's how to pull those two coordinates from A and B.

m2 = A(m,:)
n2 = B(minIdx,:)

MATLAB: Compute,store, and find min of 3d distances between two different matrices.

Here's what I use, but there are better optimized alternatives on the FEX.

 [distances,index]=min(interdists(A,B),[],2);
function Graph=interdists(A,B)
%Finds the graph of distances between point coordinates
%







% (1) Graph=interdists(A,B)
%
% in:
% 




% A:  matrix whose columns are coordinates of points, for example 
%          [[x1;y1;z1], [x2;y2;z2] ,..., [xM;yM;zM]]
%     but the columns may be points in a space of any dimension, not just 3D.
% 
% B:  A second matrix whose columns are coordinates of points in the same
%     Euclidean space. Default B=A.
% 
% 
% out:
% 
% Graph: The MxN  matrix of separation distances in l2 norm between the coordinates. 
%        Namely, Graph(i,j) will be the distance between A(:,i) and B(:,j).
%
%
% (2) interdists(A,'noself') is the same as interdists(A), except the output 
%     diagonals will be NaN instead of zero. Hence, for example, operations
%     like min(interdists(A,'noself')) will ignore self-distances.
%
% See also getgraph
noself=false;
if nargin<2  
    B=A; 
elseif ischar(B)&&strcmpi(B,'noself')
    noself=true;
    B=A;
end
N=size(A,1);
B=reshape(B,N,1,[]);
Graph=l2norm(bsxfun(@minus, A, B),1);
Graph=permute(Graph,[2,3,1]);
if noself
    n=length(Graph);
   Graph(linspace(1,n^2,n))=nan; 
function out=l2norm(X,varargin)
%out=l2norm(X,varargin)
%
%Takes the L2 norm along desired dimensions of the array X.
%
%VARARGIN can be any of the additional arguments of the sum.m function
%and follows the same conventions as this and similar functions. That is, 
%if VARARGIN={}, the function operates along the first non-singleton 
%dimension, etc...
%
%SEE ALSO sumsq
X=X.^2;
out=sum(X,varargin{:});
out=sqrt(out);

Best Answer

Related Solutions

MATLAB: Find the closest point and taking the difference of y coordinates

MATLAB: Compute,store, and find min of 3d distances between two different matrices.

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