MATLAB Answers

## Find neighboring nodes in a list of node positions

Asked by Raldi

### Raldi (view profile)

on 8 Mar 2018
Latest activity Answered by Image Analyst

### Image Analyst (view profile)

on 9 Mar 2018
Accepted Answer by Image Analyst

### Image Analyst (view profile)

I have two vectors of X and Y positions
x = randn(10,1);
y = randn(10,1);
So x(1) with y(1) together make the position of node 1
What I want to do is find all other nodes that are lets say in a distance of d=.3 from that node, and do this for all nodes in the list.
What is a good vectorized way of doing this without having a double for loop?

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## 2 Answers ### Image Analyst (view profile)

Answer by Image Analyst

### Image Analyst (view profile)

on 9 Mar 2018
Accepted Answer

If you have the Statistics and Machine Learning Toolbox, you can simply use pdist2() to find distances between all possible pairs of x,y locations:
% Set up:
x = randn(10,1);
y = randn(10,1);
xy = [x, y];
% Find distances between every possible pair of points:
distances = pdist2(xy, xy)
% Find distances less than 0.3
closeDistances = distances < 0.3 & distances > 0
% Get indexes of those close distances.
[rows, columns] = find(closeDistances)
% Done! Now, for info, print out those nodes that are close
for k = 1 : length(rows)
fprintf('Node %d and %d are close, with a distance between them of %.3f.\n',...
rows(k), columns(k), distances(rows(k), columns(k)));
end

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Answer by David Fletcher

### David Fletcher (view profile)

on 8 Mar 2018

Something like this:
x = randn(10,1);
y = randn(10,1);
%set desired distance
maxDistance=1.6;
%set node of interest
node=4;
pos=[x y];
nodeCoords=pos(node,:);
%Calculate distance from specified node (euclidean)
distFromNode=sqrt((pos(:,1)-nodeCoords(1,1)).^2+(pos(:,2)-nodeCoords(1,2)).^2);
%Create index to points within specified maxDistance
ind=distFromNode<=maxDistance;
%List of node indexes within specified distance
indexes=1:length(x);
isWithin_Index=indexes(ind)
%List of node coords within specified distance
isWithin_Position=pos(ind,:)
Though you may want to modify it so that the node doesn't report that it is within maxDistance of itself

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