how to match the number of elements of matrix used in find function matlab

Question

I have written a function to assign training examples to their closest centroids as part of a K-means clustering algorithm. It seems to me that the dimensions are satisfied and the code runs correctly at times. But frequently, I get the error

In an assignment A(:) = B, the number of elements in A and B must be the same.

for the line

idx(i) = find(dist == value);

Here is the code

function idx = findClosestCentroids(X, centroids)

K = size(centroids, 1);
idx = zeros(size(X,1), 1);
dist = zeros(K, 1);

for i = 1:size(X,1)
   for j = 1:K
       dist(j) = sum((X(i,:) - centroids(j,:)).^2);
   end
   value = min(dist);
   idx(i) = find(dist == value);
end

What is the problem here?

Answer 1

This is because you are potentially finding more than one cluster that share the same distance to a query point. find determines all values that satisfy the Boolean condition as the argument. idx(i) implies that you are assigning a single value to the location of the idx array but find may yield more than one value and that gives the assignment error that you are seeing.

Instead use the second output argument of min which determines the index of first time the smallest value occurs, which is exactly what you want to accomplish:

function idx = findClosestCentroids(X, centroids)

K = size(centroids, 1);
idx = zeros(size(X,1), 1);
dist = zeros(K, 1);

for i = 1:size(X,1)
   for j = 1:K
       dist(j) = sum((X(i,:) - centroids(j,:)).^2);
   end
   [~,idx(i)] = min(dist); %// Change
end