Why does scikit-learn's Nearest Neighbor doesn't seem to return proper cosine similarity distances?

问题

I am trying to use scikit's Nearest Neighbor implementation to find the closest column vectors to a given column vector, out of a matrix of random values.

This code is supposed to find the nearest neighbors of column 21 then check the actual cosine similarity of those neighbors against column 21.

from sklearn.neighbors import NearestNeighbors
import sklearn.metrics.pairwise as smp
import numpy as np

test=np.random.randint(0,5,(50,50))
nbrs = NearestNeighbors(n_neighbors=5, algorithm='auto', metric=smp.cosine_similarity).fit(test)
distances, indices = nbrs.kneighbors(test)

x=21   

for idx,d in enumerate(indices[x]):

    sim2 = smp.cosine_similarity(test[:,x],test[:,d])


    print "sklearns cosine similarity would be ", sim2
    print 'sklearns reported distance is', distances[x][idx]
    print 'sklearns if that distance was cosine, the similarity would be: ' ,1- distances[x][idx]

Output looks like

sklearns cosine similarity would be  [[ 0.66190748]]
sklearns reported distance is 0.616586738214
sklearns if that distance was cosine, the similarity would be:  0.383413261786

So the output of kneighbors is neither the cosine distance or the cosine similarity. What gives?

Also, as an aside, I thought sklearn's Nearest Neighbors implementation was not an Approximate Nearest Neighbors approach, yet it doesn't seem to detect the actual best neighbors in my dataset, compared to the results I get if i iterate over the matrix and check the similarities of column 211 to all the other ones. Am I misunderstanding something basic here?

回答1:

Ok the problem was that NearestNeighbors's .fit() method, by default assumes the rows are samples and the columns are features. I had to tranpose the matrix before passing it to fit.

EDIT: Also, another problem is that the callable passed as metric should be a distance callable, not a similarity callable. Otherwise you'll get the K Farthest Neighbors :/

来源：https://stackoverflow.com/questions/23032628/why-does-scikit-learns-nearest-neighbor-doesnt-seem-to-return-proper-cosine-si