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https://issues.apache.org/jira/browse/MAHOUT-1090?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13479016#comment-13479016
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Sean Owen commented on MAHOUT-1090:
-----------------------------------
That is true. I suppose the problem I have with this is that the missing
entries are assumed to be 0, and they are not in general, and so this does not
give good results. You would usually let the missing values be some mean value.
> Add a similarity implementation that computes cosine over all entries
> ---------------------------------------------------------------------
>
> Key: MAHOUT-1090
> URL: https://issues.apache.org/jira/browse/MAHOUT-1090
> Project: Mahout
> Issue Type: New Feature
> Components: Collaborative Filtering
> Affects Versions: 0.7
> Reporter: Yann Moisan
> Assignee: Sean Owen
> Priority: Minor
> Fix For: 0.8
>
>
> The aim of this feature is to use a recommender to compute similarities as
> the hadoop RowSimilarityJob. It will be faster for small dataset because
> in-memory. So we need an in-memory implementation of the Cosine Similarity
> which computes cosine over all entries (UncenteredCosineSimilarity use only
> entries that are in both vectors).
> Here is my implementation (doesn't support refresh for the moment):
> import java.util.Collection;
> import java.util.HashMap;
> import java.util.Map;
> import org.apache.mahout.cf.taste.common.Refreshable;
> import org.apache.mahout.cf.taste.common.TasteException;
> import org.apache.mahout.cf.taste.impl.similarity.AbstractItemSimilarity;
> import org.apache.mahout.cf.taste.model.DataModel;
> import org.apache.mahout.cf.taste.model.PreferenceArray;
> public class CosineSimilarity extends AbstractItemSimilarity {
> protected CosineSimilarity(DataModel dataModel) {
> super(dataModel);
> }
> @Override
> public void refresh(Collection<Refreshable> alreadyRefreshed) {
> throw new UnsupportedOperationException();
> }
> @Override
> public double itemSimilarity(long itemID1, long itemID2) throws
> TasteException {
> DataModel model = getDataModel();
> PreferenceArray xPrefs = model.getPreferencesForItem(itemID1);
> PreferenceArray yPrefs = model.getPreferencesForItem(itemID2);
> double sumXY = 0;
> double sumX2 = 0;
> double sumY2 = 0;
> Map<Long, Float> mX = new HashMap<Long, Float>();
> for (int xPrefIndex = 0; xPrefIndex < xPrefs.length(); xPrefIndex++) {
> float x = xPrefs.get(xPrefIndex).getValue();
> mX.put(xPrefs.get(xPrefIndex).getUserID(), x);
> sumX2 += x * x;
> }
> for (int yPrefIndex = 0; yPrefIndex < yPrefs.length(); yPrefIndex++) {
> float y = yPrefs.get(yPrefIndex).getValue();
> Float x = mX.get(yPrefs.get(yPrefIndex).getUserID());
> if (x != null) {
> sumXY += x * y;
> }
> sumY2 += y * y;
> }
> return sumXY / (Math.sqrt(sumX2) * Math.sqrt(sumY2));
> }
> @Override
> public double[] itemSimilarities(long itemID1, long[] itemID2s) throws
> TasteException {
> int length = itemID2s.length;
> double[] result = new double[length];
> for (int i = 0; i < length; i++) {
> result[i] = itemSimilarity(itemID1, itemID2s[i]);
> }
> return result;
> }
> }
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