Hi,
Maybe, I misunderstand the description but it seems to be the case :
(line 77)
http://svn.apache.org/viewvc/mahout/trunk/core/src/main/java/org/apache/mahout/cf/taste/impl/similarity/TanimotoCoefficientSimilarity.java?view=markup

tested by :
(testNoCorrelation)
http://svn.apache.org/viewvc/mahout/trunk/core/src/test/java/org/apache/mahout/cf/taste/impl/similarity/TanimotoCoefficientSimilarityTest.java?view=markup

Correct me if I'm wrong.

Sebastian is this turned around? yes the non-distributed implementation already has this behavior. I understood from your message that you proposed to make the distributed version return the same thing.

I think it's as simple as making its similarity computation into:

return dots == 0 ? Double.NaN : dots / (normA + normB - dots);

@Remi this is for the user-user similarity computation only.

@Sean Yes, this change is trivial. I thought it would make a nice first ticket for people who want to start contributing to Mahout (that why its marked with MAHOUT_INTRO_CONTRIBUTE)

Ah, right it affects the item-item computation in the non-distributed version too. Well I have a patch ready. I'll wait a bit until I post the 'answer'

A small patch for this issue, with the corresponding test case.
Hope it's ok.

That's fine, I think we also need to change the distributed code too, which is the one-liner I mentioned above. If there are no objections I'll put it all together and commit.

Tests pass, and this looks like a good fix, and no controversy, and has a patch from the interested party, so I just committed.

cool

We must not change the distributed code. In the distributed case, 0 is equivalent to non-existent and will already be ignored by iterateNonZero().

Ah I think I misunderstood what was to change from the start. I'll revert that bit.

Integrated in Mahout-Quality #1226 (See https://builds.apache.org/job/Mahout-Quality/1226/)
MAHOUT-902 item similarity is now NaN for no overlap

srowen : http://svn.apache.org/viewcvs.cgi/?root=Apache-SVN&view=rev&rev=1210544
Files :

• /mahout/trunk/core/src/main/java/org/apache/mahout/cf/taste/impl/similarity/TanimotoCoefficientSimilarity.java
• /mahout/trunk/core/src/main/java/org/apache/mahout/math/hadoop/similarity/cooccurrence/measures/TanimotoCoefficientSimilarity.java
• /mahout/trunk/core/src/test/java/org/apache/mahout/cf/taste/impl/similarity/TanimotoCoefficientSimilarityTest.java

Integrated in Mahout-Quality #1227 (See https://builds.apache.org/job/Mahout-Quality/1227/)
MAHOUT-902 oops do not return NaN from distributed item-item simliarity as 0 means 'ignore'

srowen : http://svn.apache.org/viewcvs.cgi/?root=Apache-SVN&view=rev&rev=1210603
Files :

• /mahout/trunk/core/src/main/java/org/apache/mahout/math/hadoop/similarity/cooccurrence/measures/TanimotoCoefficientSimilarity.java