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https://issues.apache.org/jira/browse/MAHOUT-1185?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sean Owen resolved MAHOUT-1185.
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Resolution: Not A Problem
Fix Version/s: (was: 0.8)
Diffs are computed over all pairs of prefs in the list. The diff of a pref and
itself is 0 and there is no point in computing this. Thus the outer loop is
from i=0 to length-2, and the inner loop is from j=i+1 to length-1. It is
correct as-is.
This patch would not only cause an exception (it dereferences at length) but
would incorrectly add a non-zero diff consisting just of the item's pref value
-- there's no "diff" there, it's an absolute value.
> MemoryDiffStorage.class has a bug for slope one algorithm which could cause
> incorrect recommendation results
> ------------------------------------------------------------------------------------------------------------
>
> Key: MAHOUT-1185
> URL: https://issues.apache.org/jira/browse/MAHOUT-1185
> Project: Mahout
> Issue Type: Bug
> Components: Collaborative Filtering
> Affects Versions: 0.7
> Environment: Ubuntu
> Reporter: Cunlu Zou
> Assignee: Sean Owen
> Labels: patch
> Attachments: MemoryDiffStorage.patch
>
> Original Estimate: 10m
> Remaining Estimate: 10m
>
> The function processOneUser(long averageCount, long userID) in the
> MemoryDiffStorage.class file contains a bug for calculating the itemAverage.
> Since the function tried to calculate the average difference among items (in
> a nested loop) and also the average individual item preference value in the
> same loop (the loop only from 0 to length-2, *for (int i = 0; i < length - 1;
> i++)*), the itemAverage variable does not count the last item's preference
> value for every users which could lead to an incorrect recommendation results.
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