Jake Mannix created MAHOUT-1197:
-----------------------------------
Summary: AbstractVector#cross is only appropriately efficient for
dense vectors
Key: MAHOUT-1197
URL: https://issues.apache.org/jira/browse/MAHOUT-1197
Project: Mahout
Issue Type: Bug
Components: Math
Affects Versions: 0.6
Reporter: Jake Mannix
Fix For: 0.8
Nobody overrides this implementation:
[code]
@Override
public Matrix cross(Vector other) {
Matrix result = matrixLike(size, other.size());
for (int row = 0; row < size; row++) {
result.assignRow(row, other.times(getQuick(row)));
}
return result;
}
[code]
I think you can imagine what kind of performance this has on sparse vectors (k
non-zeroes) with high cardinality (N) - scales as O(N^2) instead of O(k^2).
I think the right approach is to *not* implement this in AbstractVector at all,
and force concrete implementations to properly implement it performantly.
Alternatively, killing this method entirely might be appropriate. If anyone
was using it (and uses sparse vectors), they'd have complained about this by
now.
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