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https://issues.apache.org/jira/browse/MAHOUT-790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13088506#comment-13088506
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Lance Norskog commented on MAHOUT-790:
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bq. It really helps some algorithms to be able to pull the primary diagonal of
a matrix out as a vector. So yes, that is needed.
This sounds like a utility method? The different Matrix data structures may
want to have different implementations of viewing it; I can see a disastrous
clash between a 'sequential' Matrix and pulling diagonals in one go. It may be
one of those cases where each use of this is somewhat customized, and the
surrounding code knows the matrix implementation. That is, an algorithm for
sequential matrices is carefully coded around this fact, and so how it uses a
diagonal will also have this profile.
So, static utility method and "you know the problem space" are the two uses for
this?
I go on about this because I tried to make a generic read-only Matrix and
Vector, and then random sub-classes of those. This exercise showed the design
tensions so I'm now wary of adding more features which subclasses must consider.
> Redundancy in Matrix API, view or get?
> --------------------------------------
>
> Key: MAHOUT-790
> URL: https://issues.apache.org/jira/browse/MAHOUT-790
> Project: Mahout
> Issue Type: Improvement
> Affects Versions: 0.5
> Reporter: Ted Dunning
> Fix For: 0.6
>
> Attachments: MAHOUT-790.patch
>
>
> We have a bunch of redundant methods in our matrix interface. These include
> things that return views of parts of the matrix:
> {code}
> Matrix viewPart(int[] offset, int[] size);
> Matrix viewPart(int rowOffset, int rowsRequested, int columnOffset, int
> columnsRequested);
> Vector viewRow(int row);
> Vector viewColumn(int column);
> {code}
> and things that do the same but call refer to getting stuff
> {code}
> Vector getColumn(int column);
> Vector getRow(int row);
> double getQuick(int row, int column);
> int[] getNumNondefaultElements();
> Map<String, Integer> getColumnLabelBindings();
> Map<String, Integer> getRowLabelBindings();
> double get(String rowLabel, String columnLabel);
> {code}
> To my mind, get implies a get-by-value whereas view implies get-by-reference.
> As such, I would suggest that getColumn and getRow should disappear. On the
> other hand, getQuick and get are both correctly named.
> This raises the question of what getNumNondefaultElements really does. I
> certainly can't tell just from the signature. Is it too confusing to keep?
> Additionally, what do people think that getColumnLabelBindings and
> getRowLabelBindings return? A mutable map? Or an immutable one?
> Under the covers, viewRow and viewColumn (and the upcoming viewDiagonal) have
> default implementations that use MatrixVectorView, but AbstractMatrix doesn't
> have an implementation for getRow and getColumn.
> In sum, I suggest that:
> - getRow and getColumn go away
> - the fancy fast implementations fo getRow and getColumn that exist be
> migrated to be over-rides of viewRow and viewColumn
> - there be a constructor for AbstractMatrix that sets the internal size
> things correctly.
> - that the internal cardinality array in AbstractMatrix goes away to be
> replaced by two integers.
> - viewDiagonal() and viewDiagonal(length) and viewDiagonal(row, column) and
> viewDiagonal(int row, column, length) be added.
> I will produce a patch shortly.
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