RealVector interface could use some iterators (dense and sparse) and generic
map() and collect() methods.
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Key: MATH-312
URL: https://issues.apache.org/jira/browse/MATH-312
Project: Commons Math
Issue Type: New Feature
Affects Versions: 2.0
Environment: all
Reporter: Jake Mannix
Fix For: 2.1
As discussed on the [math] list, there are other projects out there which would
love to get a chance to standardize on using commons-math for things like
linear algebra primitives, as it would build a common base to build upon. But
to do that, some well-known and used techniques for dealing with vectors, for
one thing, are missing. Most glaringly is the treatment of sparse vectors:
giving no Iterator for non-default values means external clients lose the
advantage of sparseness - only internal methods can skip around.
Extending the RealVector interface with sparse (and dense) iterator methods
would fix this:
{code}
double getDefaultValue();
Iterator<RealVector.Entry> iterator();
Iterator<RealVector.Entry> nonDefaultIterator();
{code}
but there is another way to deal with vector data as well: instead of passing
iterators around, and worrying about all the lovely ConcurrentModification and
unsupported "remove" methods (which aren't the end of the world), we can
instead expose generic map functions:
{code}
RealVector map(UnivariateRealFunction f);
RealVector mapToSelf(UnivariateRealFunction f);
{code}
where RealVector mapToSelf(UnivariateRealFunction), which applies the function
to the vector's entries (checking whether the function preserves the default
value up front allows it to chose between the sparse or dense iterator), and
map just applies mapToSelf to a copy.
This doesn't exhaust all possible places where Iterators could be used
helpfully (there's also combining two vectors together via a
{code}map(BinaryRealFunction, RealVector other){code} which could be
specialized nonlinear forms of addition or subtraction, and {code}double
collect(UnivariateRealFunction, BinaryRealFunction){code} which uses the
iterates over all of the entries, applying the first unary function to each
entry, and then applying the binary function to combine this value with the
previous accumulated value - with "pow(2)", and "+" as the two functions, you
get L2 norm, with "abs()" and "+", you get L1 norm, etc...)
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