Version 1.3 of ai05s/ai05-0047-1.txt
!standard G.3.1 07-08-02 AI05-0047-1/03
!class binding interpretation 07-04-04
!status work item 07-04-04
!status received 07-04-04
!subject Annoyances in the array packages
I am using this AI to record a number of annoyances or bugs that I encountered during
the implementation of the array packages.
1 - The multiplication of matrices is defined to "involve inner products" (G.3.1(56/2)).
In strict mode, this requires that each component of the result comply with the
requirements of G.3.1(83/2-84/2). Technically this is known as a "componentwise"
error bound. An algorithm that gives a componentwise error bound is necessarily
cubic (i.e., in O(N**3)). There exist however algorithms for multiplying matrices
that are sub-cubic (e.g., Strassen's method) but these algorithms give normwise
error bounds, meaning that the error on one component may spill over other components.
Some variants of BLAS and other widely-used linear algebra libraries use fast matrix
multiplication algorithms, so the accuracy requirements makes it impossible to
implement Generic_Real_Arrays by interfacing to these libraries.
2 - The definition of Eigensystem does not impose any constraint on the length of the
out parameters Values and Vectors. Instead it has the mysterious sentence "The index
ranges of the parameter Vectors are those of A". It is written as if Eigensystem had
a way to change the constraints of Vectors, which is evidently false. It would seem
that it should require that Values'Length, Vectors'Length(1) and Vectors'Length(2) be
all equal and equal to A'Length(1).
3 - For some matrices, the QR iteration used to compute the eigenvalues will just not
converge (no deflation will happen). While it is always possible to let it run forever,
it is typical in this case to give up after some number of iterations. The RM makes
no provision for raising an exception. The values computed for (some of) the eigenvalues
may be really bogus, so it would be better to raise an exception than to return garbage
to the user.
4 - The index subtype for types Real_Vector and Real_Matrix is Integer. Presumably this was
intended to provide maximum flexibility in selecting the index range. It has however
unpleasant consequences. Consider:
Identity: constant Real_Matrix := ((1.0, 0.0), (0.0, 1.0));
Anyone trying to evaluate Identity'First - 1 won't like the result. Why would anyone
do that? Maybe to initialize a variable that will be used to iterate through the rows
or column of the matrix. This is sure to bite many users, and using a slightly narrower
index subtype would have been much wiser.
5 - G.3.2(75/2, 76/2) defines a function "abs" that returns the Hermitian L2-norm of a
vector. However, the specification of this function is given as:
function "abs" (Right : Complex_Vector) return Complex;
The norm of a vector is always a (nonnegative) real number, so it doesn't make much sense
to return a complex number here. This function should return a Real'Base. As a matter
of fact it did in AI95-00418, but this AI was apparently incorrectly merged into the
RM and Amendment.
6 - Section G.3.2 keeps talking about inner product, but never defines exactly what is meant
by this term. This is significant because in a complex vector space the natural inner
product is the Hermitian one, where the elements of the second vector are conjugated.
It is unclear if the function "*" conjugates the elements of Right. ISO/IEC 13813
explicitly specified that "no complex conjugation is performed". While the "* operator
defined by such a rule is not a true inner product, it is probably more appropriate in
practice as it makes the conjugations explicit in the source: the user has to write
X * Conjugate (Y) which mimics the mathematical notation where conjugation is always
made explicit. At any rate, a clarification would be useful.
7 - The eigenvectors returned by Eigensystem in Generic_Real_Arrays
and Generic_Complex_Arrays are not well specified, and this has
unfortunate effects. One problem is that the result of these subprograms
is hard to test. A more serious problem is that different implementations
may return different eigenvectors, and this may cause portability
problems. Also, even in a single implementation, changes to the internal
algorithms, or to the code generated for the annex G generics, may cause
the result of Eigensystem to change. This non-determinism is unpleasant.
Consider first the real case. Given a set of orthonormal eigenvectors,
changing the signs of any subset of these vectors also result is a set of
orthonormal eigenvectors (this is easily seen geometrically: it amount to
changing the direction of the vectors in an orthonormal basis). It would
be nice to be more specific.
Consider now the complex case. Things are more nasty here because there
is much more freedom: multiplying each vector by a complex number of
modulus 1 leaves the set of eigenvectors orthonormal. Again, it would be
nice to lift the ambiguity.
[Summary of private mail of July 2007; used by demand, er, permission.]
It appears that the eigenvectors returned by Eigensystem in
Generic_Real_Arrays and Generic_Complex_Arrays are not well specified, and
this has unfortunate effects. One problem is that the result of these
subprograms is hard to test. A more serious problem is that different
implementations may return different eigenvectors, and this may cause
portability problems. Also, even in a single implementation, changes to
the internal algorithms, or to the code generated for the annex G
generics, may cause the result of Eigensystem to change. This
non-determinism is unpleasant.
Consider first the real case. If V1, V2, ..., Vn are eigenvectors that
are mutually orthonormal, then changing the signs of any subset of these
vectors also result is a set of eigenvectors that are mutually orthonormal
(this is easily seen geometrically: it amount to changing the direction of
the vectors in an orthonormal basis). It would have been nice to be more
specific, for instance by requiring the first nonzero component of each
vector to be positive.
Consider now the complex case. Things are more nasty here because you
have much more freedom: you can multiply each vector by a complex number
of modulus 1, and still have an orthonormal set. One way to lift the
ambiguity would be to require that the first nonzero component of each
vector be a positive real. But this would require a lot of complex
divisions, which would degrade the accuracy somewhat. Not sure what to do
Another source of difficulty is if two or more eigenvalues are the same.
This results in a whole subspace of vectors and any orthogonal set in that
subspace will do. I don't see how one could overcome that easily either.
True, but it's hard to be excited about this. Even if the matrix has two
mathematically identical eigenvalues, it's hard to believe that the
numerical algorithm used will find the floating-point values of the
eigenvalues be exactly identical, so the eigenvectors won't form a whole
subspace after all. If, conversely, it turns out that the matrix didn't
have mathematically identical eigenvalues, but the numerical algorithm
produces two identical floating-point values, then the whole thing is
probably ill-conditioned, so you get what you get.
Questions? Ask the ACAA Technical Agent