Hi Martin,
On 10 Jul., 20:38, Martin Albrecht <martinralbre...@googlemail.com>
wrote:
> Which makes sense because we always return a zero matrix when we allocate in
> M4RI, so copy() is alloc + memcpy while just creating it is just an alloc.
How does that work? I learnt the hard way: Allocating memory does not
necessarily mean that the memory is initialised by zeroes. Perhaps the
other matrix classes can benefit from M4RI's way to create initialised
memory?
> I guess one approach could be a class attribute which determines whether a
> cache should be used or not (i.e. by type). Another one would decide based on
>
> some dimension (I don't know which since that depends on the implementation
> heavily).
According to Robert, the fastest way to create an empty matrix depends
not only on the class but on the dimensions. My main concern is that
doing an elaborate test - repeated each time a zero matrix is created
- would considerably slow things down. Therefore, my first suggestion
was to introduce a static method of the matrix classes (see post
above).
But I forgot that the test actually needs to be done only once for
each matrix class: It is an ideal candidate for a lazy attribute of
MatrixSpace_generic! Hence, we may do something like
@lazy_attribute
def _copy_zero(self):
if self.__is_sparse:
return False
if self.__matrix_class ==
sage.matrix.matrix_mod2_dense.Matrix_mod2_dense:
return False
if self.__matrix_class ==
sage.matrix.matrix_modn_dense.Matrix_modn_dense:
if self._nrows>... and self._ncols>...:
return False
else:
return True
...<do something for matrices over ZZ and so on>...
return True
def __call__(self,...):
...
if entries is None or entries == 0:
if self._copy_zero: # faster to copy than to create a new
one.
return self.zero_matrix().__copy__()
else:
return self.__matrix_class(self, {}, coerce=coerce,
copy=copy)
Cheers,
Simon
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