On 8/16/13 4:40 PM, GaryMak wrote:
Hi all
I hesitate to declare a <<bug>>, since it's probably something I cannot
find in the help, but the following is very strange ....
The following code always seems to output many vectors "quad" of length
4 each of whose numerical approx "quad.n()" is a vector which only has
length 3 ...
Indeed, it is a bug. The problem is that quad is a sparse vector with
zero as its last entry. You see, diagonal_matrix creates a sparse
matrix, so ZZ^indy creates a sparse vector. That means that
quad+ZZ^indy is a sparse vector. Then the bug bites us when the last
element is zero. The .n() function for a sparse vector calls this
one-liner:
vector(dict([(e[0],e[1].n(prec, digits)) for e in
self._entries.iteritems()]), sparse=True)
Notice that the size isn't specified anywhere, so it is guessed from the
biggest element of the dictionary. Since the last element was zero,
it's not in the dictionary, so the vector is shortened.
Instead, that sparse _numerical_approx function should be:
vector(R, len(self), dict([(e[0],e[1].n(prec, digits)) for e in
self._entries.iteritems()]), sparse=True)
where the ring R is somehow determined. Maybe the dictionary can be
constructed first, then the ring in which the entries live can be
determined, and then the vector call can be made.
Alternatively, a final zero element can be added to the dictionary, so
the one-liner above becomes:
entries=dict([(e[0],e[1].n(prec, digits)) for e in
self._entries.iteritems()])
if len(self)-1 not in entries:
entries[len(self)-1]=0
vector(entries, sparse=True)
since this apparently works:
v=vector({1:32,4:2,10:0},sparse=True)
Gary: are you comfortable making a trac ticket and patch for this? It
looks like the function that needs to be fixed is the very last function
in SAGE_ROOT/devel/sage/sage/modules/free_module_element.pyx:
https://github.com/sagemath/sage/blob/master/src/sage/modules/free_module_element.pyx#L4503
Thanks,
Jason
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