#8284: IntervalGraph generator and a bug in is_chordal
-----------------------------+----------------------------------------------
Reporter: ncohen | Owner: ncohen
Type: defect | Status: positive_review
Priority: critical | Milestone: sage-4.4.4
Component: graph theory | Keywords:
Author: Nathann Cohen | Upstream: N/A
Reviewer: Robert Miller | Merged:
Work_issues: |
-----------------------------+----------------------------------------------
Comment(by jason):
You might be able to get away with just creating a class that doesn't have
a defined hash function, so that the default (the memory address) is used.
The problem with using a tuple is that the two hash values below are the
same:
{{{
sage: a=(1,2)
sage: b=(1,2)
sage: hash(a)
3713081631934410656
sage: hash(b)
3713081631934410656
}}}
We can use a feature of user-defined classes, though:
"User-defined classes have __cmp__() and __hash__() methods by default;
with them, all objects compare unequal (except with themselves) and
x.__hash__() returns id(x)."
This means we can get vertices which contain objects which normally would
be considered equal in a dictionary to be stored as two different things
in a dictionary:
{{{
sage: class Vertex(object):
....: def __init__(self, value):
....: self.__value=value
....:
sage: a=Vertex((1,2))
sage: b=Vertex((1,2))
sage: a is b
False
sage: hash(a)
4528980944
sage: hash(b)
4528980816
sage: Graph({a: [b]})
Graph on 2 vertices
}}}
Does that address the problem? It introduces a problem, in that now you
can't do:
{{{g[Vertex((1,2))]}}}
to get the neighbors, since, of course, the element you are creating is
not the same as any of the vertices of the graph. Also, you have to use
v.__value to get the interval (or better, make some accessors for the
attribute (and if you want, try to disallow changing it).
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/8284#comment:14>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
--
You received this message because you are subscribed to the Google Groups
"sage-trac" group.
To post to this group, send email to [email protected].
To unsubscribe from this group, send email to
[email protected].
For more options, visit this group at
http://groups.google.com/group/sage-trac?hl=en.
