Opened 3 years ago
Closed 3 years ago
#19291 closed defect (fixed)
Graph.spanning_trees does not like loops
Reported by: | Stefan | Owned by: | |
---|---|---|---|
Priority: | major | Milestone: | sage-6.9 |
Component: | graph theory | Keywords: | |
Cc: | Merged in: | ||
Authors: | Nathann Cohen | Reviewers: | Stefan van Zwam |
Report Upstream: | N/A | Work issues: | |
Branch: | 6a86e23 (Commits) | Commit: | 6a86e239c68f0d66cc182a98e74f72b34df4f961 |
Dependencies: | Stopgaps: |
Description
G = Graph({0:{0:'a', 1:'b'}, 1:{2: 'c'}, 2:{0: 'e'}}, loops=True) S = G.spanning_trees()
goes "boom" (recursion depth exceeded). It also destroys G in the process, which shouldn't happen, I think.
Change History (5)
comment:1 Changed 3 years ago by
- Branch set to u/ncohen/19291
- Commit set to 6a86e239c68f0d66cc182a98e74f72b34df4f961
- Status changed from new to needs_review
comment:2 follow-up: ↓ 3 Changed 3 years ago by
- Reviewers set to Stefan van Zwam
- Status changed from needs_review to positive_review
I was just exploring some graph methods, I don't actually need to use it for now.
Your change forces a copy of the graph to be made regardless of whether there are loops. That's definitely safer than what happened before, but could be costly in terms of memory for big graphs. Still, I prefer the corresponding increase in safety of the code, so I will approve.
comment:3 in reply to: ↑ 2 Changed 3 years ago by
Replying to Stefan:
but could be costly in terms of memory for big graphs.
Well, considering such running time, it is likely that no one will use this method for "big graphs".
sage: g = graphs.GridGraph([2,2,2]) sage: %timeit g.spanning_trees() 10 loops, best of 3: 99.4 ms per loop sage: g = graphs.GridGraph([2,2,3]) sage: %timeit g.spanning_trees() 1 loops, best of 3: 13.4 s per loop sage: g.order(),g.size() (12, 20)
comment:4 Changed 3 years ago by
Yeah, precisely. Stefan: to answer your question, I did not mind adding a graph copy to this method because, well, given the way it is written, the amount of wasted time is so high that this copy makes no difference. If you ever need this method "seriously", it will have to be reimplemented.
Nathann
comment:5 Changed 3 years ago by
- Branch changed from u/ncohen/19291 to 6a86e239c68f0d66cc182a98e74f72b34df4f961
- Resolution set to fixed
- Status changed from positive_review to closed
Here is a fix. By the way, if you need this function for some hard computations know that there is room for much improvement in there.
Nathann
New commits:
trac #19291: Graph.spanning_trees does not like loops