ASKSAGE: Sage Q&A Forum - RSS feedhttps://ask.sagemath.org/questions/Q&A Forum for SageenCopyright Sage, 2010. Some rights reserved under creative commons license.Fri, 04 Jan 2013 18:50:38 +0100Recursive backtracking function: how to clear variables on new function call?https://ask.sagemath.org/question/9688/recursive-backtracking-function-how-to-clear-variables-on-new-function-call/I've been trying to write a simple function to make lists of all the Young diagrams/partitions between a starting partition ('top') and an ending partition ('bot'), with each step in the list a covering relation (exactly one box difference in each step).
For instance, I would like to input [2,1] for top and [1] for bot and get
[[2,1],[2],[1]]
[[2,1],[1,1],[1]]
as my outputs. (I want to do this because it's useful for some computations with Littlewood-Richardson coefficients; having the lists that give each partition in a path between top and bot will be useful for other computations.)
My code at present has an echo, and worse, I can't figure out where to initialize path_list. Where do I put path_list = [] -- or create other lists -- to make sure that:
1. I get a separate list for each path, and
2. Calling the function again clears path_list?
(2) is worst: right now, since path_list is an internally-defined variable, I get the following bad behavior:
sage: load "makepartitionlist.sage"
sage: get_path([1],[1])
[[1]]
[[1]]
sage: get_path([1],[1])
[[1], [1]]
[[1], [1]]
sage: get_path([1],[1])
[[1], [1], [1]]
[[1], [1], [1]]
because path_list is not cleared when I call get_path again (which I'd hoped putting path_list = [] in the arguments would accomplish).
The code:
def get_path(top, bot, path_list = []):
path_list.append(top)
if top == bot:
print path_list
return path_list
if not Partition(top).contains(Partition(bot)):
return None
for i in range(0,len(Partition(top).down_list())):
p = Partition(top).down_list()[i]
if Partition(p).contains(bot):
path_list.append(p)
get_path(p, bot, path_list)
I'm using Sage 5.0.1 with Sage-combinat providing the Partition function and others.kaisatFri, 04 Jan 2013 18:50:38 +0100https://ask.sagemath.org/question/9688/