问题
I would like to generate partitions for a set in a specific way: I need to filter out all partitions which are not of size N in the process of generating these partitions. The general solution is "Generate all “unique” subsets of a set (not a powerset)".
For the set S with the following subsets:
[a,b,c]
[a,b]
[c]
[d,e,f]
[d,f]
[e]
and the following 'unique' elements:
a, b, c, d, e, f
the result of the function/method running with the argument N = 2 should be:
[[a,b,c], [d,e,f]]
While the following partitions should be filtered out by the function/method:
[[a,b,c], [d,f], [e]]
[[a,b], [c], [d,e,f]]
[[a,b], [c], [d,f], [e]]
The underlying data structure is not important and could be arrays, sets or whatever.
Reason: I need to filter some partitions out before I have the full set of all partitions, because the function/method which generates all partitions is rather computationally intensive.
According to "Generating the Partitions of a Set", the number of possible partitions can be huge: 44152005855084346 for 23 elements. My data is 50-300 elements in the starting set, so I definitely need to filter out partitions that have size not equal to N before I save them anywhere.
回答1:
Once you have the partitions as given by Frederick Cheung that you linked, do:
partitions.select{|partition| partition.length == 2}
来源：https://stackoverflow.com/questions/8646396/how-do-i-generate-set-partitions-of-a-certain-size