socialranking (version 1.2.0)

powerRelationGenerator: Generate power relations

Description

Based on a list of coalitions, create a generator function that returns a new PowerRelation object with every call. NULL is returned once every possible power relation has been generated.

Alternatively, use generateRandomPowerRelation() to create random power relations.

Usage

powerRelationGenerator(coalitions, startWithLinearOrder = FALSE)
generateNextPartition(gen)
generateRandomPowerRelation(coalitions, linearOrder = FALSE, monotonic = FALSE)

Value

A generator function. Every time this generator function is called, a different PowerRelation object is returned. Once all possible power relations have been generated, the generator function returns NULL.

A generator function. If the generator is already down to its last partition, it will throw an error.

Use generateNextPartition(gen) to skip to the next partition of the generator.

Arguments

coalitions

List of coalition vectors. An empty coalition can be set with c().

startWithLinearOrder

If set to TRUE, the first PowerRelation object generated will be a linear order in the order of the list of coalitions they are given. If set to FALSE, the first PowerRelation object generated will have a single equivalence class containing all coalitions, as in, every coalition is equally powerful.

gen

A generator object returned by powerRelationGenerator().

linearOrder

logical, if TRUE, only linear orders are generated.

monotonic

logical, if TRUE, only monotonic power relations are created (see makePowerRelationMonotonic()).

Details

Using the partitions library, partitions::compositions() is used to create all possible partitions over the set of coalitions. For every partition, partitions::multinomial() is used to create all permutations over the order of the coalitions.

Note that the number of power relations (or total preorders) grows incredibly fast.

The Stirling number of second kind \(S(n,k)\) gives us the number of \(k\) partitions over \(n\) elements.

$$S(n,k) = \frac{1}{k!}\sum_{j=0}^{k} (-1)^j \binom{k}{j}(k-j)^n$$

For example, with 4 coalitions (n = 4) there are 6 ways to split it into k = 3 partitions. The sum of all partitions of any size is also known as the Bell number (\(B_n = \sum_{k=0}^n S(n,k)\), see also numbers::bell()).

Regarding total preorders \(\mathcal{T}(X)\) over a set \(X\), the Stirling number of second kind can be used to determine the number of all possible total preorders \(|\mathcal{T}(X)|\).

$$|\mathcal{T}(X)| = \sum_{k=0}^{|X|} k! * S(|X|, k)$$

In literature, it is referred to as the ordered Bell number or Fubini number.

In the context of social rankings we may consider total preorders over the set of coalitions \(2^N\) for a given set of elements or players \(N\). Here, the number of coalitions doubles with every new element. The number of preorders then are:

# of elements	# of coalitions	# of total preorders	1ms / computation
0	1	1	1ms
1	2	3	3ms
2	4	75	75ms
3	7 (w/o empty set)	47,293	47 seconds
3	8	545,835	9 minutes
4	15 (w/o empty set)	230,283,190,977,853	7,302 years
4	16	5,315,654,681,981,355	168,558 years

Examples

coalitions <- createPowerset(c('a','b'), includeEmptySet = FALSE)
# list(c('a','b'), 'a', 'b')

gen <- powerRelationGenerator(coalitions)

while(!is.null(pr <- gen())) {
  print(pr)
}
# (ab ~ a ~ b)
# (ab ~ a) > b
# (ab ~ b) > a
# (a ~ b) > ab
# ab > (a ~ b)
# a > (ab ~ b)
# b > (ab ~ a)
# ab > a > b
# ab > b > a
# a > ab > b
# b > ab > a
# a > b > ab
# b > a > ab

# from now on, gen() always returns NULL
gen()
# NULL

# Use generateNextPartition() to skip certain partitions
gen <- powerRelationGenerator(coalitions)

gen <- generateNextPartition(gen)
gen <- generateNextPartition(gen)
gen()
# ab > (a ~ b)

gen <- generateNextPartition(gen)
gen()
# ab > a > b

coalitions <- createPowerset(c('a','b'), includeEmptySet = FALSE)
# list(c('a','b'), 'a', 'b')

gen <- powerRelationGenerator(coalitions)
gen()
# (ab ~ a ~ b)

gen()
# (ab ~ b) > a

# skipping partition of size two, where the first partition has
# 2 coalitions and the second partition has 1 coalition
gen <- generateNextPartition(gen)
gen()
# ab > (a ~ b)

# only remaining partition is one of size 3, wherein each
# equivalence class is of size 1
gen <- generateNextPartition(gen)
gen()
# ab > a > b

# went through all partitions, it will only generate NULL now
gen <- generateNextPartition(gen)
stopifnot(is.null(gen()))


# create random power relation
generateRandomPowerRelation(coalitions)

# make sure it's monotonic, i.e., {1} > {1,2} cannot exist
# because {1} is a subset of {1,2}
generateRandomPowerRelation(coalitions, monotonic = TRUE)