next_subset {neighbours}

Select Next Subset

Description

Select next subset of size k from a set of size n.

Usage

next_subset(a, n, k)

Arguments

a

a numeric vector (integers)

n

an integer: the size of the set to choose from

k

an integer: the subset size

Details

Given a subset a of size k taken from a set of size n, compute the next subset by alphabetical order.

Uses algorithm NEXKSB of Nijenhuis and Wilf (1975).

Value

a numeric vector (the next subset) or NULL (when there is no next subset)

Author(s)

Enrico Schumann

References

Nijenhuis, A. and Wilf, H. S. (1975) Combinatorial Algorithms for Computers and Calculators. Academic Press.

See Also

choose computes the number of combinations
combn creates all combinations
expand.grid

Examples

n <- 4
k <- 2
t(combn(n, k))
##      [,1] [,2]
## [1,]    1    2
## [2,]    1    3
## [3,]    1    4
## [4,]    2    3
## [5,]    2    4
## [6,]    3    4

a <- 1:k
print(a)
while (!is.null(a))
    print(a <- next_subset(a, n = n, k = k))
## [1] 1 2
## [1] 1 3
## [1] 1 4
## [1] 2 3
## [1] 2 4
## [1] 3 4