OBsMD (version 12.0)

combinations: Enumerate the Combinations of the Elements of a Vector

Description

combinations enumerates the possible combinations of a specified size from the elements of a vector.

Usage

combinations(n, r, v=1:n, set=TRUE, repeats.allowed=FALSE)

Value

Returns a matrix where each row contains a vector of length r.

Arguments

n

Size of the source vector

r

Size of the target vectors

v

Source vector. Defaults to 1:n

set

Logical flag indicating whether duplicates should be removed from the source vector v. Defaults to TRUE.

repeats.allowed

Logical flag indicating whether the constructed vectors may include duplicated values. Defaults to FALSE.

Author

Original versions by Bill Venables . Extended to handle repeats.allowed by Gregory R. Warnes .

Details

Caution: The number of combinations increases rapidly with n and r!.

To use values of n above about 45, you will need to increase R's recursion limit. See the expression argument to the options command for details on how to do this.

References

Venables, Bill. "Programmers Note", R-News, Vol 1/1, Jan. 2001. https://cran.r-project.org/doc/Rnews/

Examples

combinations(3,2,letters[1:3])
combinations(3,2,c(1:3),repeats=TRUE)
combinations(6,3,1:6,repeats=TRUE)



# To use large 'n', you need to change the default recusion limit
options(expressions=1e5)
cmat <- combinations(100,2)
dim(cmat) # 4950 by 2