combinations  R Documentation 
combinations
enumerates the possible combinations of a
specified size from the elements of a vector.
combinations(n, r, v=1:n, set=TRUE, repeats.allowed=FALSE)
n 
Size of the source vector 
r 
Size of the target vectors 
v 
Source vector. Defaults to 
set 
Logical flag indicating whether duplicates should be
removed from the source vector 
repeats.allowed 
Logical flag indicating whether the
constructed vectors may include duplicated values. Defaults to

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.
Returns a matrix where each row contains a vector of length r
.
Original versions by Bill Venables
Bill.Venables@cmis.csiro.au. Extended to handle
repeats.allowed
by Gregory R. Warnes
greg@warnes.net.
Venables, Bill. "Programmers Note", RNews, Vol 1/1, Jan. 2001. https://cran.rproject.org/doc/Rnews/
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
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