[R] all possible subsets of r out of n

[Yuelin Li]

Has anyone written a function that returns all possible
subsets of r elements out of a vector that contains n
elements?  I have not been able to find an answer in the FAQ
or the archives.

Say I have a vector: letters[1:10].  I want to get a matrix
of 120 rows that contains all possible subsets of 3 letters
out of the 10.  And I would like to be able to generalize to
any r out of n, like function(x=letters[1:10], r=3) that
returns:

a, b, c
a, b, d
a, b, e
...

etc.

Many thanks in advance,

- Yuelin Li.

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