Generate all combinations of length 2 using 3 letters

Question

The simplest way to generate such combinations would be to use combn function as follows:

print(combn(letters[1:3],2))

The output that it generates is:

    [,1] [,2] [,3]
[1,] "a"  "a"  "b" 
[2,] "b"  "c"  "c"

This doesn't generate combinations like aa, bb (same character repeated). This even doesn't generate ba if ab is generated.

I want to generate all such combinations of length 2 for a vector [a,b,c].

Is there a simple way to do so in R?

Use `expand.grid` i.e. `expand.grid(rep(list(letters[1:3]), 2))` — akrun, Apr 26 '16 at 06:01
... though be aware of the default `stringsAsFactors = TRUE` — Henry, Apr 26 '16 at 06:11
Here's an alternative (which, unsurprisingly, is less elegant than the suggestion by @akrun ) `unique(t(combn(rep(letters[1:3],2),2)))` — RHertel, Apr 26 '16 at 06:21

score 2 · Answer 1 · answered Apr 26 '16 at 07:24

2

The paste0 function is vectorized and so succeeds with outer:

outer(c('a','b','c'), c('a','b','c'), paste0)

     [,1] [,2] [,3]
[1,] "aa" "ab" "ac"
[2,] "ba" "bb" "bc"
[3,] "ca" "cb" "cc"

answered Apr 26 '16 at 07:24

IRTFM

1 Answers1