为此专门构建了一些软件包.基本前提是我们需要重复长度为m的组合，其中m可能大于输入向量.我们从经典的gtools开始:

There are a few packages specifically built for this. The basic premise is that we need combinations with repetition of length m where m could be larger than the input vector. We start with the classic gtools:

library(gtools)
combinations(2, 3, letters[1:2], repeats.allowed = TRUE)
    [,1] [,2] [,3]
[1,] "a"  "a"  "a" 
[2,] "a"  "a"  "b" 
[3,] "a"  "b"  "b" 
[4,] "b"  "b"  "b"

然后是arrangements，它是iterpc(在上面的注释中由@Gregor链接的程序包)的替代物:

And then there is arrangements which is a replacement for iterpc (the package linked by @Gregor in the comments above):

library(arrangements)
arrangements::combinations(2, 3, letters[1:2], replace = TRUE)
     [,1] [,2] [,3]
[1,] "a"  "a"  "a" 
[2,] "a"  "a"  "b" 
[3,] "a"  "b"  "b" 
[4,] "b"  "b"  "b"

最后是我写的RcppAlgos:

library(RcppAlgos)
comboGeneral(letters[1:2], 3, TRUE)
     [,1] [,2] [,3]
[1,] "a"  "a"  "a" 
[2,] "a"  "a"  "b" 
[3,] "a"  "b"  "b" 
[4,] "b"  "b"  "b"

combn是一个很棒的功能，它作为R的基本软件包之一提供，但是其缺点之一是它不允许重复(这是此处所要求的).我写了一个非常全面的概述，概述了与此类似的问题，可以在这里找到:遍历R中的组合法学.

combn is an awesome function that ships as one of the base packages with R, however one of its shortcomings is that it doesn't allow repetition (which is what is required here). I wrote a pretty comprehensive overview for problems exactly like this one that can be found here: A Walk Through a Slice of Combinatorics in R.