R找到所有可能的唯一组合

Question

我正在尝试在 R 中找到所有可能的唯一组合。似乎已经问了很多类似的问题，但我找不到相同的问题。

我的问题是从向量 x 中找到 m 个元素的组合，但 m 可能大于 x。比如从letters[1:2]中挑选3个元素，希望可以return:

combn(letters[1:2],3)
       [,1]  [,2]  [,3]  [,4] 
[1,]   "a"   "a"   "a"    "b"
[2,]   "a"   "a"   "b"    "b"
[3,]   "a"   "b"   "b"    "b"

而是组合函数 n < m 中的错误。类似的函数有gtools:permutations, expand.grid.

如果之前有人问过同样的问题，但我没有听清，再次表示歉意。谢谢

Answer 1

combn1=function(x,m){
  n=ifelse(length(x)==1,ifelse(is.numeric(x),x,1),length(x))
  if(n>=m) return(combn(x,m))
  a=do.call(expand.grid, rep(list(x),m))
  b=t(unique(t(apply(a,1,sort))))
  `dimnames<-`(b,NULL)
}

combn1(letters[1],3)
     [,1]
[1,] "a" 
[2,] "a" 
[3,] "a" 
> combn1(letters[1:2],3)
     [,1] [,2] [,3] [,4]
[1,] "a"  "a"  "a"  "b" 
[2,] "a"  "a"  "b"  "b" 
[3,] "a"  "b"  "b"  "b" 
> combn1(letters[1:3],3)
     [,1]
[1,] "a" 
[2,] "b" 
[3,] "c"

Answer 2

有一些专门为此构建的软件包。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。我们从经典开始 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 替代 iterpc （@Gregor 在上面的评论中链接的包）：

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 RcppAlgos，这是我创作的：

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 是一个很棒的函数，作为 R 的基础包之一提供，但它的缺点之一是它不允许重复（这是这里所需要的） .我写了一个非常全面的概述来解决与这个问题完全相同的问题，可以在这里找到：A Walk Through a Slice of Combinatorics in R.

Answer 3

这可能有效，它列出了所有不重复的唯一组合

> v1 = letters[1:4]
> v1 
[1] "a" "b" "c" "d"

> do.call("c",lapply(seq_along(v1),function(i) utils::combn(v1,i,FUN=list)))


[[1]]
[1] "a"

[[2]]
[1] "b"

[[3]]
[1] "c"

[[4]]
[1] "d"

[[5]]
[1] "a" "b"

[[6]]
[1] "a" "c"

[[7]]
[1] "a" "d"

[[8]]
[1] "b" "c"

[[9]]
[1] "b" "d"

[[10]]
[1] "c" "d"

[[11]]
[1] "a" "b" "c"

[[12]]
[1] "a" "b" "d"

[[13]]
[1] "a" "c" "d"

[[14]]
[1] "b" "c" "d"

[[15]]
[1] "a" "b" "c" "d"

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