函数combn()一次生成m个x元素的所有组合。对于nCm小(其中n是x的元素数)，这是非常快速且高效的，但很快就会耗尽内存。例如:
> combn(c(1:50), 12, simplify = TRUE)
Error in matrix(r, nrow = len.r, ncol = count) :
invalid 'ncol' value (too large or NA)
我想知道是否可以修改功能combn()使其仅生成k个选定的组合。我们将此新函数称为selectedcombn()。然后我们将有:
> combn(c("a", "b", "c", "d"), m=2)
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] "a" "a" "a" "b" "b" "c"
[2,] "b" "c" "d" "c" "d" "d"
>chosencombn(c("a", "b", "c", "d"), m=2, i=c(1,4,6))
[,1] [,2] [,3]
[1,] "a" "b" "c"
[2,] "b" "c" "d"
>chosencombn(c("a", "b", "c", "d"), m=2, i=c(4,5))
[,1] [,2]
[1,] "b" "b"
[2,] "c" "d"
我知道这样的功能将需要使用组合的排序，以便人们可以立即找到给定组合的位置。
是否存在这样的顺序？可以对其进行编码以获得与combn()一样高效的功能吗？
最佳答案
软件包"trotter"对此很有用，因为它不会将排列保留在内存中。
library(trotter)
combs = cpv(2, c("a", "b", "c", "d"))
sapply(c(1, 4, 6), function(i) combs[i])
# [,1] [,2] [,3]
#[1,] "a" "b" "c"
#[2,] "b" "c" "d"