问题描述
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:
The function combn() generates all combinations of the elements of x taken m at a time. It is very fast and efficient for nCm small (where n is the number of elements of x) but it quickly runs out of memory. For example:
> 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:
I would like to know if the function combn() can be modified such that it generates only k chosen combinations. Let's call this new function chosencombn(). Then we would have:
> 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，这种功能需要使用组合的顺序，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?可以对它进行编码以获得像combn()一样高效的功能吗?
I understand that such a function would require the use of an ordering of the combinations such that one can find the place of a given combination immediately. Does such an ordering exist? Can it be coded to obtain a function as efficient as combn()?
推荐答案
包裹猪蹄" 很有用为此，因为它不会将排列保留在内存中.
Package "trotter" is useful for this as it does not keep the permutations in memory.
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"
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