R组中的组合

Question

我有一个关于组合的问题.

我的迷你样本看起来像这样:

sample <- data.frame(
  group=c("a","a","a","a","b","b","b"),
  number=c(1,2,3,2,4,5,3)
)

如果我将函数应用于combn数据框,它会给出以下结果,即"数字"列下的值的所有组合,无论该值属于哪个组:

         [,1] [,2]
   [1,]    1    2
   [2,]    1    3
   [3,]    1    2
   [4,]    1    4
   [5,]    1    5
   [6,]    1    3
   [7,]    2    3
   [8,]    2    2
   [9,]    2    4
  [10,]    2    5
  [11,]    2    3
  [12,]    3    2
  [13,]    3    4
  [14,]    3    5
  [15,]    3    3
  [16,]    2    4
  [17,]    2    5
  [18,]    2    3
  [19,]    4    5
  [20,]    4    3
  [21,]    5    3

我用于上述结果的代码如下:

t(combn((sample$number), 2))

但是,我想在组内得到组合结果(即"a","b").因此,我想得到的结果应如下所示:

     [,1] [,2] [,3]
[1,]   a    1    2
[2,]   a    1    3
[3,]   a    1    2
[4,]   a    2    3
[5,]   a    2    2
[6,]   a    3    2
[7,]   b    4    5
[8,]   b    4    3
[9,]   b    5    3

除了组合,我想得到指示该组的列.

Answer 1

我们可以使用 group by 函数data.table

library(data.table)
setDT(sample)[, {i1 <-  combn(number, 2)
                   list(i1[1,], i1[2,]) }, by =  group]
#    group V1 V2
#1:     a  1  2
#2:     a  1  3
#3:     a  1  2
#4:     a  2  3
#5:     a  2  2
#6:     a  3  2
#7:     b  4  5
#8:     b  4  3
#9:     b  5  3

或者一个紧凑的选择是

setDT(sample)[, transpose(combn(number, 2, FUN = list)), by = group]

或者使用base R

 lst <- by(sample$number, sample$group, FUN = combn, m= 2)
 data.frame(group = rep(unique(as.character(sample$group)), 
                        sapply(lst, ncol)), t(do.call(cbind, lst)))