在data.table中提取组内所有可能的对

Question

Currently, I am struggling with a problem related to obtaining all possible permutation within group using data.table.

To explain my problem, let me show you an example.

x <- c(1, 1, 1, 2, 2)
y <- c('red', 'blue', 'black', 'orange', 'red')
dt1 <- as.data.table(cbind(x,y))
dt1
   x      y
1: 1    red
2: 1   blue
3: 1  black
4: 2 orange
5: 2    red

Now I want to see every possible pair of color(y) within group(x). So my ideal result would be....

x   y1      y2
1   black   blue
1   black   red
1   blue    black
1   blue    red
1   red     black
1   red     blue
2   orange  red
2   red     orange

To find a solution for this, I did googling it and I found a function, permutation, which is what I am looking for but I find it hard to squeeze it into data.table framework.

y <- c('red', 'blue', 'black')
permutations(n=3, r=2, v=y, repeats.allowed=F)

     [,1]    [,2]   
[1,] "black" "blue" 
[2,] "black" "red"  
[3,] "blue"  "black"
[4,] "blue"  "red"  
[5,] "red"   "black"
[6,] "red"   "blue" 

So I tried to do the following but obviously it has errors..

dt1[, .(j = lapply(.SD, permutations, n=.N, r=2, v=y, repeats.allowed=F)), by=x]

Any suggestion for this? I will really appreciate it.

Answer 1

First off, don't use as.data.table(cbind(...)) to create the data table. You will get unexpected column classes due to cbind coercing to matrix. Use

dt1 <- data.table(x, y)

That said, you can do

dt1[, {
        p <- gtools::permutations(.N, 2, y, repeats=FALSE)
        .(y1 = p[, 1], y2 = p[, 2])
    }, by = x]

which gives

  x y1 y2 1: 1 black blue 2: 1 black red 3: 1 blue black 4: 1 blue red 5: 1 red black 6: 1 red blue 7: 2 orange red 8: 2 red orange 

There is no need to loop since we are operating on groups. permutations creates a matrix, so we create our desired result columns from the resulting matrix columns of permutations .