CODEWITHSUNDEEP
rcombinatorics
tretelrusch
tretelrusch

Reputation: 61

R – How to calculate combinations of pairs?

From a number of items (n), how do I compute all possible combinations of pairs in R? I can calculate the number of possible combinations with (n-1)*(n-3)*(n-5)*…, but how to get the combinations themselves?

For example, I have

x <- c("A", "B", "C", "D")

That would make 4-1=3 combinations of pairs, so I want to have something like 

("AB","CD")
("AC","BD")
("AD","BC")

Thanks a lot!

Upvotes: 1

Views: 3415

Answers (2)

eipi10
eipi10

Reputation: 93871

First, let's get all pairs of pairs:

dat = t(combn(combn(x, 2, paste, collapse=""), 2))

     [,1] [,2]
 [1,] "AB" "AC"
 [2,] "AB" "AD"
 [3,] "AB" "BC"
 [4,] "AB" "BD"
 [5,] "AB" "CD"
 [6,] "AC" "AD"
 [7,] "AC" "BC"
 [8,] "AC" "BD"
 [9,] "AC" "CD"
[10,] "AD" "BC"
[11,] "AD" "BD"
[12,] "AD" "CD"
[13,] "BC" "BD"
[14,] "BC" "CD"
[15,] "BD" "CD"

We now have all pairs of pairs. But it looks like you want only pairs of pairs where each letter appears only once. The code below identifies rows that meet this condition (though it seems like the code is more complicated than it needs to be).

dat[sapply(strsplit(apply(dat, 1, paste, collapse=""), ""), function(i) length(unique(i)) == 4), ]

     [,1] [,2]
[1,] "AB" "CD"
[2,] "AC" "BD"
[3,] "AD" "BC"

Upvotes: 5

Felipe Alvarenga
Felipe Alvarenga

Reputation: 2652

For that you can use either combn or expand.grid

x <- c("A","B","C","D") 

expand.grid(x, x) # generates both way combinations

   Var1 Var2
1     A    A
2     B    A
3     C    A
4     D    A
5     A    B
6     B    B
7     C    B
8     D    B
9     A    C
10    B    C
11    C    C
12    D    C
13    A    D
14    B    D
15    C    D
16    D    D

t(combn(x, 2)) # order does not matter

     [,1] [,2]
[1,] "A"  "B" 
[2,] "A"  "C" 
[3,] "A"  "D" 
[4,] "B"  "C" 
[5,] "B"  "D" 
[6,] "C"  "D"

Upvotes: 3

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