12

In a data frame, I have one column containing character strings. Let's say it looks like this:

x <- unique(df[,1])
x
"A" "A" "B" "B" "B" "C"

I'd like to get all possible combinations of the unique character strings as sets of 2 without caring about their order, so A, B is the same as B, A, and I don't want to get same values as combination like A, A. So far, I got until this point:

comb <- expand.grid(x, x)
comb <- comb[which(comb[,1] != comb[,2]),]

But this still leaves the problem of having rows with the same combination of strings in a different order. How do I get rid of this?

AnjaM
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    `x` can't **possibly** be what you show; they would be unique if you had applied `unique()` to `df[,1]` would they not? – Gavin Simpson Sep 03 '12 at 09:40
  • You're right, I mixed it up with printing the content of the data frame column itself. `x` of course doesn't contain duplicates. – AnjaM Sep 03 '12 at 10:15

2 Answers2

22

There's the combn function in the utils package:

t(combn(LETTERS[1:3],2))
#      [,1] [,2]
# [1,] "A"  "B" 
# [2,] "A"  "C" 
# [3,] "B"  "C"

I'm a little confused as to why your x has duplicated values.

BenBarnes
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13

I think you are looking for combn:

x <- c("A", "A", "B", "B", "B", "C")
combn(x,2)

Gives:

     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12] [,13] [,14] [,15]
[1,] "A"  "A"  "A"  "A"  "A"  "A"  "A"  "A"  "A"  "B"   "B"   "B"   "B"   "B"   "B"  
[2,] "A"  "B"  "B"  "B"  "C"  "B"  "B"  "B"  "C"  "B"   "B"   "C"   "B"   "C"   "C"

And if you want only unique values in x (I have no idea why you have duplicate values in x in the first place if it's the result of a unique() call):

> combn(unique(x),2)
     [,1] [,2] [,3]
[1,] "A"  "A"  "B" 
[2,] "B"  "C"  "C"
Sacha Epskamp
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  • Thanks, I didn't know about `combn`. I accepted the answer of BenBarnes because it gives exactly the output I wanted, but obviously you also answered my question. – AnjaM Sep 03 '12 at 10:13