unique pairs or combinations from a vector - r

Question

Where am I going wrong with my function.
I am trying to create a function which will count all the unique pairs in a vector, say I have the following input:
ar <- c(10, 20, 20, 30, 30, 30, 40, 50)
The number of unique pairs is 20 = 1, 30 = 1 so I can just sum these up and the total number of unique pairs is 2.
However everything I am trying is creating 30 as having 2 unique pairs (since 30 occurs 3 times in the vector.
n <- 9
ar <- c(10, 20, 20, 30, 30, 30, 40, 50)
CountThePairs <- function(n, ar){
for(i in 1:length(ar)){
sum = ar[i] - ar[]
pairs = length(which(sum == 0))
}
return(sum)
}
CountThePairs(n = NULL, ar)
Is there an easier way of doing this? I prefer the base R version but interested in package versions also.

Answer 1

Here's a simpler way using floor and table form base R -
ar <- c(10, 20, 20, 30, 30, 30, 40, 50)
sum(floor(table(ar)/2))
[1] 2
Example 2 - Adding one more 30 to vector so now there are 2 pairs of 30 -
ar <- c(10, 20, 20, 30, 30, 30, 30, 40, 50)
sum(floor(table(ar)/2))
[1] 3
If 2 30 pairs count as one "unique" pair then original solution by #tmfmnk was correct -
sum(table(ar) >= 2)

Answer 2

You could use sapply on the unique values of the vector to return a logical vector if that value is repeated. The sum of that logical value is then the number of unique pairs.
ar <- c(10, 20, 20, 30, 30, 30, 40, 50)
is_pair <- sapply(unique(ar), function(x) length(ar[ar == x]) > 1)
sum(is_pair)
#[1] 2
I'm not sure what behaviour you want if there are four 30's - does this count as one unique pair still or is it now two? If the latter, you would need a slightly different solution:
n_pair <- sapply(unique(ar), function(x) length(ar[ar == x]) %/% 2)
sum(n_pair)
#[1] 2