Question

I have a data frame with two columns. The first column defines subsets of the data. I want to find all values in the second column that only appear in one subset in the first column.

For example, from:

df=data.frame(
  data_subsets=rep(LETTERS[1:2],each=5),
  data_values=c(1,2,3,4,5,2,3,4,6,7))

data_subsets data_values
      A           1
      A           2
      A           3
      A           4
      A           5
      B           2
      B           3
      B           4
      B           6
      B           7

I would want to extract the following data frame.

data_subsets   data_values
    A              1
    A              5
    B              6
    B              7

I have been playing around with duplicated but I just can’t seem to make it work. Any help is appreciated. There are a number of topics tackling similar problems, I hope I didn’t overlook the answer in my searches!

EDIT

I modified the approach from @Matthew Lundberg of counting the number of elements and extracting from the data frame. For some reason his approach was not working with the data frame I had, so I came up with this, which is less elegant but gets the job done:

counts=rowSums(do.call("rbind",tapply(df$data_subsets,df$data_values,FUN=table)))
extract=names(counts)[counts==1]
df[match(extract,df$data_values),]

Answer 1

First, find the count of each element in df$data_values:

 x <- sapply(df$data_values, function(x) sum(as.numeric(df$data_values == x)))

> x
 [1] 1 2 2 2 1 2 2 2 1 1

Now extract the rows:

> df[x==1,]
   data_subsets data_values
1             A           1
5             A           5
9             B           6
10            B           7

Note that you missed “A 5” above. There is no “B 5”.