· r-2 rstats

R/dplyr: Extracting data frame column value for filtering with %in%

I've been playing around with dplyr over the weekend and wanted to extract the values from a data frame column to use in a later filtering step.

I had a data frame:


library(dplyr)
df = data.frame(userId = c(1,2,3,4,5), score = c(2,3,4,5,5))

And wanted to extract the userIds of those people who have a score greater than 3. I started with:


highScoringPeople = df %>% filter(score > 3) %>% select(userId)
> highScoringPeople
  userId
1      3
2      4
3      5

And then filtered the data frame expecting to get back those 3 people:


> df %>% filter(userId %in% highScoringPeople)
[1] userId score 
<0 rows> (or 0-length row.names)

No rows! I created vector with the numbers 3-5 to make sure that worked:


> df %>% filter(userId %in% c(3,4,5))
  userId score
1      3     4
2      4     5
3      5     5

That works as expected so highScoringPeople obviously isn't in the right format to facilitate an 'in lookup'. Let's explore:


> str(c(3,4,5))
 num [1:3] 3 4 5

> str(highScoringPeople)
'data.frame':	3 obs. of  1 variable:
 $ userId: num  3 4 5

Now it's even more obvious why it doesn't work - highScoringPeople is still a data frame when we need it to be a vector/list.

One way to fix this is to extract the userIds using the $ syntax instead of the select function:


highScoringPeople = (df %>% filter(score > 3))$userId

> str(highScoringPeople)
 num [1:3] 3 4 5

> df %>% filter(userId %in% highScoringPeople)
  userId score
1      3     4
2      4     5
3      5     5

Or if we want to do the column selection using dplyr we can extract the values for the column like this:


highScoringPeople = (df %>% filter(score > 3) %>% select(userId))[[1]]

> str(highScoringPeople)
 num [1:3] 3 4 5

Not so difficult after all.

  • LinkedIn
  • Tumblr
  • Reddit
  • Google+
  • Pinterest
  • Pocket