Over a million developers have joined DZone.

R/dplyr: Extracting Data Frame Column Value for Filtering With %in%

DZone's Guide to

R/dplyr: Extracting Data Frame Column Value for Filtering With %in%

· Big Data Zone
Free Resource

Need to build an application around your data? Learn more about dataflow programming for rapid development and greater creativity. 

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:

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
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.

Check out the Exaptive data application Studio. Technology agnostic. No glue code. Use what you know and rely on the community for what you don't. Try the community version.


Published at DZone with permission of Mark Needham, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.


Dev Resources & Solutions Straight to Your Inbox

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.


{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}