Using dplyr::filter, how can the output be limited to just first 500 rows?

Question

dplyr is a great and fast library.

Using the %>% operator enables powerful manipulation.

In my first step, I need to limit the output to only 500 rows max (for display purposes).

How can I do that?

par<-filter(pc,Child_Concept_GID==as.character(mcode)) %>% select(Parent_Concept_GID)

what I need is something like

filter(pc,CONDITION,rows=500)

Is there direct way or a nice workaround without making the first step a separate step (outside the %>% "stream")

Answer 1

There are a couple of ways to do this. Assuming you are pipe-lining your data (using %>%)

• top_n(tn) works with grouped data. It will not return tn rows, if the data is sorted with arrange()
• head(500) takes the first 500 rows (can be used after arrange(), for example)
• sample_n(size=500) can be used to select 500 arbitrary rows

If you are looking for the R equivalent to SQL's LIMIT, use head().

Answer 2

I think you're actually looking for slice() here.

filter(pc, condition) %>% slice(1:500) 

This does not rank the results. It merely pulls a slice, by position. In this case positions 1 through 500.

If this is coming from a relational db, head is a better option.