Question

I have the data frame

df1 = data.frame(Site=c(rep("A",5),rep("B",7)),Species=sample(1:100, size=12))

and want to filter the dataframe so that only the first 3 rows for each site are return. ie. returning the dataframe

df2=data.frame(Site=c(rep("A",3),rep("B",3)),Species=c(3,84,45,38,39,22))

Any ideas? Thanks

Answer 1

You can easily select the first three rows using ddply:

library(plyr)
ddply(df1, "Site", function(df) df[1:min(nrow(df), 3),])

Using min(nrow(df), 3) means that if the site only appears twice, we won’t try and select three rows.