Select nth element in data frame by factor

Question

I\'ve got a dataframe with a text column name and factor city. It is ordered alphabetically firstly by city and then name. No

Answer 1

You can use plyr for this:

dat <- structure(list(name = c("John", "Josh", "Matt", "Bob", "Kate", 

"Lily", "Matt"), city = c("Atlanta", "Atlanta", "Atlanta", "Boston", "Boston", "Boston", "Boston")), .Names = c("name", "city"), class = "data.frame", row.names = c(NA, -7L))

library(plyr)

ddply(dat, .(city), function(x, n) x[n,], n=3)

> ddply(dat, .(city), function(x, n) x[n,], n=3)
  name    city
1 Matt Atlanta
2 Lily  Boston
> ddply(dat, .(city), function(x, n) x[n,], n=4)
  name   city
1    
2 Matt Boston
> 

There are plenty of other options too using base R or data.table or sqldf...