Question

I’ve got a dataframe with a text column name and factor city. It is ordered alphabetically firstly by city and then name. Now I need to get a data frame which contains only nth element in each city, keeping this ordering. How can it be done in a pretty way without loops?

I have:

name    city
John    Atlanta
Josh    Atlanta
Matt    Atlanta
Bob     Boston
Kate    Boston
Lily    Boston
Matt    Boston

I want a function, which returns n’th element by city, i.e., if it is 3rd, then:

name    city
Matt    Atlanta
Lily    Boston

It should return NULL for name if it is out of range for the selected city, i.e., for 4th:

name    city
NULL    Atlanta
Matt    Boston

Using only base R please?

Answer 1

In base R using by:

Set up some test data, including an additional out of range value:

test <- read.table(text="name    city
John    Atlanta
Josh    Atlanta
Matt    Atlanta
Bob     Boston
Kate    Boston
Lily    Boston
Matt    Boston
Bob     Seattle
Kate    Seattle",header=TRUE)

Get the 3rd item in each city:

do.call(rbind,by(test,test$city,function(x) x[3,]))

Result:

        name    city
Atlanta Matt Atlanta
Boston  Lily  Boston
Seattle <NA>    <NA>

To get exactly what you want, here is a little function:

nthrow <- function(dset,splitvar,n) {
    result <- do.call(rbind,by(dset,dset[splitvar],function(x) x[n,]))
    result[,splitvar][is.na(result[,splitvar])] <- row.names(result)[is.na(result[,splitvar])]
    row.names(result) <- NULL
    return(result)
}

Call it like:

nthrow(test,"city",3)

Result:

  name    city
1 Matt Atlanta
2 Lily  Boston
3 <NA> Seattle