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

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     

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  Seattle