SortDataFrame

library(data.table)
library(plyr)
library(dplyr)
library(magrittr)

## data generation
N = 10000
dataset = sample(state.abb, N, replace = TRUE) %>%
  tbl_dt() %>% setnames("state") %>%
  mutate(hospital = paste0("Hospital", 1:N),
    birthrate = runif(N, 0, 0.1), deathrate = runif(N, 0, 0.2))

## order by birthrate and deathrate
datasetSort = dataset %>% arrange(state, birthrate, deathrate)

# other method
datasetSort_2 = dataset[order(dataset$state, dataset$birthrate,
  dataset$deathrate),]

## The hospitals with top 5 birthrate in each state
datasetBirthRate5 = datasetSort %>% split(use_series(., state)) %>% lapply(head, n = 5)

# other method
datasetBirthRate5_2 = lapply(split(datasetSort_2,
  datasetSort_2$state), function(x) head(x, 5))

## The hospital with the highest birthrate in each state
datasetMaxBR = datasetSort %>% group_by(state) %>%
  filter(birthrate==max(birthrate))

# other method
datasetMaxBR_2 = do.call(ribind,
  lapply(split(datasetSort_2, datasetSort_2$state),
  function(x) x[which.max(x$birthrate)]))

## The hospital with the lowest deathrate in each state
datasetMinDR = datasetSort %>% group_by(state) %>%
  filter(deathrate==min(deathrate))

# other method
datasetMaxBR_2 = do.call(rbind,
  lapply(split(datasetSort_2, datasetSort_2$state),
  function(x) x[which.min(x$deathrate)]))