dplyr_example.R

library(gapminder)
library(tidyverse)

##======================================================================
## Returns the first 5 countries with the worst drop in life expectancy.
##======================================================================

gapminder %>%
    select(country, year, continent, lifeExp) %>%
    group_by(continent, country) %>%
    mutate(le_delta = lifeExp - lag(lifeExp)) %>%
    summarize(worst_le_delta = min(le_delta, na.rm = TRUE)) %>%
    arrange(worst_le_delta) %>%
    head(5)

##===================================
## The above results in:
##===================================

# A tibble: 5 x 3
# Groups:   continent [1]
#  continent country   worst_le_delta
#  <fct>     <fct>              <dbl>
#1 Africa    Rwanda             -20.4
#2 Africa    Zimbabwe           -13.6
#3 Africa    Lesotho            -11.0
#4 Africa    Swaziland          -10.4
#5 Africa    Botswana           -10.2
##===================================