if (!require('tidyverse')) install.packages('tidyverse'); library('tidyverse')
if (!require('gapminder')) install.packages('gapminder'); library('gapminder')

gap <- gapminder %>% filter(year == 1987 | year == 1992)

gap %>%
  group_by(year, continent) %>% 
  summarise(average = mean(gdpPercap)) %>% 
  spread(continent, average)


gap %>%
  group_by(continent, year) %>% 
  summarise(average = mean(gdpPercap)) %>% 
  spread(year, average)