How can I group and count observations made by decade in a dataset that has the years set as individual values?

Question

I have the folllowing dataset:

# A tibble: 90 × 2
   decade     n
    <dbl> <int>
 1   1930    13
 2   1931    48
 3   1932    44
 4   1933    76
 5   1934    73
 6   1935    63
 7   1936    54
 8   1937    51
 9   1938    41
10   1939    42
# … with 80 more rows

With more years that continue until 2010. I wish to "group" by decade like 1930s, 1940s, etc... and to have on another column the count of n in each year until the end of the decade.

For example:

# A tibble: 90 × 2
   decade     n
    <dbl>        <int>
 1   1930-1939    449
 2   1940-1949    516 

Thanks!

Answer 1

We could use the modulo operator %%:

library(dplyr)

df %>% 
  group_by(decade = decade - decade %% 10) %>% 
  summarise(n = sum(n))

  decade     n
   <dbl> <int>
1   1930   505