slider-even-years.R

library(slider)
library(dplyr)
library(lubridate)
library(tidyr)

x <- as.Date(c(paste0(2008:2014, "-01-01"), paste0(2008:2014, "-01-02")))
x <- sort(x)

df <- data.frame(x = x)
df <- mutate(df, year = year(x))

df
#>             x year
#> 1  2008-01-01 2008
#> 2  2008-01-02 2008
#> 3  2009-01-01 2009
#> 4  2009-01-02 2009
#> 5  2010-01-01 2010
#> 6  2010-01-02 2010
#> 7  2011-01-01 2011
#> 8  2011-01-02 2011
#> 9  2012-01-01 2012
#> 10 2012-01-02 2012
#> 11 2013-01-01 2013
#> 12 2013-01-02 2013
#> 13 2014-01-01 2014
#> 14 2014-01-02 2014

df_nested <- nest(df, data = -year)

df_nested
#> # A tibble: 7 x 2
#>    year data            
#>   <dbl> <list>          
#> 1  2008 <tibble [2 × 1]>
#> 2  2009 <tibble [2 × 1]>
#> 3  2010 <tibble [2 × 1]>
#> 4  2011 <tibble [2 × 1]>
#> 5  2012 <tibble [2 × 1]>
#> 6  2013 <tibble [2 × 1]>
#> 7  2014 <tibble [2 × 1]>

# Current year + 2 years before
# Step forward by 2 years between each result
# Only compute on "complete" sets of years
slide(df_nested, identity, .before = 2, .step = 2, .complete = TRUE)
#> [[1]]
#> NULL
#> 
#> [[2]]
#> NULL
#> 
#> [[3]]
#> # A tibble: 3 x 2
#>    year data            
#>   <dbl> <list>          
#> 1  2008 <tibble [2 × 1]>
#> 2  2009 <tibble [2 × 1]>
#> 3  2010 <tibble [2 × 1]>
#> 
#> [[4]]
#> NULL
#> 
#> [[5]]
#> # A tibble: 3 x 2
#>    year data            
#>   <dbl> <list>          
#> 1  2010 <tibble [2 × 1]>
#> 2  2011 <tibble [2 × 1]>
#> 3  2012 <tibble [2 × 1]>
#> 
#> [[6]]
#> NULL
#> 
#> [[7]]
#> # A tibble: 3 x 2
#>    year data            
#>   <dbl> <list>          
#> 1  2012 <tibble [2 × 1]>
#> 2  2013 <tibble [2 × 1]>
#> 3  2014 <tibble [2 × 1]>

indices <- seq(2008, 2014, by = 2)
starts <- indices[-length(indices)]
stops <- indices[-1]

starts
#> [1] 2008 2010 2012
stops
#> [1] 2010 2012 2014

hop_index(
  .x = df_nested, 
  .i = df_nested$year, 
  .starts = starts, 
  .stops = stops, 
  .f = identity
)
#> [[1]]
#> # A tibble: 3 x 2
#>    year data            
#>   <dbl> <list>          
#> 1  2008 <tibble [2 × 1]>
#> 2  2009 <tibble [2 × 1]>
#> 3  2010 <tibble [2 × 1]>
#> 
#> [[2]]
#> # A tibble: 3 x 2
#>    year data            
#>   <dbl> <list>          
#> 1  2010 <tibble [2 × 1]>
#> 2  2011 <tibble [2 × 1]>
#> 3  2012 <tibble [2 × 1]>
#> 
#> [[3]]
#> # A tibble: 3 x 2
#>    year data            
#>   <dbl> <list>          
#> 1  2012 <tibble [2 × 1]>
#> 2  2013 <tibble [2 × 1]>
#> 3  2014 <tibble [2 × 1]>