count new elements in vector list

Question

I want to count new elements that weren't present in previous years. In the example

Sample data:

var1 <- list('2003' = 1:3, '2004' = c(4:3), '2005' = c(6,4,1), '2006' = 1:4 )

I would like to get the output

newcount <- list('2003' = 0, '2004' = 1, '2005' = 1, '2006' = 0)

Unsuccessful code:

newcount <- mapply(setdiff, var1, seq_along(var1), function(i) 
            {if (i > 1) {Reduce(union, var1[i-1], accumulate=T)}}, length)

Answer 1

Almost there, but its better to use vector indexing to work with the offset and add the always-known initial element afterwards:

lapply(c(list(`2003`=integer(0)),
       mapply(setdiff,var1[-1], 
              Reduce(union,var1,accumulate=TRUE)[-length(var1)])),length)
$`2003`
[1] 0

$`2004`
[1] 1

$`2005`
[1] 1

$`2006`
[1] 0