Count days per year

Question

I have two dates

begin <- as.Date("2007-05-20")
end   <- as.Date("2010-06-13")

How can I count the days per year?

The output should maybe look like this

year   days
2007   226
2008   366
2009   365
2010   164

Answer 1

Let's first create a proper date sequence:

R> bd <- as.Date("2007-05-20")
R> ed <- as.Date("2010-06-13")
R> seqd <- seq(bd, ed, by="1 day")
R> head(seqd)
[1] "2007-05-20" "2007-05-21" "2007-05-22" "2007-05-23" "2007-05-24" "2007-05-25"

We then create a helper function which, given a date, returns its year:

R> getYear <- function(d) as.POSIXlt(d)$year + 1900
R> getYear(head(seqd))
[1] 2007 2007 2007 2007 2007 2007

After that, we just call table() on what the helper returns from the date sequence:

R> table(getYear(seqd))

2007 2008 2009 2010 
 226  366  365  164 

Answer 2

Or, alternatively create a data.frame with an additional column year (using Dirk's data):

dat = data.frame(seqd, year = strftime(seqd, "%Y"))
> head(dat)
        seqd year
1 2007-05-20 2007
2 2007-05-21 2007
3 2007-05-22 2007
4 2007-05-23 2007
5 2007-05-24 2007
6 2007-05-25 2007

and then use count from the plyr package:

require(plyr)
> count(dat, "year")
  year freq
1 2007  226
2 2008  366
3 2009  365
4 2010  164