Count number of observations per day, month and year in R

Question

I have the a dataframe in the following form (its too big to post here entirerly):

      listing_id    date    city    type    host_id availability
1   703451  25/03/2013  amsterdam   Entire home/apt 3542621 245
2   703451  20/04/2013  amsterdam   Entire home/apt 3542621 245
3   703451  28/05/2013  amsterdam   Entire home/apt 3542621 245
4   703451  15/07/2013  amsterdam   Entire home/apt 3542621 245
5   703451  30/07/2013  amsterdam   Entire home/apt 3542621 245
6   703451  19/08/2013  amsterdam   Entire home/apt 3542621 245

and so on...

I would like three new data frames. One counting the number of observations for a particular year (2013,2012, 2011 and so on) another per month (07/2013, 06/2013 and so on) and another per day (28/05/2013, 29/05/2013 and so on). I just want to count how many occurances there are per unit of time.

How would I do that?

Answer 1

Using data.table, this is pretty straightforward:

library(data.table)
dt <- fread("listing_id    date    city    type    host_id availability
703451  25/03/2013  amsterdam   Entire_home/apt 3542621 245
703451  20/04/2013  amsterdam   Entire_home/apt 3542621 245
703451  28/05/2013  amsterdam   Entire_home/apt 3542621 245
703451  15/07/2013  amsterdam   Entire_home/apt 3542621 245
703451  30/07/2013  amsterdam   Entire_home/apt 3542621 245
703451  19/08/2013  amsterdam   Entire_home/apt 3542621 245")
dt$date <- as.Date(dt$date, "%d/%m/%Y")

dt[, .N, by=year(date)] 
#    year N
# 1: 2013 6

dt[, .N, by=.(year(date), month(date))] 
#    year month N
# 1: 2013     3 1
# 2: 2013     4 1
# 3: 2013     5 1
# 4: 2013     7 2
# 5: 2013     8 1

dt[, .N, by=date] # or: dt[, .N, by=.(year(date), month(date), day(date)] 
#          date N
# 1: 2013-03-25 1
# 2: 2013-04-20 1
# 3: 2013-05-28 1
# 4: 2013-07-15 1
# 5: 2013-07-30 1
# 6: 2013-08-19 1