Extract unique values from column in dataframe, grouped by ID

Question

I have a dataframe data0 with id's and dates like this:

id    date
1   2016-10-20
1   2016-10-19
1   2016-10-20
2   2016-10-21
2   2016-10-22
3   2016-10-21
3   2016-10-21
3   2016-10-22

Reproduce:

data0 <- structure(list(id = c(1, 1, 1, 2, 2, 3, 3, 3), date = structure(c(17094, 17093, 17094, 17095, 17096, 17095, 17095, 17096), class = "Date")), .Names = c("id", "date"), row.names = c(NA, -8L), class = "data.frame")

How can I summarize dates by id so that I come out with a structure with counts like this? :

id  2016-10-19  2016-10-20  2016-10-21  2016-10-22
1       1            2      
2                                 1           1
3                                 2           1

Answer 1

data0 <- structure(list(id = c(1, 1, 1, 2, 2, 3, 3, 3),
                        date = structure(c(17094, 17093, 17094, 17095, 17096, 17095, 17095, 17096), class = "Date")),
                   .Names = c("id", "date"), row.names = c(NA, -8L), class = "data.frame")

Use the built in table function.

> table(data0)
   date
id  2016-10-19 2016-10-20 2016-10-21 2016-10-22
  1          1          2          0          0
  2          0          0          1          1
  3          0          0          2          1

Answer 2

Another way around using xtabs :

data0$col <- 1
xtabs(col~id+date, data0)

#   date
#id  2016-10-19 2016-10-20 2016-10-21 2016-10-22
#  1          1          2          0          0
#  2          0          0          1          1
#  3          0          0          2          1