Calculate number of pending tasks at given time points (ideally with dplyr) - r

Question

I have a database containing a list of events. Each event has an associated start date, and a date when the event ended or was completed, eg:
dataset <- tibble(
eventid = sample(1:100, 25, replace=TRUE),
start_date = sample(seq(as.Date('2011/01/01'), as.Date('2012/01/01'), by="day"), 25),
completed_date = sample(seq(as.Date('2012/01/01'), as.Date('2014/01/01'), by="day"), 25)
)
> dataset
# A tibble: 25 x 3
eventid start_date completed_date
<int> <date> <date>
1 57 2011-01-14 2013-01-07
2 97 2011-01-21 2011-03-03
3 58 2011-01-26 2011-02-05
4 25 2011-03-22 2013-07-20
5 8 2011-04-20 2012-07-16
6 81 2011-04-26 2013-03-04
7 42 2011-05-02 2012-01-16
8 77 2011-05-03 2012-08-14
9 78 2011-05-21 2013-09-26
10 49 2011-05-22 2013-01-04
# ... with 15 more rows
>
I am trying to produce a rolling "snapshot" of how many tasks were pending a different points in time, e.g. month by month. Expected result:
# A tibble: 25 x 2
month count
<date> <int>
1 2011-01-01 0
2 2011-02-01 3
3 2011-03-01 2
4 2011-04-01 2
5 2011-05-01 4
6 2011-06-01 8
I have attempted to group my variables using group_by(period=floor_date(start_date,"month")), but I'm a bit stuck and would appreciate a pointer in the right direction!
I would prefer a solution using dplyr if possible.
Thanks!

Answer 1

You can expand rows for each month included in the range of dates with map2 from purrr. map2 will iterate over multiple inputs simultaneously. In this case, it will iterate through the start and end dates at the same time.
In each iteration, if will create a monthly sequence using seq (or seq.Date) from start to end month (determined from floor_date). The result is nested for each row of data (since one row can have multiple months in the sequence). So, unnest is needed afterwards.
The transmute will add a new variable called month_year (and drop the old ones) and use substr to extract the year and month only (no day). This is the first through seventh character of the date.
Then, you can group_by the month-year and count up the number of pending projects for each month_year.
I included set.seed to reproduce from data below.
library(dplyr)
library(tidyr)
library(purrr)
library(lubridate)
dataset %>%
mutate(month = map2(floor_date(start_date, "month"),
floor_date(completed_date, "month"),
seq.Date,
by = "month")) %>%
unnest(month) %>%
transmute(month_year = substr(month, 1, 7)) %>%
group_by(month_year) %>%
summarise(count = n())
Output
month_year count
<chr> <int>
1 2011-01 1
2 2011-02 3
3 2011-03 9
4 2011-04 10
5 2011-05 13
6 2011-06 15
7 2011-07 16
8 2011-08 18
9 2011-09 19
10 2011-10 20
# … with 22 more rows
If you want to exclude the completed month (except when start month and completed month are the same, if that can exist), you can subtract 1 month from the sequence of months created. In this case, you can use pmax so that if both start and end months are the same, it will still count the month).
Here is the modified mutate with map2:
mutate(month = map2(floor_date(start_date, "month"),
pmax(floor_date(completed_date, "month") - 1, floor_date(start_date, "month")),
seq.Date,
by = "month"))
Data
set.seed(123)
dataset <- tibble(
eventid = sample(1:100, 25, replace=TRUE),
start_date = sample(seq(as.Date('2011/01/01'), as.Date('2012/01/01'), by="day"), 25),
completed_date = sample(seq(as.Date('2012/01/01'), as.Date('2014/01/01'), by="day"), 25)
)