Mapping ymd_hms time into 15 minute time intervals

I am looking for a data.table solution to the following question.

Suppose I have the following dataset:

library(data.table)
library(lubridate)

DT <- data.table(date = c("2019-01-01 00:03:04", 
                          "2019-01-01 00:07:03",
                          "2019-01-01 00:15:23",
                          "2019-01-01 00:16:28",
                          "2019-01-01 00:21:30"))
DT
         date
1: 2019-01-01 00:03:04
2: 2019-01-01 00:07:03
3: 2019-01-01 00:15:23
4: 2019-01-01 00:16:28
5: 2019-01-01 00:21:30

I am looking to map each date into 15 minute interval for each day. So the final dataset would look like:

   date                    date_15min_grp
1: 2019-01-01 00:03:04              1
2: 2019-01-01 00:07:03              1
3: 2019-01-01 00:15:23              2
4: 2019-01-01 00:16:28              2
5: 2019-01-01 00:21:30              2

The groups would be unique at the day level, so 2019-01-01 00:03:04 and 2019-01-02 00:03:04 would both return values of 1. My current solution is to extract the hour/minute/second component of each day. I would then add the values (after conversion into minutes), divide them by 60, and try to map each value to the nearest "group". But this seems to be slow and inelegant.

A data.table/lubridate would be greatly appreciated.

Thanks so much!

Solution

Please find below a solution using data.table and lubridate

Reprex

Code

library(data.table)
library(lubridate)

DT[, date_15min_grp := fcase(minute(date) < 15, 1,
                             minute(date) < 30, 2,
                             minute(date) < 45, 3,
                             default = 4)][]

Output

#>                   date date_15min_grp
#> 1: 2019-01-01 00:03:04              1
#> 2: 2019-01-01 00:07:03              1
#> 3: 2019-01-01 00:15:23              2
#> 4: 2019-01-01 00:16:28              2
#> 5: 2019-01-01 00:21:30              2

^{Created on 2021-11-30 by the reprex package (v2.0.1)}

AS A FOLLOW-UP TO YOUR COMMENT

Code

library(data.table)
library(lubridate)

DT[, date_15min_grp := fcase(minute(date) < 15, hour(date)*4 + 1,
                             minute(date) < 30, hour(date)*4 + 2,
                             minute(date) < 45, hour(date)*4 + 3,
                             minute(date) < 60, hour(date)*4 + 4)][]

Output

#>                   date date_15min_grp
#> 1: 2019-01-01 00:03:04              1
#> 2: 2019-01-01 00:07:03              1
#> 3: 2019-01-01 00:15:23              2
#> 4: 2019-01-01 00:16:28              2
#> 5: 2019-01-01 00:21:30              2

^{Created on 2021-12-01 by the reprex package (v2.0.1)}