Expand start and end dates to unbalanced monthly panel with dplyr

Question

I have start and end dates for events that I want to expand into a monthly panel, and I wanted to know if there was any tool in dplyr for solving this problem. The following code does what I want to do with ddply(). It first creates an example tibble data.frame (called "wide") where "id" represents an individual and "HomeNum" is an event for that individual. The next line creates a "date" variable that is a monthly series from "StartDate" to "FinishDate" within each "id" by "HomeNum" group.

library(plyr)
library(dplyr)
library(tibble)
wide = 
    tibble(
        id = c(1, 1, 2, 2, 2),
        HomeNum = c(0,1,0,1,2),
        StartDate = as.Date(c("2001-01-01", "2001-03-01", "2000-04-01", "2001-02-01", "2002-08-01")),
        FinishDate = as.Date(c("2001-02-01", "2002-05-01", "2001-01-01", "2002-07-01", "2002-12-01"))
    )
panel = 
    ddply(wide, 
          ~id+HomeNum, 
          transform, 
          date = seq.Date(StartDate, FinishDate, by = "month")
    )

I assume that dplyr, as the "the next iteration of plyr", must have some way to implement a similar solution (and output a tibble), but the following did not work:

panel = 
    wide %>% 
    group_by(id, HomeNum) %>% 
    mutate(date = seq.Date(StartDate, FinishDate, by = "month")) 

and returned

Error in mutate_impl(.data, dots) :
    Column `date` must be length 1 (the group size), not 2

Frankly, I am surprised that the ddply() solution works and does not throw a similar error.

My implementation with ddply() is similar to answers to this question.

Answer 1

You can coerce the elements of date to lists and unnest.

library(tidyverse)
wide %>%
  group_by(id, HomeNum) %>%
  mutate(date = list(seq.Date(StartDate, FinishDate, by = "month"))) %>%
  unnest(date)