Expand rows by date range using start and end date

Using data.table:

require(data.table) ## 1.9.2+
setDT(df)[ , list(idnum = idnum, month = seq(start, end, by = "month")), by = 1:nrow(df)]

# you may use dot notation as a shorthand alias of list in j:
setDT(df)[ , .(idnum = idnum, month = seq(start, end, by = "month")), by = 1:nrow(df)]

setDT converts df to a data.table. Then for each row, by = 1:nrow(df), we create idnum and month as required.

Using dplyr :

test %>%
    group_by(idnum) %>%
    summarize(start=min(start),end=max(end)) %>%
    do(data.frame(idnum=.$idnum, month=seq(.$start,.$end,by="1 month")))

Note that here I don't generate a sequence between start and end for each row, instead it is a sequence between min(start) and max(end) for each idnum. If you want the former :

test %>%
    rowwise() %>%
    do(data.frame(idnum=.$idnum, month=seq(.$start,.$end,by="1 month")))

Updated2

With new versions of purrr (0.3.0) and dplyr (0.8.0), this can be done with map2

library(dplyr)
library(purrr)
 test %>%
     # sequence of monthly dates for each corresponding start, end elements
     transmute(idnum, month = map2(start, end, seq, by = "1 month")) %>%
     # unnest the list column
     unnest %>% 
     # remove any duplicate rows
     distinct

Updated

Based on @Ananda Mahto's comments

 res1 <- melt(setNames(lapply(1:nrow(test), function(x) seq(test[x, "start"],
 test[x, "end"], by = "1 month")), test$idnum))

Also,

  res2 <- setNames(do.call(`rbind`,
          with(test, 
          Map(`expand.grid`,idnum,
          Map(`seq`, start, end, by='1 month')))), c("idnum", "month"))


  head(res1)
 #  idnum      month
 #1    17 1993-01-01
 #2    17 1993-02-01
 #3    17 1993-03-01
 #4    17 1993-04-01
 #5    17 1993-05-01
 #6    17 1993-06-01

tidyverse answer

Data

df <- structure(list(idnum = c(17L, 17L, 17L), start = structure(c(8401, 
8401, 8401), class = "Date"), end = structure(c(8765, 8765, 8765
), class = "Date")), class = "data.frame", .Names = c("idnum", 
"start", "end"), row.names = c(NA, -3L))

Answer and output

library(tidyverse)
df %>%
  nest(start, end) %>%
  mutate(data = map(data, ~seq(unique(.x$start), unique(.x$end), 1))) %>%
  unnest(data)

# # A tibble: 365 x 2
   # idnum       data
   # <int>     <date>
 # 1    17 1993-01-01
 # 2    17 1993-01-02
 # 3    17 1993-01-03
 # 4    17 1993-01-04
 # 5    17 1993-01-05
 # 6    17 1993-01-06
 # 7    17 1993-01-07
 # 8    17 1993-01-08
 # 9    17 1993-01-09
# 10    17 1993-01-10
# # ... with 355 more rows