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.

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

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")))