Can you make dplyr::mutate and dplyr::lag default = its own input value?

Question

This is similar to this dplyr lag post, and this dplyr mutate lag post, but neither of those ask this question about defaulting to the input value. I am using dplyr to muta

Answer 1

In the OP's code ...

...
d) group_by(ip) %>%
e) mutate(shifted = dplyr::lag(fulldate, default=fulldate)) %>%
...

The default= argument should have a length of one. Replacing the OP's code with default = first(fulldate) should work in this case (since the first element won't have a lag and so is where we need to apply the default value).

Related cases:

• Similarly, with a "lead", we'd want dplyr::lead(x, default=last(x)).
• With a lag or lead of more than one step (n greater than 1), default= cannot do it and we'd probably need to switch to if_else or case_when or similar. (I'm not sure about the current tidyverse idiom.)

Answer 2

How about

ifelse(is.na(lag(value)), value, lag(value))

Answer 3

I think Frank's solution works pretty well. Here is the complete example:

library(dplyr, warn.conflicts = F)

test <- data.frame(ip=c("192.168.1.2","192.168.1.2","192.168.1.4","192.168.1.4","192.168.1.4","192.168.1.7"),
                   hour=c(2017070700,2017070700,2017070700,2017070701,2017070702,2017070700),
                   snap=c(0,15,0,45,30,15))


test %>%
  mutate(snap = formatC(snap, width = 2, flag = 0)) %>%
  mutate(fulldateint = paste(hour, snap, sep = "")) %>%
  mutate(fulldate = as.POSIXct(strptime(fulldateint, "%Y%m%d%H%M"))) %>%
  group_by(ip) %>%
  mutate(shifted = lag(fulldate, default = first(fulldate))) %>%
  mutate(diff = fulldate - shifted) %>% 
  ungroup() %>% 
  select(ip, fulldate, shifted, diff)

#> # A tibble: 6 x 4
#>            ip            fulldate             shifted      diff
#>        <fctr>              <dttm>              <dttm>    <time>
#> 1 192.168.1.2 2017-07-07 00:00:00 2017-07-07 00:00:00    0 secs
#> 2 192.168.1.2 2017-07-07 00:15:00 2017-07-07 00:00:00  900 secs
#> 3 192.168.1.4 2017-07-07 00:00:00 2017-07-07 00:00:00    0 secs
#> 4 192.168.1.4 2017-07-07 01:45:00 2017-07-07 00:00:00 6300 secs
#> 5 192.168.1.4 2017-07-07 02:30:00 2017-07-07 01:45:00 2700 secs
#> 6 192.168.1.7 2017-07-07 00:15:00 2017-07-07 00:15:00    0 secs