How to do if {multiple statements for multiple variables} within group_by data frame

[lzhang2020]

Hello, all:
I am trying to do something like:

df%>%arrange(id,drugclass, start_date, end_date)%>%
group_by(id,drugclass)%>%
if(row_number==1) then {a1=a3; a2=a4}
else if ( ) {a1=a5; a2=a6}

something like this. Can I do it?

I am trying to do it by mutate ifelse. But ifelse only allows one output variable. I can't do something for both a1 and a2.
Any suggestions? thanks a lot!

[FJCC]

Can you post a small sample of your data? The output of the command

dput(head(df))

would be very helpful. Simplifying the data to the minimum number of required columns would be best.

[woodward]

If general you'll have to do it with two ifelse functions inside the mutate. Or even better use dplyr::case_when which is much more flexible. If your conditional boolean expression is complicated you could calculate this first

df%>%
  arrange(id,drugclass, start_date, end_date)%>%
  group_by(id,drugclass)%>%
  mutate(
    cond = row_number == 1, # do it like this for complex conditions
    a1 = case_when(
      cond ~ a3,
      TRUE ~ a5 # otherwise
    ),
    a2 = case_when(
      cond ~ a4,
      TRUE ~ a6 # otherwise
    )
  )