Merge nearest date, and related variables from a another dataframe by group

Question

I have two dataframes each with multiple rows per ID. I need to return the closest date and related data from the second dataframe based on the ID and date of the first data

Answer 1

Here's my take using dplyr, based on the accepted answer. I wanted to have a bit more freedom on the grouping column.

match_by_group_date <- function(df1, df2, grp, datecol) {
  
  grp1 <- df1 %>% pull({{grp}}) %>% unique()
  grp2 <- df2 %>% pull({{grp}}) %>% unique()
  
  li <-
  lapply(intersect(grp1, grp2), function(tt) {
    d1 <- filter(df1, {{grp}}== tt)
    d2 <- filter(df2, {{grp}}==tt) %>% mutate(indices = 1:n())
    d2_date <- d2 %>% pull({{datecol}}) %>% as.POSIXct()
    print(d2_date)
    d1 <- mutate(d1, indices = map_dbl({{datecol}}, function(d) which.min(abs(d2_date - as.POSIXct(d)))))
    
    left_join(d1,d2, by=c(quo_name(enquo(grp)), "indices"))
  })
  
  # bind rows
  return(bind_rows(li))
}