constructing an identifier string for each row in data

Question

I have the following data:

library(data.table)
d = data.table(a = c(1:3), b = c(2:4))

and would like to get this result (in a way that would work with arbitrary number of columns):

d[, c := paste0('a_', a, '_b_', b)]
d
#   a b       c
#1: 1 2 a_1_b_2
#2: 2 3 a_2_b_3
#3: 3 4 a_3_b_4

The following works, but I'm hoping to find something shorter and more legible.

d = data.table(a = c(1:3), b = c(2:4))
d[, c := apply(mapply(paste, names(.SD), .SD, MoreArgs = list(sep = "_")),
               1, paste, collapse = "_")]

Answer 1

one way, only slightly cleaner:

d[, c :=  apply(d, 1, function(x) paste(names(d), x, sep="_", collapse="_")) ]

     a b       c
1: 1 2 a_1_b_2
2: 2 3 a_2_b_3
3: 3 4 a_3_b_4

Answer 2

Here is an approach using do.call('paste'), but requiring only a single call to paste

I will benchmark on a situtation where the columns are integers (as this seems a more sensible test case

N <- 1e4

d <- setnames(as.data.table(replicate(5, sample(N), simplify = FALSE)), letters[seq_len(5)])

f5 <- function(d){
  l <- length(d)
  o <- c(1L, l + 1L) + rep_len(seq_len(l) -1L, 2L * l)
  do.call('paste',c((c(as.list(names(d)),d))[o],sep='_'))}


microbenchmark(f1(d), f2(d),f5(d))
Unit: milliseconds
  expr       min        lq    median        uq       max neval
 f1(d)  41.51040  43.88348  44.60718  45.29426  52.83682   100
 f2(d) 193.94656 207.20362 210.88062 216.31977 252.11668   100
 f5(d)  30.73359  31.80593  32.09787  32.64103  45.68245   100