loop.R

f1 <- function(n) {
  x <- numeric()
  for (i in 1:n) {
    x <- c(x, i)
  }
  x
}

f1.5 <- function(n) {
  x <- numeric()
  for (i in 1:n) {
    x[[i]] <- i
  }
  
  x
}

f2 <- function(n) {
  x <- numeric(n)
  for (i in 1:n) {
    x[[i]] <- i
  }
  x
}

bench::mark(f1(1e4), f1.5(1e4), f2(1e4))
#> Warning: Some expressions had a GC in every iteration; so filtering is disabled.
#> # A tibble: 3 × 6
#>   expression       min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>  <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 f1(10000)      109ms 113.34ms      8.77     382MB    77.2 
#> 2 f1.5(10000)    936µs   1.05ms    726.      1.68MB    67.6 
#> 3 f2(10000)      212µs 216.77µs   4495.     96.77KB     6.00

I switched both to [[; as I'd expect, that made very little difference.