24.6 Avoiding copies

A pernicious source of slow R code is growing an object with a loop. Whenever you use c(), append(), cbind(), rbind(), or paste() to create a bigger object, R must first allocate space for the new object and then copy the old object to its new home. If you’re repeating this many times, like in a for loop, this can be quite expensive. You’ve entered Circle 2 of the R inferno.

You saw one example of this type of problem in Section 23.2.2, so here I’ll show a slightly more complex example of the same basic issue. We first generate some random strings, and then combine them either iteratively with a loop using collapse(), or in a single pass using paste(). Note that the performance of collapse() gets relatively worse as the number of strings grows: combining 100 strings takes almost 30 times longer than combining 10 strings.

random_string <- function() {
paste(sample(letters, 50, replace = TRUE), collapse = "")
}
strings10 <- replicate(10, random_string())
strings100 <- replicate(100, random_string())

collapse <- function(xs) {
out <- ""
for (x in xs) {
out <- paste0(out, x)
}
out
}

bench::mark(
loop10  = collapse(strings10),
loop100 = collapse(strings100),
vec10   = paste(strings10, collapse = ""),
vec100  = paste(strings100, collapse = ""),
check = FALSE
)[c("expression", "min", "median", "itr/sec", "n_gc")]
#> # A tibble: 4 x 4
#>   expression      min   median itr/sec
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl>
#> 1 loop10      27.95µs  30.31µs    32282.
#> 2 loop100     681.4µs 706.48µs     1410.
#> 3 vec10        5.68µs   5.93µs   163946.
#> 4 vec100      37.85µs  39.03µs    25399.

Modifying an object in a loop, e.g., x[i] <- y, can also create a copy, depending on the class of x. Section 2.5.1 discusses this issue in more depth and gives you some tools to determine when you’re making copies.