loops-are-not-that-bad.R

library(tidyverse)

n <- 1000
v <- 1:n

vector_c <- function(n) {
    out <- c()
    for (i in 1:n) {
        out <- c(out, i)
    }
    out
}

vector_unalloc_two_bracket <- function(n) {
    out <- c()
    for (i in 1:n) {
        out[[i]] <- i
    }
    unlist(out)
}

vector_prealloc_two_bracket <- function(n) {
    out <- vector("numeric", n)
    for (i in 1:n) {
        out[[i]] <- i
    }
    out
}

vector_c_unlist <- function(n) {
    results <- vector(mode = "list", length = n)
    results[[1]] <- v[1]
    for (i in 1:(n-1)){
        results[[i]] <- c(results[i], v[i+1])
    }
    results |> unlist()
}


bench::mark(
    vector_c = vector_c(n),
    prealloc_two_bracket = vector_prealloc_two_bracket(n),
    unalloc_two_bracket = vector_unalloc_two_bracket(n),
    vector_c_unlist = vector_c_unlist(n),
    filter_gc = FALSE
) %>% 
    select("expression", "median", "itr/sec", "mem_alloc") %>% 
    arrange(desc(median)) %>% 
    mutate(times_faster = as.double(first(median)/median))

# A tibble: 4 × 5
  expression             median `itr/sec` mem_alloc times_faster
                           
1 vector_c              581.3µs     1111.    1.97MB         1   
2 vector_c_unlist       236.9µs     3783.   56.79KB         2.45
3 unalloc_two_bracket   230.9µs     3750.  190.35KB         2.52
4 prealloc_two_bracket   26.8µs    35908.   26.84KB        21.7