## ----style, echo = FALSE, results = 'asis'--------------------------------------------------------
options(width=100)
knitr::opts_chunk$set(
    eval=as.logical(Sys.getenv("KNITR_EVAL", "TRUE")),
    cache=as.logical(Sys.getenv("KNITR_CACHE", "TRUE")))

## ----setup, echo=FALSE----------------------------------------------------------------------------
suppressPackageStartupMessages({
    library(BiocParallel)
    library(microbenchmark)
})

## ----vectorize------------------------------------------------------------------------------------
x <- 1:10
log(x)     ## NOT for (i in seq_along(x)) x[i] <- log(x[i])

## ----pre-allocate---------------------------------------------------------------------------------
result <- numeric(10)
result[1] <- runif(1)
for (i in 2:length(result))
       result[i] <- runif(1) * result[i - 1]
result

## ----inefficient----------------------------------------------------------------------------------
f0 <- function(n, a=2) {
    ## stopifnot(is.integer(n) && (length(n) == 1) &&
    ##           !is.na(n) && (n > 0))
    result <- numeric()
    for (i in seq_len(n))
        result[[i]] <- a * log(i)
    result
}

## ----system-time----------------------------------------------------------------------------------
system.time(f0(10000))
n <- 1000 * seq(1, 20, 2)
t <- sapply(n, function(i) system.time(f0(i))[[3]])
plot(t ~ n, type="b")

## ----correct-init---------------------------------------------------------------------------------
n <- 10000
system.time(expected <- f0(n))
head(expected)

## ----hoist----------------------------------------------------------------------------------------
f1 <- function(n, a=2) {
    result <- numeric()
    for (i in seq_len(n))
        result[[i]] <- log(i)
    a * result
}
identical(expected, f1(n))

library(microbenchmark)
microbenchmark(f0(n), f1(n), times=5)

## ----preallocate-and-fill-------------------------------------------------------------------------
f2 <- function(n, a=2) {
    result <- numeric(n)
    for (i in seq_len(n))
        result[[i]] <- log(i)
    a * result
}
identical(expected, f2(n))
microbenchmark(f0(n), f2(n), times=5)

## ----use-apply------------------------------------------------------------------------------------
f3 <- function(n, a=2)
    a * sapply(seq_len(n), log)

identical(expected, f3(n))
microbenchmark(f0(n), f2(n), f3(n), times=10)

## ----use-vectorize--------------------------------------------------------------------------------
f4 <- function(n, a=2)
    a * log(seq_len(n))
identical(expected, f4(n))
microbenchmark(f0(n), f3(n), f4(n), times=10)

## ----vectorized-scale-----------------------------------------------------------------------------
n <- 10 ^ (5:8)                         # 100x larger than f0
t <- sapply(n, function(i) system.time(f4(i))[[3]])
plot(t ~ n, log="xy", type="b")

## ----parallel-sleep-------------------------------------------------------------------------------
library(BiocParallel)

fun <- function(i) {
    Sys.sleep(1)
    i
}

## serial
f0 <- function(n)
    lapply(seq_len(n), fun)

## parallel
f1 <- function(n)
    bplapply(seq_len(n), fun)