A parallelized version of for loop

# A general procedure for parallelized for loop
# Even with 4 cores, the gain is interesting

# Let take this example
A <- 1:10000
system.time({
out <- NULL
for (i in A) {
  # I do something with i, an element of A
  # this part must be copied
  j <- sqrt(i)
  out <- c(out, j)
  # I go out of the loop
}
})

library("parallel")
system.time({
B <- split(A, cut(A, detectCores(), labels = FALSE))

out <- unname(unlist(mclapply(B, FUN= function(Bi) {
  outi <- NULL
  for (i in Bi) {
    # I do something with i, an element of A
    j <- sqrt(i)
    outi <- c(outi, j)
    # I go out of the loop
  }
  return(outi)
})))
})

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