R語言中for循環(huán)的并行處理方式

更新時間：2021年04月21日 11:11:08 作者：SANGF

這篇文章主要介紹了R語言中for循環(huán)的并行處理方式，具有很好的參考價值，希望對大家有所幫助。一起跟隨小編過來看看吧

前言

本文用于記錄筆者在將R語言中的for語句并行化處理中的一些問題。

實驗

這里使用foreach和doParallel包提供的函數(shù)實現(xiàn)for語句的并行處理。

for語句腳本

func <- function(x, y, z) {
  return(x^y/z)
}
# >>> main <<<
x <- 2
y <- 3
z <- 1:100000
start <- (proc.time())[3][[1]]
a <- 0
for (i_z in z) {
  a <- a + func(x, y, i_z)
}
end <- (proc.time())[3][[1]]
print(paste('Result = ', round(a, 2), ', time = ', (end-start), 's', sep=''))

輸出：

[1] "Result = 96.72, time = 0.177s"

并行化版本

library(foreach)
library(doParallel)
func <- function(x, y, z) {
  return(x^y/z)
}
# >>> main <<<
x <- 2
y <- 3
z <- 1:100000
start <- (proc.time())[3][[1]]
cl <- makeCluster(12)
registerDoParallel(cl)
a <- foreach(z=z, .combine='rbind') %dopar% func(x, y, z)
a <- sum(a)
stopCluster(cl)
end <- (proc.time())[3][[1]]
print(paste('Result = ', round(a, 2), ', time = ', (end-start), 's', sep=''))

輸出：

[1] "Result = 96.72, time = 37.988s"

總結(jié)

1、這里發(fā)現(xiàn)并行化所用時間大于非并行化所用過的時間，是因為需要執(zhí)行的操作（func函數(shù)）過于簡單，而foreach處理時會有額外的資源消耗。此時foreach額外消耗的資源遠大于需要執(zhí)行的操作所需的資源，因此會導致并行化后反而使用的時間增加了。所以對于一些復雜的操作比較適合使用并行化的策略。

2、foreach函數(shù)的.packages參數(shù)可以為并行化函數(shù)傳遞額外需要的包。

3、foreach中的參數(shù)為需要在func中循環(huán)的變量，其他固定的變量則在func中傳入。參數(shù)可以是data.frame類型。

補充：R語言--for循環(huán)語句的使用

R語言for循壞語句的使用（多個for)

對于多個for循還語句，R語言的執(zhí)行順序（以3個for為例）：從外向內(nèi)單個執(zhí)行，里邊循還完整，再往外一層，直到全部完成。話不多說，上例子：

代碼：

library(data.table)
mm<-data.table()
m<-c(1,2,3,4,5)
n<-c('a','b','c','d','e')
o<-c(6,7,8,9,10)
for (i1 in m){
  for ( i2 in n){
     for (i3 in o){
       print(c(i1,i2,i3))
       aa<-data.table(i1,i2,i3)
       bb<-rbind(mm,aa)
       
     }
  }
}

執(zhí)行結(jié)果：

[1] "1" "a" "6"
[1] "1" "a" "7"
[1] "1" "a" "8"
[1] "1" "a" "9"
[1] "1"  "a"  "10"
[1] "1" "b" "6"
[1] "1" "b" "7"
[1] "1" "b" "8"
[1] "1" "b" "9"
[1] "1"  "b"  "10"
[1] "1" "c" "6"
[1] "1" "c" "7"
[1] "1" "c" "8"
[1] "1" "c" "9"
[1] "1"  "c"  "10"
[1] "1" "d" "6"
[1] "1" "d" "7"
[1] "1" "d" "8"
[1] "1" "d" "9"
[1] "1"  "d"  "10"
[1] "1" "e" "6"
[1] "1" "e" "7"
[1] "1" "e" "8"
[1] "1" "e" "9"
[1] "1"  "e"  "10"
[1] "2" "a" "6"
[1] "2" "a" "7"
[1] "2" "a" "8"
[1] "2" "a" "9"
[1] "2"  "a"  "10"
[1] "2" "b" "6"
[1] "2" "b" "7"
[1] "2" "b" "8"
[1] "2" "b" "9"
[1] "2"  "b"  "10"
[1] "2" "c" "6"
[1] "2" "c" "7"
[1] "2" "c" "8"
[1] "2" "c" "9"
[1] "2"  "c"  "10"
[1] "2" "d" "6"
[1] "2" "d" "7"
[1] "2" "d" "8"
[1] "2" "d" "9"
[1] "2"  "d"  "10"
[1] "2" "e" "6"
[1] "2" "e" "7"
[1] "2" "e" "8"
[1] "2" "e" "9"
[1] "2"  "e"  "10"
[1] "3" "a" "6"
[1] "3" "a" "7"
[1] "3" "a" "8"
[1] "3" "a" "9"
[1] "3"  "a"  "10"
[1] "3" "b" "6"
[1] "3" "b" "7"
[1] "3" "b" "8"
[1] "3" "b" "9"
[1] "3"  "b"  "10"
[1] "3" "c" "6"
[1] "3" "c" "7"
[1] "3" "c" "8"
[1] "3" "c" "9"
[1] "3"  "c"  "10"
[1] "3" "d" "6"
[1] "3" "d" "7"
[1] "3" "d" "8"
[1] "3" "d" "9"
[1] "3"  "d"  "10"
[1] "3" "e" "6"
[1] "3" "e" "7"
[1] "3" "e" "8"
[1] "3" "e" "9"
[1] "3"  "e"  "10"
[1] "4" "a" "6"
[1] "4" "a" "7"
[1] "4" "a" "8"
[1] "4" "a" "9"
[1] "4"  "a"  "10"
[1] "4" "b" "6"
[1] "4" "b" "7"
[1] "4" "b" "8"
[1] "4" "b" "9"
[1] "4"  "b"  "10"
[1] "4" "c" "6"
[1] "4" "c" "7"
[1] "4" "c" "8"
[1] "4" "c" "9"
[1] "4"  "c"  "10"
[1] "4" "d" "6"
[1] "4" "d" "7"
[1] "4" "d" "8"
[1] "4" "d" "9"
[1] "4"  "d"  "10"
[1] "4" "e" "6"
[1] "4" "e" "7"
[1] "4" "e" "8"
[1] "4" "e" "9"
[1] "4"  "e"  "10"
[1] "5" "a" "6"
[1] "5" "a" "7"
[1] "5" "a" "8"
[1] "5" "a" "9"
[1] "5"  "a"  "10"
[1] "5" "b" "6"
[1] "5" "b" "7"
[1] "5" "b" "8"
[1] "5" "b" "9"
[1] "5"  "b"  "10"
[1] "5" "c" "6"
[1] "5" "c" "7"
[1] "5" "c" "8"
[1] "5" "c" "9"
[1] "5"  "c"  "10"
[1] "5" "d" "6"
[1] "5" "d" "7"
[1] "5" "d" "8"
[1] "5" "d" "9"
[1] "5"  "d"  "10"
[1] "5" "e" "6"
[1] "5" "e" "7"
[1] "5" "e" "8"
[1] "5" "e" "9"
[1] "5"  "e"  "10"

以上為個人經(jīng)驗，希望能給大家一個參考，也希望大家多多支持腳本之家。如有錯誤或未考慮完全的地方，望不吝賜教。

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