保存foreach dopar循环的多个输出 [英] Saving multiple outputs of foreach dopar loop

问题描述

我想知道是否可以/如何作为foreach dopar循环的一部分返回多个输出.

I would like to know if/how it would be possible to return multiple outputs as part of foreach dopar loop.

让我们举一个非常简单的例子.假设我想在foreach循环中进行2次操作，并且想为i的每个值返回或保存这两项操作的结果.

Let's take a very simplistic example. Let's suppose I would like to do 2 operations as part of the foreach loop, and would like to return or save the results of both operations for each value of i.

仅返回一个输出，就很简单:

For only one output to return, it would be as simple as:

library(foreach)
library(doParallel)
cl <- makeCluster(3)
registerDoParallel(cl)

oper1 <- foreach(i=1:100000) %dopar% {
    i+2
}

oper1将是一个包含100000个元素的列表，每个元素都是针对每个i值的操作i+2的结果.

oper1 would be a list with 100000 elements, each element is the result of the operation i+2 for each value of i.

假设现在我想分别返回或保存两个不同操作的结果，例如i+2和i+3.我尝试了以下方法:

Suppose now I would like to return or save the results of two different operations separately, e.g. i+2 and i+3. I tried the following:

oper1 = list()
oper2 <- foreach(i=1:100000) %dopar% {
    oper1[[i]] = i+2
    return(i+3)
}

希望i+2的结果将保存在列表oper1中，并且第二个操作i+3的结果将由foreach返回.但是，列表oper1中没有任何内容！在这种情况下，只有i+3的结果会从循环中返回.

hoping that the results of i+2 will be saved in the list oper1, and that the results of the second operation i+3 will be returned by foreach. However, nothing gets populated in the list oper1! In this case, only the result of i+3 gets returned from the loop.

有没有办法将两个输出都返回或保存在两个单独的列表中?

Is there any way of returning or saving both outputs in two separate lists?

推荐答案

不要尝试将副作用与foreach或任何其他并行程序包一起使用.而是从列表中的foreach循环的主体返回所有值.如果希望最终结果是两个列表的列表，而不是100,000个列表的列表，则指定一个组合函数以对结果进行转置:

Don't try to use side-effects with foreach or any other parallel program package. Instead, return all of the values from the body of the foreach loop in a list. If you want your final result to be a list of two lists rather than a list of 100,000 lists, then specify a combine function that transposes the results:

comb <- function(x, ...) {
  lapply(seq_along(x),
    function(i) c(x[[i]], lapply(list(...), function(y) y[[i]])))
}

oper <- foreach(i=1:10, .combine='comb', .multicombine=TRUE,
                .init=list(list(), list())) %dopar% {
  list(i+2, i+3)
}

oper1 <- oper[[1]]
oper2 <- oper[[2]]

请注意，此合并功能要求使用.init参数为第一次调用合并功能设置x的值.

Note that this combine function requires the use of the .init argument to set the value of x for the first invocation of the combine function.

这篇关于保存foreach dopar循环的多个输出的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！