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/如何将多个输出作为 foreach dopar 循环的一部分返回.

让我们举一个非常简单的例子.假设我想在 foreach 循环中执行 2 个操作，并且想为 i 的每个值返回或保存这两个操作的结果.

只返回一个输出，就这么简单:

库(foreach)库(doParallel)cl <- makeCluster(3)registerDoParallel(cl)oper1 <- foreach(i=1:100000) %dopar% {我+2}

oper1 将是一个包含 100000 个元素的列表，每个元素都是对 i 的每个值进行 i+2 操作的结果.

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:

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

希望i+2的结果保存在列表oper1中，第二次操作的结果i+3> 将由 foreach 返回.但是，列表 oper1 中没有填充任何内容！在这种情况下，循环只返回 i+3 的结果.

有没有办法将两个输出返回或保存在两个单独的列表中?

解决方案

不要尝试对 foreach 或任何其他并行程序包使用副作用.相反，从列表中的 foreach 循环体中返回所有值.如果您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，请指定一个组合函数来转置结果:

comb <- function(x, ...) {lapply(seq_along(x),函数(i) c(x[[i]], lapply(list(...), function(y) y[[i]])))}操作 <- foreach(i=1:10, .combine='comb', .multicombine=TRUE,.init=list(list(), list())) %dopar% {列表(i+2, i+3)}oper1 <- oper[[1]]oper2 <- oper[[2]]

请注意，此组合函数需要使用 .init 参数为组合函数的第一次调用设置 x 的值.

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

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.

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

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)
}

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.

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

解决方案

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]]

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.

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