foreach包中列表的结构 [英] Structure of lists in foreach package

问题描述

我在foreach软件包中发现了一个功能/错误，我不理解.也许有人可以向我解释这种行为:

I have found a feature/bug in the foreach package, which I do not understand. Perhaps someone can explain me this behaviour:

我用foreach包创建了一个for循环(我将它们与mutlicore计算一起使用，但是在下面的示例中，该错误在两种变体中均出现).此循环运行r次.在每次运行中，都会返回带有c条目的列表.因此，我希望有一个包含r个条目的列表，并且每个条目都由c个列表组成.

I created a for-loop with the foreach package (I use them together with mutlicore calculations, but here just in a sequentiell example, the bug appears in both variants). This loop runs r times. In every run a list with c entries is returned. So I expect a list with r entries, and every entry consists of c lists.

我的代码如下:

library(foreach)

clusters <- 10
runs <- 100

temp <- foreach(r = 1:runs,
                .combine = 'list',
                .multicombine = TRUE) %do% {

              signal_all <- lapply(1:clusters, function(x){

                return(1)

              })

              return(signal_all)
            } ## end do

使用此代码，所有功能均可按预期运行，请参见下图:

With this code, all works as expected, see the following picture:

但是当增加runs <- 101时，输出temp是这样的:

But when increasing runs <- 101, the output temp is this:

预期的列表结构被破坏.但是在注释.combine = 'list'行时，所有操作都按预期进行.

The expected list structure is destroyed. But when commenting out the line .combine = 'list' all works as expected.

library(foreach)

clusters <- 10
runs <- 100

temp <- foreach(r = 1:runs,
                .multicombine = TRUE) %do% {

              signal_all <- lapply(1:clusters, function(x){

                return(1)

              })

              return(signal_all)
            } ## end do

有人可以解释这种行为吗? 感谢您的帮助！

Can someone explain this behaviour? Thanks for any help!

推荐答案

同时，我找到了解决方案.

Meanwhile I have found a solution.

foreach函数知道某些组合函数(例如c或cbind)带有许多参数，并且将最多使用100个参数(默认情况下)调用它们以提高性能.使用参数.maxcombine可以手动设置它们.

The foreach function knows that some comine-functions (e.g. c or cbind) take many arguments, and will call them with up to 100 arguments (by default) in order to improve performance. With the argument .maxcombine you can set them manually.

library(foreach)

clusters <- 10
runs <- 101

temp <- foreach(r = 1:runs,
                .combine = 'list',
                .maxcombine = runs,
                .multicombine = T) %do% {

              signal_all <- lapply(1:clusters, function(x){

                return(1)

              })

              return(signal_all)
            } ## end do

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