在返回NA的R中使用foreach循环 [英] Using foreach loop in r returning NA
问题描述
我想在R中使用"foreach"循环(包foreach + doParallel),但是在我的工作中我发现该循环返回一些NA,而经典的"for"循环返回我想要的值:
I would like to use the "foreach" loop in R (package foreach + doParallel) but in my work i found that the loop returns some NA and the classic "for" loop returns the value I want :
library(foreach)
library(doParallel)
ncore=as.numeric(Sys.getenv('NUMBER_OF_PROCESSORS'))-1
registerDoParallel(cores=ncore)
B=2
a = vector()
b = vector()
foreach(i = 1:B, .packages = "ez",.multicombine = T,.inorder = T, .combine = 'c')%dopar%{
a[i] = i + 1
return(a)
}
for(i in 1:B){
b[i] = i + 1
b
}
您可以看到是否尝试过,对象"a"返回一个具有2,NA和3的向量,而对象"b"则返回了2和3(这就是我想要的).
As you can see if you try it, the object "a" returns a vector with 2, NA and 3 while the object "b" returns 2 and 3 (that's what I want).
我实际上不明白为什么我的结果中会有一个"NA" ...
I actually can't understand why there's a "NA" in my results...
推荐答案
这是因为foreach
不会更改全局对象a
.尝试与list
结合使用.将会更容易了解正在发生的事情.我已经将B
增加到了3
.
This is because foreach
does not change the global object a
. Try to combine with list
. It will be easier to understand what is happening. I have increased B
to 3
.
> B=3
>
> a = vector()
>
> foreach(i = 1:B, .multicombine = T, .inorder = T, .combine = 'list') %dopar% {
+ a[i] = i + 1
+ return(a)
+ }
[[1]]
[1] 2
[[2]]
[1] NA 3
[[3]]
[1] NA NA 4
我们可以看到,在每次迭代中,都会提取空向量a
并填充其中一个值.如果您c
合并结果,则会得到NA
值.
We can see that in each iteration an empty vector a
is taken and one value of it is filled. If you c
combine the result you get NA
values.
> foreach(i = 1:B, .multicombine = T, .inorder = T, .combine = 'c') %dopar% {
+ a[i] = i + 1
+ return(a)
+ }
[1] 2 NA 3 NA NA 4
在此示例中,您可以这样做.
In this example you could do.
> a <- foreach(i = 1:B, .multicombine = T, .inorder = T, .combine = 'c') %dopar% {
+ i + 1
+ }
> a
[1] 2 3 4
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