嵌套函数环境选择 [英] Nested function environment selection

问题描述

我正在编写一些用于执行重复任务的函数，但是我试图将加载数据的时间减至最少.基本上，我有一个函数可以获取一些信息并进行绘制.然后，我有了第二个函数，该函数将循环遍历并将多个图输出到.pdf.在这两个函数中，我都有以下代码行:

I am writing some functions for doing repeated tasks, but I am trying to minimize the amount of times I load the data. Basically I have one function that takes some information and makes a plot. Then I have a second function that will loop through and output multiple plots to a .pdf. In both functions I have the following line of code:

if(load.dat) load("myworkspace.RData")

其中load.dat是逻辑，我需要的数据存储在myworkspace.RData中.当我调用循环并输出多个图的包装函数时，我不想在每次对内部函数的调用中重新加载工作区.我以为我只需要在包装函数中加载一次工作区，然后内部函数就可以访问该数据，但是我却指出了错误.

where load.dat is a logical and the data I need is stored in myworkspace.RData. When I am calling the wrapper function that loops through and outputs multiple plots I do not want to reload the workspace in every call to the inner function. I thought I could just load the workspace once in the wrapper function, then the inner function could access that data, but I got an error stating otherwise.

所以我的理解是，当一个函数在其本地环境中找不到该变量时(该函数在调用该函数时创建)，该函数将在父环境中查找该变量.

So my understanding was when a function cannot find the variable in its local environment (created when the function gets called), the function will look to the parent environment for the variable.

我假设内部函数调用的父环境将是外部函数调用.显然这是不正确的:

I assumed the parent environment to the inner function call would be the outer function call. Obviously this is not true:

func1 <- function(...){
  print(var1)
}

func2 <- function(...){
  var1 <- "hello"
  func1(...)
}

> func2()
Error in print(var1) : object 'var1' not found

在阅读了许多问题之后，使用了语言手册和此确实很有帮助的博客文章，我提出了以下建议:

After reading numerous questions, the language manual, and this really helpful blog post, I came up with the following:

var1 <- "hello"
save(list="var1",file="test.RData")
rm(var1)

func3 <- function(...){
  attach("test.RData")
  func1(...)
  detach("file:test.RData")
}

> func3()
[1] "hello"

是否有更好的方法可以做到这一点?当func2调用func1时，为什么func1在由func2创建的本地环境中不查找未定义的变量?

Is there a better way to do this? Why doesn't func1 look for undefined variables in the local environment created by func2, when it was func2 that called func1?

注意:我不知道如何命名这个问题.如果有人有更好的建议，我将对其进行更改并进行编辑.

Note: I did not know how to name this question. If anyone has better suggestions I will change it and edit this line out.

推荐答案

要说明词汇作用域，请考虑以下内容:

To illustrate lexical scoping, consider the following:

首先，我们创建一个沙箱环​​境，只是为了避免常见的R_GlobalEnv:

First let's create a sandbox environment, only to avoid the oh-so-common R_GlobalEnv:

sandbox <-new.env()

现在，我们在其中添加了两个函数:f，用于查找名为x的变量；和g，它定义了本地x并调用f:

Now we put two functions inside it: f, which looks for a variable named x; and g, which defines a local x and calls f:

sandbox$f <- function()
{
    value <- if(exists("x")) x else "not found."
    cat("This is function f looking for symbol x:", value, "\n")
}

sandbox$g <- function()
{
    x <- 123
    cat("This is function g. ")
    f()
}

技术性:在控制台中输入功能定义会导致将封闭环境设置为R_GlobalEnv，因此我们手动强制f和g的机柜匹配它们所属"的环境:

Technicality: entering function definitions in the console causes then to have the enclosing environment set to R_GlobalEnv, so we manually force the enclosures of f and g to match the environment where they "belong":

environment(sandbox$f) <- sandbox
environment(sandbox$g) <- sandbox

呼叫g. f找不到本地变量x=123:

Calling g. The local variable x=123 is not found by f:

> sandbox$g()
This is function g. This is function f looking for symbol x: not found. 

现在，我们在全局环境中创建一个x并调用g.函数f首先在沙箱中查找x，然后在沙箱的父项中查找，恰好是R_GlobalEnv:

Now we create a x in the global environment and call g. The function f will look for x first in sandbox, and then in the parent of sandbox, which happens to be R_GlobalEnv:

> x <- 456
> sandbox$g()
This is function g. This is function f looking for symbol x: 456 

只需检查f首先在其外壳中查找x，我们就可以在其中放置x并调用g:

Just to check that f looks for x first in its enclosure, we can put a x there and call g:

> sandbox$x <- 789
> sandbox$g()
This is function g. This is function f looking for symbol x: 789 

结论:R中的符号查找遵循封闭环境链，而不是执行嵌套函数调用期间创建的评估框架.

Conclusion: symbol lookup in R follows the chain of enclosing environments, not the evaluation frames created during execution of nested function calls.

只需将链接添加到

Just adding a link to this very interesting answer from Martin Morgan on the related subject of parent.frame() vs parent.env()

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