R 中的全局和局部变量 [英] Global and local variables in R

问题描述

我是 R 的新手，我对 R 中局部和全局变量的使用感到很困惑.

I am a newbie for R, and I am quite confused with the usage of local and global variables in R.

我在互联网上读了一些帖子，说如果我使用 = 或 <- 我将在当前环境中分配变量，并使用 <<- 我可以在函数内部访问全局变量.

I read some posts on the internet that say if I use = or <- I will assign the variable in the current environment, and with <<- I can access a global variable when inside a function.

但是，我记得在 C++ 中，每当您在方括号 {} 内声明变量时都会出现局部变量，所以我想知道这对 R 是否相同?或者只是为了 R 中的函数，我们有局部变量的概念.

However, as I remember in C++ local variables arise whenever you declare a variable inside brackets {}, so I'm wondering if this is the same for R? Or is it just for functions in R that we have the concept of local variables.

我做了一个小实验，似乎表明只有括号是不够的，我有什么问题吗?

I did a little experiment, which seems to suggest that only brackets are not enough, am I getting anything wrong?

{
   x=matrix(1:10,2,5)
}
print(x[2,2])
[1] 4

推荐答案

在函数内部声明的变量是该函数的局部变量.例如:

Variables declared inside a function are local to that function. For instance:

foo <- function() {
    bar <- 1
}
foo()
bar

给出以下错误:Error: object 'bar' not found.

如果你想让 bar 成为一个全局变量，你应该这样做:

If you want to make bar a global variable, you should do:

foo <- function() {
    bar <<- 1
}
foo()
bar

在这种情况下，bar 可以从函数外部访问.

In this case bar is accessible from outside the function.

然而，与 C、C++ 或许多其他语言不同，括号不能确定变量的范围.例如，在以下代码片段中:

However, unlike C, C++ or many other languages, brackets do not determine the scope of variables. For instance, in the following code snippet:

if (x > 10) {
    y <- 0
}
else {
    y <- 1
}

y 在 if-else 语句之后仍然可以访问.

y remains accessible after the if-else statement.

正如您所说，您还可以创建嵌套环境.您可以查看这两个链接以了解如何使用它们:

As you well say, you can also create nested environments. You can have a look at these two links for understanding how to use them:

1. http://stat.ethz.ch/R-manual/R-devel/library/base/html/environment.html
2. http://stat.ethz.ch/R-manual/R-devel/library/base/html/get.html

这里有一个小例子:

test.env <- new.env()

assign('var', 100, envir=test.env)
# or simply
test.env$var <- 100

get('var') # var cannot be found since it is not defined in this environment
get('var', envir=test.env) # now it can be found

这篇关于R 中的全局和局部变量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！