哈德利高级R书中的非标准评估 [英] Non standard evaluation in Hadley's advanced R book

问题描述

在Hadley的高级R书中，有一段代码可以我无法理解输出.

In Hadley's Advanced R book, there is a piece of code that I cannot understand the output.

f <- function(x) substitute(x)
g <- function(x) deparse(f(x))
g(1:10)
g(x)
g(x + y ^ 2 / z + exp(a * sin(b)))

为什么它们都返回"x"?尤其是当

Why do they all return "x"? Especially when

g <- function(x) deparse(substitute(x))

按预期返回"1:10"，"x"和"x + y ^ 2 / z + exp(a * sin(b))".

推荐答案

首先，一些背景信息:promise是未经评估的参数.一个promise由两部分组成:1)导致这种延迟计算的代码/表达式(此代码可以由substitute或pryr::promise_info查看)，以及2)创建该代码/表达式的环境并且应该进行评估(pryr::promise_info可以查看此环境).

First, some background information: A promise is an unevaluated argument. A promises comprises of two parts: 1) the code / expression that gives rise to this delayed computation (this code can be viewed by substitute or pryr::promise_info), and 2) the environment where this code / expression is created and should be evaluated in (this environment can be viewed by pryr::promise_info).

如果将g()函数更改为

g <- function(x) deparse(f(whatever))

您将永远得到无论如何".这是因为g()调用f(whatever)时，它将一个promise对象传递给f() －该对象具有代码whatever和g()的执行环境.然后，f()中的substitute查看此promise对象并返回该promise的代码/表达式，在这种情况下为whatever.

you would always get "whatever". This is because when g() calls f(whatever), it passes a promise object to f()--this object has the code whatever and the environment of g()'s execution environment. Then, the substitute within f() looks at this promise object and returns the code / expression of that promise, which is whatever in this case.

可以通过运行以下代码来确认promise对象的代码和环境:

The code and the environment of the promise object can be confirmed by running the following code:

library(pryr) # need to install it
f <- function(x) {
  print(promise_info(x))
  substitute(x)
}

g <- function(x) {
  print(environment())
  f(whatever)
}
g(1:10)

最重要的是，您将获得传递给f(whatever)的所有信息.这就是为什么分离这些功能不是一个好主意的原因.一种解决方法是使用

The bottom line is you'll get back whatever you pass to f(whatever). That's why it's not a good idea to separate these functions. One work around would be to use

g <- function(...) deparse(f(...))

通过这种方式，参数将传递给f()，而不会在g()中重命名.

This way the parameter is passed through to f() and not renamed in g().

另一方面，g <- function(x) deparse(substitute(x)); g(1:10)产生1:10的原因是，在这种情况下，substitute正在查看承诺对象x(与上述情况下的承诺对象whatever相反).承诺x在此处具有代码1:10和环境R_GlobalEnv. (同样，可以使用g <- function(x) { print(promise_info(x) ; deparse(substitute(x))进行检查.因此substitute(x)会按预期返回1:10.

On the other hand, g <- function(x) deparse(substitute(x)); g(1:10) produces 1:10 because, in this case, substitute is looking at promise object x (in contrasts to the promise object whatever in the above case). Promise x here has the code 1:10 and the environment R_GlobalEnv. (Again, this can be checked using g <- function(x) { print(promise_info(x) ; deparse(substitute(x)). So substitute(x) returns 1:10 as expected.

这篇关于哈德利高级R书中的非标准评估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！