在`$`登录R后使用替代项 [英] Working with substitute after `$` sign in R

问题描述

我想知道下面的代码中的 G 是否可以像 D $ post == 1 之后在 $ 之后工作有效吗?

I was wondering if there is a way that G in my code below could work after $ just like D$post == 1 works?

D <- data.frame(post = 1:10, out = 2:11)

G <- substitute(post == 1)

D$G    ## can we make `G` to work like `D$post`?

D$post == 1  ## Works

推荐答案

您可以执行以下操作:

G <- substitute(post == 1)
E <- substitute(D$G, list(G = G))
#D$post == 1

该表情看起来像您想要的，对吗?嗯，事实并非如此，您可以在尝试对其进行评估时看到:

That expression looks like what you want, right? Well, it isn't, as you can see when you try to evaluate it:

eval(E)
#Error in D$post == 1 : invalid subscript type 'language'

让我们更详细地检查该表达式:

Let's inspect the expression in more detail:

as.list(E)
#[[1]]
#`$`
#
#[[2]]
#D
#
#[[3]]
#post == 1

好的，我们有一个函数调用(对 $ )，带有两个参数( D 和 post == 1 ).第二个参数是一个表达式，而 $ 需要一个名称.

OK, we have one function call (to $) with two arguments (D and post == 1). The second argument is an expression whereas $ expects a name.

让我们将其与它的外观进行比较:

Let's compare this with how it should look like:

as.list(quote(D$post == 1))
#[[1]]
#`==`
#
#[[2]]
#D$post
#
#[[3]]
#[1] 1

as.list(quote(D$post == 1)[[2]])
#[[1]]
#`$`
#
#[[2]]
#D
#
#[[3]]
#post

因此， D $ post == 1 实际上是对两个嵌套函数的调用，并且将其解析为:

So, D$post == 1 is actually a call to two nested functions and get's parsed to this:

`==`(`$`(D, post), 1)

我希望这可以弄清楚为什么"[w]在$符号后进行替换"并不是那么简单.

I hope this clarifies why "[w]orking with substitute after $ sign" is not so simple.

如果您了解如何解析表达式，仅说明这仍然可行:

Just to show that is still possible, if you understand how the expression is parsed:

E <- quote(D$x)
E[[3]] <- G[[2]]
G[[2]] <- E
eval(G)
#[1]  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE

但是，这样的代码确实很难维护和调试.不要这样

However, such code is really hard to maintain and debug. Don't do this.

如@joran所示，您可以使用像 with 这样的函数来对data.frame中的表达式 post == 1 求值，该函数基本上只是 eval(G，D，parent.frame()).但是，这是一条危险且湿滑的道路，可能会导致巨龙.让我在 help("subset")此处引用相关警告:

As @joran shows, you can use functions like with that evaluate the expression post == 1 within the data.frame, which is basically just a wrapper for eval(G, D, parent.frame()). However, that's a dangerous and slippery path that can lead to dragons. Let me quote the relevant warning from help("subset") here:

这是旨在交互使用的便捷功能.为了编程时，最好使用标准的子集功能，例如[，尤其是参数子集的非标准评估可能会带来意想不到的后果.

This is a convenience function intended for use interactively. For programming it is better to use the standard subsetting functions like [, and in particular the non-standard evaluation of argument subset can have unanticipated consequences.

这篇关于在`$`登录R后使用替代项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！