`levels<-`( 这是什么魔法? [英] `levels&lt;-`( What sorcery is this?

问题描述

在回答另一个问题时，@Marek 发布了以下解决方案:https://stackoverflow.com/a/10432263/636656

In an answer to another question, @Marek posted the following solution: https://stackoverflow.com/a/10432263/636656

dat <- structure(list(product = c(11L, 11L, 9L, 9L, 6L, 1L, 11L, 5L, 
                                  7L, 11L, 5L, 11L, 4L, 3L, 10L, 7L, 10L, 5L, 9L, 8L)), .Names = "product", row.names = c(NA, -20L), class = "data.frame")

`levels<-`(
  factor(dat$product),
  list(Tylenol=1:3, Advil=4:6, Bayer=7:9, Generic=10:12)
  )

产生的输出:

 [1] Generic Generic Bayer   Bayer   Advil   Tylenol Generic Advil   Bayer   Generic Advil   Generic Advil   Tylenol
[15] Generic Bayer   Generic Advil   Bayer   Bayer  

这只是一个向量的打印输出，所以为了存储它你可以做更令人困惑的事情:

This is just the printout of a vector, so to store it you can do the even more confusing:

res <- `levels<-`(
  factor(dat$product),
  list(Tylenol=1:3, Advil=4:6, Bayer=7:9, Generic=10:12)
  )

显然这是对 levels 函数的某种调用，但我不知道这里做了什么.这种巫术的术语是什么，我如何提高我在这个领域的魔法能力?

Clearly this is some kind of call to the levels function, but I have no idea what's being done here. What is the term for this kind of sorcery, and how do I increase my magical ability in this domain?

推荐答案

这里的答案很好，但缺少一个重要的点.让我试着描述一下.

The answers here are good, but they are missing an important point. Let me try and describe it.

R 是一种函数式语言，不喜欢改变它的对象.但它确实允许赋值语句，使用替换函数:

R is a functional language and does not like to mutate its objects. But it does allow assignment statements, using replacement functions:

levels(x) <- y

相当于

x <- `levels<-`(x, y)

诀窍是，这种重写是由<-完成的；它不是由 levels<- 完成的.levels<- 只是一个接受输入并给出输出的常规函数​​；它不会改变任何东西.

The trick is, this rewriting is done by <-; it is not done by levels<-. levels<- is just a regular function that takes an input and gives an output; it does not mutate anything.

这样做的一个后果是，根据上述规则，<- 必须是递归的:

One consequence of that is that, according to the above rule, <- must be recursive:

levels(factor(x)) <- y

是

factor(x) <- `levels<-`(factor(x), y)

是

x <- `factor<-`(x, `levels<-`(factor(x), y))

这种纯函数转换(直到最后，赋值发生的地方)与命令式语言中的赋值等效，这有点美妙.如果我没记错的话，函数式语言中的这种结构称为镜头.

It's kind of beautiful that this pure-functional transformation (up until the very end, where the assignment happens) is equivalent to what an assignment would be in an imperative language. If I remember correctly this construct in functional languages is called a lens.

但是，一旦你定义了像 levels<- 这样的替换函数，你就会得到另一个意想不到的意外收获:你不仅有能力进行分配，你还有一个方便的函数在一个因子中，并给出不同水平的另一个因子.真的没有什么任务"！

But then, once you have defined replacement functions like levels<-, you get another, unexpected windfall: you don't just have the ability to make assignments, you have a handy function that takes in a factor, and gives out another factor with different levels. There's really nothing "assignment" about it!

因此，您所描述的代码只是利用了对 levels<- 的另一种解释.我承认 levels<- 这个名字有点令人困惑，因为它暗示了一个分配，但这不是正在发生的事情.代码只是设置了一种管道:

So, the code you're describing is just making use of this other interpretation of levels<-. I admit that the name levels<- is a little confusing because it suggests an assignment, but this is not what is going on. The code is simply setting up a sort of pipeline:

• 从 dat$product

将其转换为因子

更改级别

将其存储在 res

就个人而言，我认为这行代码很漂亮;)

Personally, I think that line of code is beautiful ;)

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