如何为S4类定义子集运算符? [英] How to define the subset operators for a S4 class?

问题描述

我很难为S4类定义正确的方法来定义[，$和[[子集运算符.

I am having trouble figuring out the proper way to define the [, $, and [[ subset operators for an S4 class.

谁能为我提供一个为S4类定义这三个的基本示例?

Can anyone provide me with a basic example of defining these three for an S4 class?

推荐答案

发现通用名称，以便我们了解我们的目标

Discover the generic so that we know what we are aiming for

> getGeneric("[")
standardGeneric for "[" defined from package "base"

function (x, i, j, ..., drop = TRUE) 
standardGeneric("[", .Primitive("["))
<bytecode: 0x32e25c8>
<environment: 0x32d7a50>
Methods may be defined for arguments: x, i, j, drop
Use  showMethods("[")  for currently available ones.

定义一个简单的类

setClass("A", representation=representation(slt="numeric"))

并实现方法

setMethod("[", c("A", "integer", "missing", "ANY"),
    ## we won't support subsetting on j; dispatching on 'drop' doesn't
    ## make sense (to me), so in rebellion we'll quietly ignore it.
    function(x, i, j, ..., drop=TRUE)
{
    ## less clever: update slot, return instance
    ## x@slt = x@slt[i]
    ## x
    ## clever: by default initialize is a copy constructor, too
    initialize(x, slt=x@slt[i])
})

实际情况:

> a = new("A", slt=1:5)
> a[3:1]
An object of class "A"
Slot "slt":
[1] 3 2 1

有多种支持(隐式)签名的策略，例如，您可能还希望支持逻辑索引和字符索引值，可能同时适用于i和j.最直接的是外观"模式，其中每种方法对子集索引的常见类型(例如integer)进行某种初步强制，以允许对索引条目进行重新排序和重复，然后使用callGeneric进行调用一个方法完成子类的工作.

There are different strategies for supporting the (implicitly) many signatures, for instance you'd likely also want to support logical and character index values, possibly for both i and j. The most straight-forward is a "facade" pattern where each method does some preliminary coercion to a common type of subset index, e.g., integer to allow for re-ordering and repetition of index entries, and then uses callGeneric to invoke a single method that does the work of subsetting the class.

[[没有概念上的区别，只是希望尊重返回内容的语义，而不是[所隐含的对象的另一个实例.对于$，我们有

There are no conceptual differences for [[, other than wanting to respect the semantics of returning the content rather than another instance of the object as implied by [. For $ we have

> getGeneric("$")
standardGeneric for "$" defined from package "base"

function (x, name) 
standardGeneric("$", .Primitive("$"))
<bytecode: 0x31fce40>
<environment: 0x31f12b8>
Methods may be defined for arguments: x
Use  showMethods("$")  for currently available ones.

和

setMethod("$", "A",
    function(x, name)
{
    ## 'name' is a character(1)
    slot(x, name)
})

使用

> a$slt
[1] 1 2 3 4 5

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