为自定义 S3 对象编写“as.data.frame"方法

问题描述

在许多变体中，类似的问题已被问过多次......但我没有找到明确的建议:

In many variants, similar questions have been asked many times ...but i do not find a clear advice about:

将 S3 方法导出为函数"

我使用 roxygen2 编写了一个自定义 S3 类，将其命名为 'my_item'.这是构造函数:

I wrote a custom S3 class with roxygen2, call it 'my_item'.This is the constructor function:

my_item <- function(n) structure(list(n=n),class='my_item')

我需要的是一种定义 "list of my_items => data.frame" 转换函数的方法:

What I need is a way to define a "list of my_items => data.frame" cast function:

 #' @method as.data.frame my_item
 #' @export

 as.data.frame.my_item <- function(x) ...

只要我以这种方式用 my_item 调用它，就可以了:

As soon as I call it with a my_item in this way, it is fine:

as.data.frame(my_item('a'))

但是对对象列表应用相同的调用是行不通的，因为 class(list) 是空的:

But applying the same call to a list of objects cannot work, because class(list) is empty:

as.data.frame(list(my_item('a'),my_item('b')))

这也行不通，因为函数/方法没有导出

This cannot work also because the function/method is not exported

as.data.frame.my_item(list(my_item('a'),my_item('b')))

这不适用于显式命名空间限定:

This does not work with explicit namespace qualification:

my_pkg::as.data.frame.my_item(...)

Error: 'as.data.frame.my_item' is not an exported object from 'namespace:my_pkg'

在包 zoo 中，这对于 plot 通用函数是可能的.

---

In the package zoo, this is possible for plot generic function.

参见 plot.zoo，一种 S3 方法，作为函数导出:

See plot.zoo, an S3 method, exported as a function:

在动物园::NAMESPACE

export(
   ...
   "plot.zoo"
   ...
)
S3method("plot", "zoo")
S3method("as.data.frame", "zoo")

由此产生的包范围是:

library(zoo)

methods('as.data.frame')

[1] as.data.frame.aovproj*        as.data.frame.array           as.data.frame.AsIs
[4] as.data.frame.character       as.data.frame.chron*          as.data.frame.complex
[7] as.data.frame.data.frame      as.data.frame.data.table*     as.data.frame.Date
[10] as.data.frame.dates*          as.data.frame.default         as.data.frame.difftime
[13] as.data.frame.factor          as.data.frame.ftable*         as.data.frame.integer
[16] as.data.frame.ITime*          as.data.frame.list            as.data.frame.logical
[19] as.data.frame.logLik*         as.data.frame.matrix          as.data.frame.model.matrix
[22] as.data.frame.noquote         as.data.frame.numeric         as.data.frame.numeric_version
[25] as.data.frame.ordered         as.data.frame.POSIXct         as.data.frame.POSIXlt
[28] as.data.frame.raw             as.data.frame.shingle*        as.data.frame.table
[31] as.data.frame.times*          as.data.frame.ts              as.data.frame.vector
[34] as.data.frame.yearmon*        as.data.frame.yearqtr*        as.data.frame.zoo*
see '?methods' for accessing help and source code

methods('plot')

[1] plot.acf*           plot.data.frame*    plot.decomposed.ts* plot.default        plot.dendrogram*
[6] plot.density*       plot.ecdf           plot.factor*        plot.formula*       plot.function
[11] plot.hclust*        plot.histogram*     plot.HoltWinters*   plot.isoreg*        plot.lm*
[16] plot.medpolish*     plot.mlm*           plot.ppr*           plot.prcomp*        plot.princomp*
[21] plot.profile.nls*   plot.raster*        plot.shingle*       plot.spec*          plot.stepfun
[26] plot.stl*           plot.table*         plot.times*         plot.trellis*       plot.ts
[31] plot.tskernel*      plot.TukeyHSD*      plot.zoo
see '?methods' for accessing help and source code

这说明plot.zoo导出了，as.data.frame.zoo*没有导出

This shows that plot.zoo is exported, as.data.frame.zoo* is not exported

所以这个问题可能是错误的.

So probably the question is wrong.

更好的方法是:

在使用lists-of-list-based-S3-objects"时如何实现cast-protocol ('as-...')?>

推荐答案

我采取了不同的方法:

• 通过 as.my_item_list() 函数将 'my_item'+'_list' 类添加到通用列表中
• 定义一个 as.data.frame.my_item_list 方法来处理我的自定义" 列表
• 使用泛型调用 DF 转换:as.data.frame(as.my_item_list(some_list))

• adding 'my_item'+'_list' class to a generic list via an as.my_item_list() function
• defining an as.data.frame.my_item_list method working on my "custom" lists
• invoke DF conversion with generics: as.data.frame(as.my_item_list(some_list))

在以下示例中，我将 my_item 类名替换为较短的 ob

In the following example, I substituted my_item class name with a shorter ob

定义

# ---( the 'ob' S3 constructor )---------------------------------------------

ob <- function(a,b) structure(list(a=a,b=b), class='ob')
is.ob <- function(x) inherits(x,'ob')

#---( the 'ob_list' cast for lists )-----------------------------------------

as.ob_list <- function(x, ...) UseMethod('as.ob_list')
as.ob_list.list <- function(x, ...) {
    stopifnot(all(sapply(x,is.ob)))
    class(x) <- append(class(x),'ob_list',after=0)
    x
}

#---( as.data.frame.* methods )----------------------------------------------

as.data.frame.ob <- function(x, ...) data.frame(t(as.matrix(unlist(x))))
as.data.frame.ob_list <- function(x, ...) do.call('rbind', lapply(x,as.data.frame))

测试

o1 <- ob(1,10)
o2 <- ob(2,20)
o3 <- ob(3,30)

ol <- list(o1,o2,o3)

df <- as.data.frame(as.ob_list(ol))

print(df)

  a  b
1 1 10
2 2 20
3 3 30

看起来不错！

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