如何使用[]或[[]]约定引用bibentry类中的特定标记？

Question

我正试图从bib文件中获取特定标签的值.每当我尝试和我们entry["bibtype"]或entry[["bibtype"]]我得到一个错误,但entry$bibtype工作正常.

entry<-bibentry(
  bibtype = "Manual",
  title = "R: A Language and Environment for Statistical Computing",
  author = person("R Development Core Team"),
  organization = "R Foundation for Statistical Computing",
  address = "Vienna, Austria",
  year = 2010,
  isbn = "3-900051-07-0",
  url = "http://www.R-project.org/")

# the first two fail
entry["bibtype"]
entry[["bibtype"]]
entry$bibtype

foo <- list("bar" = "baz")

#all of these work
foo["bar"]
foo[["bar"]]
foo$bar

我得到的错误是:

Error in switch(attr(paper, "bibtype"), Article = formatArticle(paper),  : 
  EXPR must be a length 1 vector

有人解决了这个吗？无论如何强迫bibentries接受这个？

Answer 1

TL;博士

"bibtype"不是"bibentry"对象中的命名组件,而是作为属性实现.如果$这些对象是选中的"东西",那么这些对象的方法就是一个特殊情况来访问属性,而[和[[方法没有这个"特征".

使用rref如下定义

> attributes(unclass(rref)[[1]])$bibtype
[1] "Manual"
> attr(unclass(rref)[[1]], "bibtype")
[1] "Manual"

请参阅长版本以获得更广泛的展示

您需要对象的属性,但这些属性似乎无法通过常用方法使用,例如使用以下示例?bibentry:

## R reference
rref <- bibentry(bibtype = "Manual",
        title = "R: A Language and Environment for Statistical Computing",
        author = person("R Development Core Team"),
        organization = "R Foundation for Statistical Computing",
        address = "Vienna, Austria",
        year = 2010,
        isbn = "3-900051-07-0",
        url = "http://www.R-project.org/")

我们注意到"bibtype"输出中的属性str()

> str(rref)
Class 'bibentry'  hidden list of 1
 $ :List of 7
  ..$ title       : chr "R: A Language and Environment for Statistical Computing"
  ..$ author      :Class 'person'  hidden list of 1
  .. ..$ :List of 5
  .. .. ..$ given  : chr "R Development Core Team"
  .. .. ..$ family : NULL
  .. .. ..$ role   : NULL
  .. .. ..$ email  : NULL
  .. .. ..$ comment: NULL
  ..$ organization: chr "R Foundation for Statistical Computing"
  ..$ address     : chr "Vienna, Austria"
  ..$ year        : chr "2010"
  ..$ isbn        : chr "3-900051-07-0"
  ..$ url         : chr "http://www.R-project.org/"
  ..- attr(*, "bibtype")= chr "Manual"

但是我们无法访问该属性

> attr(rref, "bibtype")
NULL
> attr(rref[[1]], "bibtype")
NULL

第一个失败是因为就R而言,类的对象"bibentry"在R中实现的方式(或者更确切地说应用于它们的方法)attributes()也attr()看不到这个特定的属性.唯一可见的属性是:

> attributes(rref)
$class
[1] "bibentry"

> attributes(rref[1])
$class
[1] "bibentry"

如果我们unclass() rref,那么我们需要意识到对象是一个包含bibentries的组件的列表.在这种情况下,它rref是一个包含单个组件的列表,它是类"bibentry"的对象,它是7个组件的列表.

一个天真的人会认为你可以这样做:

attr(rref[[1]], "bibtype")

这将从第一个"bibentry"对象rref(只有一个)中获取并查找其中的属性.这不起作用:

> attributes(rref[[1]])
$class
[1] "bibentry"

因为对象的方法[和[[实现方式"bibentry":

> utils:::`[[.bibentry`
function (x, i) 
{
    rval <- unclass(x)[i]
    class(rval) <- class(x)
    rval
}
<bytecode: 0x1a75938>
<environment: namespace:utils>

[单一[方法以完全相同的方式实现.]这意味着你可以做这样的愚蠢的事情:

> rref[[1]][[1]][[1]][[1]]
R Development Core Team (2010). _R: A Language and Environment for
Statistical Computing_. R Foundation for Statistical Computing, Vienna,
Austria. ISBN 3-900051-07-0, <URL: http://www.R-project.org/>.

这没有任何意义,但[[1]]每次只是指同一个对象.我不知道这是否是故意的; ?bibentry具有

Note:

     The bibentry functionality is still experimental.

但它不是最理想的.

现在,您需要遵循实现utils:::`[.bibentry`和unclass()对象,然后开始子集化并访问属性:

> attributes(unclass(rref)[[1]])
$names
[1] "title"        "author"       "organization" "address"      "year"        
[6] "isbn"         "url"         

$bibtype
[1] "Manual"

> attributes(unclass(rref)[[1]])$bibtype
[1] "Manual"
> attr(unclass(rref)[[1]], "bibtype")
[1] "Manual"

对比该utils:::`$.bibentry`方法的实现与utils:::`[[.bibentry`:

> utils:::`$.bibentry`
function (x, name) 
{
    is_attribute <- name %in% bibentry_attribute_names
    rval <- if (is_attribute) 
        lapply(unclass(x), attr, name)
    else lapply(unclass(x), "[[", name)
    if (length(rval) == 1L) 
        rval <- rval[[1L]]
    rval
}
<bytecode: 0x1b0fd70>
<environment: namespace:utils>

此方法专门用于处理属性.这在R中看起来似乎有些非标准行为.这就解释了原因

> rref$bibtype
[1] "Manual"

但是(天真的)基本相同

> str(rref[["bibtype"]])
Class 'bibentry'  hidden list of 1
 $ : NULL

失败,因为没有在未"bibtype"显示的列表中命名的组件,因此返回NULL组件,而在打印时发生错误:

> foo <- rref[["bibtype"]]
> foo
Error in switch(attr(paper, "bibtype"), Article = formatArticle(paper),  : 
  EXPR must be a length 1 vector