什么是“{”在R类中 [英] What is "{" class in R?

问题描述

这是代码：

mf = function(..., expr) {
    expr = substitute(expr)
    print(class(expr))
    print(str(expr))
    expr
}
mf(a = 1, b = 2, expr = {matrix(NA, 4, 4)})

输出：

[1] "{"
length 2 {  matrix(NA, 4, 4) }
 - attr(*, "srcref")=List of 2
  ..$ :Class 'srcref'  atomic [1:8] 1 25 1 25 25 25 1 1
  .. .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> 
  ..$ :Class 'srcref'  atomic [1:8] 1 26 1 41 26 41 1 1
  .. .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> 
 - attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> 
 - attr(*, "wholeSrcref")=Class 'srcref'  atomic [1:8] 1 0 1 42 0 42 1 1
  .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> 
NULL
{
matrix(NA, 4, 4)
}

显然， substitute（expr）的结果产生类{。这个班级是什么？为什么长度为2的 {matrix（NA，4，4）} 这些奇怪的attrs是什么意思？

Apparently the result of substitute(expr) produces something of the class "{". What is this class exactly? Why is {matrix(NA, 4, 4)} of length 2? What do these strange attrs mean?

推荐答案

{一块代码。只需看这些课程，请注意这些

The { is the class for a block of code. Just looking at the classes, note the difference between these

mf(a = 1, b = 2, expr = {matrix(NA, 4, 4)})
# [1] "{"
mf(a = 1, b = 2, expr = matrix(NA, 4, 4))
# [1] "call"

一类 {可以容纳多个语句。 length（）表示块中有多少个语句（包括块的开始）。例如

A class of { can hold multiple statements. The length() indicates how many statements are in the block (including the start of the block). For example

length(quote({matrix(NA, 4, 4)}))
# [1] 2
length(quote({matrix(NA, 4, 4); matrix(NA,3,3)}))
# [1] 3
length(quote({}))
# [1] 1

属性srcref和srcfile是如何R轨迹，其功能定义为试图提供有用的错误消息。您可以看到？srcfile 帮助页面了解更多信息。

The attributes "srcref" and "srcfile" are how R tracks where functions are defined for trying to give informative error messages. You can see the ?srcfile help page for more information about that.

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