r - R中S4类的一元加

Question

我正在 R 中试验 S4 类，我试图为我的对象定义一个加号( + )运算符，即重载加号运算符。我设法重载了二进制文件 + ，但我不知道如何重载一元加号。这是我试图实现的最小工作(一元运算符不起作用)示例:

setClass("HWtest",
         representation(expr  = "character"),
         prototype = list(expr  = NA_character_)
)

H <- new("HWtest", expr="Hello")
W <- new("HWtest", expr="World")

setMethod("+", signature(e1="HWtest", e2="HWtest"),
          function(e1,e2){
            new("HWtest", 
                expr = paste(e1@expr," + ",e2@expr))
          }
)

现在我可以使用 +运营商，它工作顺利:

H+W
An object of class "HWtest"
Slot "expr":
[1] "Hello  +  World"

现在一元加号当然不起作用，所以必须重载

+H
Error in +H : invalid argument to unary operator

所以我尝试通过以下方式重载它:

setMethod("+", signature(e="HWtest"),
          function(e){
            new("HWtest", 
                expr = paste("+ ",e@expr))
          }
)

但这会产生错误:

Error in match.call(fun, fcall) : 
  argument 1 matches multiple formal arguments

是否可以重载一元加号？如果是这样，我将如何为这个最小的例子做到这一点？

Answer 1

尝试添加这个(除了你的二进制重载):

setMethod("+", signature(e1 = "HWtest", e2 = "missing"),
          function(e1){
            new("HWtest", 
                expr = paste("+ ", e1@expr))
          }
)

R> +H
An object of class "HWtest"
Slot "expr":
[1] "+  Hello"

R> H + W
An object of class "HWtest"
Slot "expr":
[1] "Hello  +  World"

引用帮助文件 ?groupGeneric ,

[...] when a unary operator is encountered the Ops method is called with one argument and e2 is missing.

正如弗兰克指出的那样， ?setMethod帮助文件包含一些关于使用 missing 的有用信息。作为方法签名(强调):

Two additional special class names can appear: "ANY", meaning that this argument can have any class at all; and "missing", meaning that this argument must not appear in the call in order to match this signature.