r语言 - S3类通用和特定函数

Question

我想从对象列表中转换一些数据。我创建了一个通用函数和特定的函数，这样它就会根据对象的类别使用正确的函数。但它似乎没有从通用到特定的功能。如果我输入一个特定的函数，它可以工作，但它首先违背了使用泛型函数的目的。

请帮助。

names <- c("Astro","Barnstormer", "Big Railroad","Buzz", "Soak Station", "Cin.       Castle", 
       "Jamboree", "Dumbo", "Enchanted", "Haunted Mansion", "Jungle Cruise", "Mad Tea", 
       "Aladdin", "Winnie","Monsters, Inc", "Peter Pan", "7 Dwarfs", "Space Mountain")
Letters <-  LETTERS[1:20]
#Giving it different classes
items <- list(names=names, letters=Letters)
class(items$names) <- append(class(items$names), "names ")
class(items$letters) <- append(class(items$letters), "letters")
class(items) <- append(class(items),"items") 
# Generic method
setNames <- function(x, newValue)
{
   print("Find the correct method")
   UseMethod("setNames", x)
 }
#Default method
 setNames.Default <- function(x)
{
   print("You got it wrong")
   return(x)
}
#Specific method to class
 setNames.names <- function(x, newValue)
 {
    x$names <- newValue
    return(x)
 }
  # This works
 t <- setNames.names(items, "abc")
  # This doesn't work. 
 t <- setNames(items, "abc")

Answer 1

我将items的类设置为c("names", "list")，并对您的函数进行了一些更改。这能达到你的目的吗?

#Giving it different classes
items <- list(names=names, letters=Letters)
class(items) <- c("names", "list")
"setNames" <- function(x, ...) {
  UseMethod("setNames")
}

setNames.names <- function(x, newValue) {
  x$names <- newValue
  return(x)
}
setNames.default <- function(x, ...)
{
   warning("unknown class")
   return(x)
}
# This works
t <- setNames.names(items, "abc")
# Check if it works 
t2 <- setNames(items, "abc")
identical(t, t2)
[1] TRUE

和不起作用的情况

foo <- items
class(foo) <- "list"
setNames(foo, "abc")
$names
 [1] "Astro"             "Barnstormer"       "Big Railroad"     
 [4] "Buzz"              "Soak Station"      "Cin.       Castle"
 [7] "Jamboree"          "Dumbo"             "Enchanted"        
[10] "Haunted Mansion"   "Jungle Cruise"     "Mad Tea"          
[13] "Aladdin"           "Winnie"            "Monsters, Inc"    
[16] "Peter Pan"         "7 Dwarfs"          "Space Mountain"   
$letters
 [1] "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N" "O" "P" "Q" "R" "S"
[20] "T"
Warning message:
In setNames.default(foo, "abc") : unknown class