我注意到定义 as.matrix
或as.data.frame
作为 S4 类的 S3 方法
使例如lm (formula, objS4)
和prcomp (object)
开箱即用。如果它们被定义为 S4 方法,则这不起作用。
为什么方法定义为 S3 或 S4 方法很重要?
as.data.frame
的示例:
setClass ("exampleclass", representation (x = "data.frame"))
object <- new ("exampleclass", x = iris)
setMethod ("as.data.frame", signature="exampleclass", definition= function (x, ...) x@x )
## [1] "as.data.frame"
as.data.frame (object)
## Sepal.Length Sepal.Width Petal.Length Petal.Width Species
## 1 5.1 3.5 1.4 0.2 setosa
## 2 4.9 3.0 1.4 0.2 setosa
## 3 4.7 3.2 1.3 0.2 setosa
## ...snip...
lm (Petal.Length ~ Petal.Width, object)
## error in as.data.frame.default(data) :
## cannot coerce class 'structure("exampleclass", package = ".GlobalEnv")' into a data.frame
as.data.frame.exampleclass <- function (x, ...) x@x
lm (Petal.Length ~ Petal.Width, object)
## Call:
## lm(formula = Petal.Length ~ Petal.Width, data = object)
##
## Coefficients:
## (Intercept) Petal.Width
## 1.084 2.230
由于情况可能有点复杂,lm
仅当在由数据构建的环境中计算公式时才会发生强制转换,这是一个具有相同行为的更简单的情况:
setMethod ("as.matrix", signature="exampleclass", definition= function (x, ...) as.matrix (x@x[, 1:4]) )
prcomp (object)
## error in as.vector(data) :
## No method to coerce this S4 class into a vector
as.matrix.exampleclass <- function (x, ...) as.matrix (x@x [, 1:4])
prcomp (object)
## Standard deviations:
## [1] 2.0562689 0.4926162 0.2796596 0.1543862
##
## Rotation:
## PC1 PC2 PC3 PC4
## Sepal.Length 0.36138659 -0.65658877 0.58202985 0.3154872
## Sepal.Width -0.08452251 -0.73016143 -0.59791083 -0.3197231
## Petal.Length 0.85667061 0.17337266 -0.07623608 -0.4798390
## Petal.Width 0.35828920 0.07548102 -0.54583143 0.7536574
在这里,stats:::prcomp.default
被调用,它以简单的 x <- as.matrix (x)
开头。对于上面的 S4 定义,这会失败,但对于 S3 定义却有效。
最佳答案
我从评论中得知,lm
只是显式调用as.data.frame
。如果你看一下as.data.frame
:
> as.data.frame
function (x, row.names = NULL, optional = FALSE, ...)
{
if (is.null(x))
return(as.data.frame(list()))
UseMethod("as.data.frame")
}
<bytecode: 0x29140b8>
<environment: namespace:base>
您将看到它调用 S3 泛型,并来自 methods documentation
An S4 method alone will not be seen if the S3 generic function is called directly. However, >primitive functions and operators are exceptions: The internal C code will look for S4 >methods if and only if the object is an S4 object. In the examples, the method for
[
for >class "myFrame" will always be called for objects of this class.
关于r - `as.matrix` 和 `as.data.frame` S3 方法与 S4 方法,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/15593021/