CODEWITHSUNDEEP
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Tim
Tim

Reputation: 99478

"as." in names of functions for conversion between data types in R

as.matrix will convert a data frame to a matrix. as.data.frame will do the opposite conversion.

ts will convert a vector, matrix or data frame to a time series object.

So I wonder why as. appear in some functions' names, and not in the others? Thanks!

Upvotes: 0

Views: 92

Answers (2)

Luca Braglia
Luca Braglia

Reputation: 3243

Following Tim's comment request...

Generally speaking

  • matrix and ts (like other function with class-like-name) are used when you create an object from scratch (eg a data.frame from a set of vector, a matrix from a vector specifying dims).
  • as.matrix and as.ts (like other function like as.classname) are used to coerce an object of a given class to classname.

matrix and as.matrix fits the general rule quite well.

matrix is tipically efficient in creating an object from scratch. You can see that after a few manipulation it call low level code (via .Internal) to provide the correct data structure.

> matrix
function (data = NA, nrow = 1, ncol = 1, byrow = FALSE, dimnames = NULL)
{
    if (is.object(data) || !is.atomic(data))
        data <- as.vector(data)
    .Internal(matrix(data, nrow, ncol, byrow, dimnames, missing(nrow), missing(ncol)))
}

as.* function aim (eg as.matrix's one) is totally different because they have to cope with different/complex data structures, all to be coerced to that considered. Therefore are somewhat more high-level function (pure R mainly)

> as.matrix
function (x, ...)
UseMethod("as.matrix")
<bytecode: 0x3a5a2f0>
<environment: namespace:base>

which objects can be converted to matrix?

> methods(as.matrix)
[1] as.matrix.data.frame as.matrix.default
[3] as.matrix.dist*      as.matrix.ftable*
[5] as.matrix.noquote    as.matrix.POSIXlt
[7] as.matrix.raster*

   Non-visible functions are asterisked

try from the console

as.matrix.data.frame
as.matrix.ftable
getAnywhere(as.matrix.ftable)

HTH :)

Upvotes: 3

IRTFM
IRTFM

Reputation: 263411

Naming a function as.new_class_name is part of the S3 function dispatch mechanism. If you want to define a new class and a function to provide coercion to that class, then you make an as.new_class_name function and register it using setClass. see the examples at:

 ?setAs
 ?setClass

The interpreter will then be able to properly dispatch to as.new_class_name (assuming you have defined it) when it encounters such a call.

Upvotes: 2

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