27

One of the kool things about R is if I type the function name I get to see the implementation. But this one is confusing me, recursively:

> library(xts)
> align.time
function (x, ...) 
{
    UseMethod("align.time")
}
<environment: namespace:xts>

x is an XTS object, so doesn't that mean it will call the XTS align.time method... but that is what I'm looking at! (Typing xts::align.time gives exactly the same response.)

Mischa Vreeburg
  • 1,576
  • 1
  • 13
  • 18
Darren Cook
  • 27,837
  • 13
  • 117
  • 217
  • 2
    See also this very similar question: http://stackoverflow.com/q/5835312/602276 – Andrie Nov 25 '11 at 09:57
  • 2
    Circle 7 of http://www.burns-stat.com/pages/Tutor/R_inferno.pdf might help you get a sense of what generic functions and methods are all about. – Patrick Burns Nov 25 '11 at 10:54
  • @PatrickBurns Thanks, that looks like a useful read. – Darren Cook Nov 25 '11 at 12:58
  • Thanks for the replies, all very helpful (I gave the tick to Andrie as he needs the points more than Gavin ;-). I'll try and remember methods() and triple colons next time I get stuck. – Darren Cook Nov 25 '11 at 13:01

2 Answers2

37

The short answer is that you are looking for the function xts:::align.time.xts.

The longer answer is that you can find which methods exist for align.time by calling methods:

> methods(align.time)
[1] align.time.POSIXct* align.time.POSIXlt* align.time.xts*    

   Non-visible functions are asterisked

This tells you that there is a method align.time.xts that is not exported from the namespace. At this point you can probably guess that it can be found in package xts, but you can confirm that with getAnywhere:

> getAnywhere("align.time.xts")
A single object matching 'align.time.xts' was found
It was found in the following places
  registered S3 method for align.time from namespace xts
  namespace:xts
with value

function (x, n = 60, ...) 
{
    if (n <= 0) 
        stop("'n' must be positive")
    .xts(x, .index(x) + (n - .index(x)%%n), tzone = indexTZ(x), 
        tclass = indexClass(x))
}
<environment: namespace:xts>

You can, of course, read the source directly, but since the function is not exported, you need to use package:::function (i.e. three colons):

> xts:::align.time.xts
function (x, n = 60, ...) 
{
    if (n <= 0) 
        stop("'n' must be positive")
    .xts(x, .index(x) + (n - .index(x)%%n), tzone = indexTZ(x), 
        tclass = indexClass(x))
}
<environment: namespace:xts>
Andrie
  • 176,377
  • 47
  • 447
  • 496
8

align.time() is exported from the xts namespace, so xts::align.time and align.time are the same thing. You need to note that there is an align.time() method for objects of class "xts" provided in the package and that is not exported from the namespace (it is just registered as an S3 method):

> xts:::align.time.xts
function (x, n = 60, ...) 
{
    if (n <= 0) 
        stop("'n' must be positive")
    .xts(x, .index(x) + (n - .index(x)%%n), tzone = indexTZ(x), 
        tclass = indexClass(x))
}
<environment: namespace:xts>

It is this method that is being called when you pass an "xts" object to align.time().

When you call align.time() UseMethod() sets up the search for and call of the appropriate "align.time" method, if available, for the class of object supplied as the first argument. UseMethod is doing exactly what you think it is doing, you have just confused yourself by looking at the same function (the generic) in two different ways.

Gavin Simpson
  • 170,508
  • 25
  • 396
  • 453
  • 1
    :-) right back at you. The OP should also take into account the first derivative of our respective rep profiles when deciding on awarding the accept. You're motoring ahead of late! ;-) – Gavin Simpson Nov 25 '11 at 17:25