attr vs attributes

attributes(my_obj)[[my_attr_name]] <- my_attr_value

...fails when my_obj doesn't already have an attribute named my_attr_name.

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
Of Murat Tasan
Sent: Friday, May 10, 2013 8:16 AM
To: Duncan Murdoch
Cc: r-help at r-project.org
Subject: Re: [R] attr vs attributes

thanks, both.

the only real difference between the two approaches that i can see is when
assigning _new_ attributes to an object where the attribute name is itself
variable.
something like this:

attributes(my_obj)[[my_attr_name]] <- my_attr_value

...fails when my_obj doesn't already have an attribute named my_attr_name.
BUT(!) it does work just fine when the attribute named my_attr_name is
already attached to my_obj.

i guess in the end attr(...) is just easier, but i can also see a spot of
confusion in the 'attributes(x)[[y]] <- z' working sometimes for folks and
sometimes not.

(to be clear, it's pretty easy to figure out when it does and doesn't work,
but if there's a new programmer coming along and it works the first time,
when the upstream code changes a bit (i.e. the attribute name changes) and
this attribute-setting line then fails, it could be very confusing :-)

thanks for the thoughts!

cheers,

-m




On Fri, May 10, 2013 at 10:58 AM, Duncan Murdoch
<murdoch.duncan at gmail.com>wrote:

On 10/05/2013 10:50 AM, Rui Barradas wrote:

Hello,

There's at least one example where only the form attr(x, "foo") <- "bar"
would work, not the other form. If you want to set attributes
programatically, use the first form, like in the function below. Note
that the example is artificial.


setAttr <- function(x, attrib, value){
        attr(x, attrib) <- value
        x
}

x <- 1:4
setAttr(x, "foo", "bar")


You cannot make

attribute(x)$attrib <- value

But

attributes(x)[[attrib]] <- value

would be fine.  I don't know why Murat thought there would be different
consistency checks; I'd assume the main difference would be that attr()
would be quicker.  (It does a lot less: attributes(x)[[attrib]] <- value
essentially does

temp <- attributes(x)
temp[[attrib]] <- value
attributes(x) <- temp

and there are a lot of wasted operations there.)

Duncan Murdoch

Hope this helps,

Rui Barradas

Em 09-05-2013 18:35, Murat Tasan escreveu:

hi all -- i looked through the R Language Definition document, but

couldn't

find any particular warning or example that would clarify the best use

of

attribute setting for R objects.

let x be some R object, and i'd like to add attribute "foo" with value
"bar".

case 1:

attr(x, "foo") <- "bar"

case 2:

attributes(x)$foo <- "bar"

in both cases, attributes(x) reveals the appropriate setting has taken
place.
i'm assuming that attr(...) is 'safer' in the sense that perhaps
consistency checks are made?
(almost like a generic accessor/setter method?)
but i also haven't seen any examples where case 2 (above) would bad...

is

there are trivial such example out there?

BTW -- the cause for interest here is when dealing with dendrogram

objects,

for which much of the useful data are stored as attributes, and where
running dendrapply means having to explicitly set attribute values to
retain the tree structure in the resulting object.

cheers,

-m

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