R help - Nov 2007 - a newbie question about "data"

hi, I am reading Modern Applied Statistics with S 4th ed$B!#(B
page4 have these two lines:
> library(MASS)
> data(chem) # needed in R only
 but I find withou the line " data(chem)"
I can still access chem, isn't it?
is it unnecessary or something i missed here?
thanks for the replay in advance.

On 8/11/2007, at 4:26 PM, envisage wrote:
> hi, I am reading Modern Applied Statistics with S 4th ed?
> page4 have these two lines:
>> library(MASS)
>> data(chem) # needed in R only
>  but I find withou the line " data(chem)"
> I can still access chem, isn't it?
> is it unnecessary or something i missed here?
> thanks for the replay in advance.
I don't think you're missing anything.  It appears that things have  
changed
and that ``chem'' has become what Prof. Ripley refers to as a ``first  
class
object'' in the MASS package.  That is it's ``there''.

If you start R and execute

	library(MASS)
	ls(2)

you will see ``chem'' amongst the names listed.

		cheers,

			Rolf Turner
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The book came out in 2002 and a lot has happened with R in the time since then. 
In particular it is now possible for R to have 'lazy loading' of
objects.  If the person setting up the package has used this option (as they all
now should), when the package is loaded R essentially is made aware that the
data (and other) objects are there, so it is visible, but it is only read into
memory if it is used.  The little tag that alerts R to the existence of the
object and triggers the automatic loading on demand is called a
'promise' and the action itself is called a 'delayed assign'.

Previously this all had to be done manually.  data() could be used either to
discover what data objects were available in packages, or to load them in to
memory.  Many old-style packages still require you to do this.  There are cases
where it is justified, I suppose, but they are quite hard to think of...
 
With R you have to keep abreast of developments, and it's all happening
pretty fast.
 
Bill Venables.
 
PS Since you are new to R, it is not considered very friendly to send messages
to the R groups anonymously.


Bill Venables
CSIRO Laboratories
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AUSTRALIA
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http://www.cmis.csiro.au/bill.venables/
<http://www.cmis.csiro.au/bill.venables/>

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org
<mailto:r-help-bounces at r-project.org> ] On Behalf Of envisage
Sent: Thursday, 8 November 2007 1:27 PM
To: r-help at r-project.org
Subject: [R] a newbie question about "data"

hi, I am reading Modern Applied Statistics with S 4th ed$B!#(J
page4 have these two lines:
> library(MASS)
> data(chem) # needed in R only
 but I find withou the line " data(chem)"
I can still access chem, isn't it?
is it unnecessary or something i missed here?
thanks for the replay in advance.