What's data() for?

On 14/05/2010 5:35 AM, (Ted Harding) wrote:

On 13-May-10 23:43:58, yjmha69 wrote:

Hi there,

library(faraway)
pima

    pregnant glucose diastolic triceps insulin  bmi diabetes age test
1          6     148        72      35       0 33.6    0.627  50    1
2          1      85        66      29       0 26.6    0.351  31    0

data(pima)
pima

    pregnant glucose diastolic triceps insulin  bmi diabetes age test
1          6     148        72      35       0 33.6    0.627  50    1
2          1      85        66      29       0 26.6    0.351  31    0

As you can see, I can already use pima without running data(pima),
after running data(pima), it looks the same. So what's the reason to
use data(pima) ?

Thanks
YJM

The difference is that data(pima) will load the dataset pima
(which can be found in the package "faraway") without the use
of library(faraway). It won't load anything else from faraway.

That won't work.  Unless you attach faraway, R won't know what "pima" refers 
to, and will just give an error.

The difference between data(pima) and pima is that, in this case, there isn't 
really much of one, but in other cases there might be.  Prior to the 
introduction of lazy loading of data, it always made a difference:  the pima 
object wouldn't be loaded into memory until requested by data(pima).  With 
lazy loading, a stub for the object is always in memory, with the main part 
of the object only loaded on first use.  Many packages (including faraway) 
use lazy loading of data so data() is to some extent unnecessary:  but there 
are some circumstances under which lazy loading won't work, so a few packages 
don't use it, and I believe it is not the default.

Duncan Murdoch

When you use library(faraway) you will load everything in the
package faraway, including of course the dataset pima (which is
why you see no difference, since that dataset is the same whichever
way you load it).

So with data() you put less load on your system, and also avoid
possible conflicts between what you already have in your environment
and what would be brought in when you do library(faraway).

Ted.

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Date: 14-May-10                                       Time: 10:35:15
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