[R] What's data() for?

[Duncan Murdoch]

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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> E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
> Fax-to-email: +44 (0)870 094 0861
> Date: 14-May-10                                       Time: 10:35:15
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