[R] saving a 'get' object in R

[David Stevens]

Greg - I appreciate your taking the time to explain. This is very 
helpful. My case was a bit unusual as I'm helping a colleague with code 
to use on a regular but individual basis. I want them to name a data set 
once at the top of the script and have that name propagate through 
several objects that end up being saved at the end with file names tied 
to the data set name. Then tomorrow, they'll do the same thing on a 
different data set so each session would be pretty simple, with only 2-3 
named objects at the end, though the data sets are big and cumbersome.

I'll dig into this more and apologize to those who thought this problem 
was too trivial for the r-help forum.

David

On 6/24/2014 4:00 PM, Greg Snow wrote:
> The main reason to avoid assign/get is that there are better ways.
> You can use a list or environment more directly without using
> assign/get.  Also the main reason to use assign/get is to work with
> global variables which are frowned on in general programming (and
> prone to hard to find bugs).
>
> Consider the following code snippet:
>
> x <- 1:10
> functionThatUsesAssign(x)
>
> what is the value of x after running this code?  If you use global
> variables then the answer is "I don't know", and if x was something
> that took several hours to compute and I just accidentally overwrote
> it, then I am probably not happy.
>
> Using assign often leads to the temptation to try code like:
>
> assign( "x[5]", 25 )
>
> which does something (no warnings, no errors) but generally not what
> was being attempted.
>
> Another common (mis)use of assign/get is to create a sequence of
> variables like "data01", "data02", "data03", ...  and then do the same
> thing for each of the data objects.  This is much better done by
> putting all the objects into a single list, then you can easily
> iterate over the list using lapply/sapply or still access the
> individual pieces.  Then when it comes time to save these objects, you
> only need to save the list, not all the individual objects, same for
> deleting, copying, moving, etc.  And you don't have a bunch of
> different objects cluttering your workspace, just a single list.
>
> On Tue, Jun 24, 2014 at 2:57 PM, David Stevens <david.stevens at usu.edu> wrote:
>> Thanks to all for the replies. I tried all three and they work great. I was
>> misinterpreting the list = parameter in save(...) and I get your point about
>> overwriting existing objects.  I've heard about not using assign/get before.
>> Can anyone point me to why and what alternatives there are?
>>
>> Regards
>>
>> David
>>
>>
>> On 6/24/2014 2:50 PM, Henrik Bengtsson wrote:
>>> I recommend to use saveRDS()/readRDS() instead.  More convenient and
>>> avoids the risk that load() has of overwriting existing variables with
>>> the same name.
>>>
>>> /Henrik
>>>
>>> On Tue, Jun 24, 2014 at 1:45 PM, Greg Snow <538280 at gmail.com> wrote:
>>>> I think that you are looking for the `list` argument in `save`.
>>>>
>>>> save( list=foo, file=paste0(foo, '.Rdata') )
>>>>
>>>> In general it is best to avoid using the assign function (and get when
>>>> possible).  Usually there are better alternatives.
>>>>
>>>> On Tue, Jun 24, 2014 at 2:35 PM, David Stevens <david.stevens at usu.edu>
>>>> wrote:
>>>>> R community,
>>>>>
>>>>> Apologies if this has been answered. The concept I'm looking for is to
>>>>> save() an object retrieved using get() for an object
>>>>> that resulted from using assign. Something like
>>>>>
>>>>> save(get(foo),file=paste(foo,'rData',sep=''))
>>>>>
>>>>> where assign(foo,obj) creates an object named foo with the contents of
>>>>> obj
>>>>> assigned. For example, if
>>>>>
>>>>> x <- data.frame(v1=c(1,2,3,4),v2=c('1','2','3','4'))
>>>>> foo = 'my.x'
>>>>> assign(foo,x)
>>>>> # (... then modify foo as needed)
>>>>> save(get(foo),file=paste(foo,'.rData',sep=''))
>>>>>
>>>>> # though this generates " in save(get(foo), file = paste(foo, ".rData",
>>>>> sep
>>>>> = "")) :
>>>>> object ‘get(foo)’ not found", whereas
>>>>>
>>>>> get(foo)
>>>>>
>>>>> at the command prompt yields the contents of my.x
>>>>>
>>>>> There's a concept I'm missing here. Can anyone help?
>>>>>
>>>>> Regards
>>>>>
>>>>> David Stevens
>>>>>
>>>>> --
>>>>> David K Stevens, P.E., Ph.D.
>>>>> Professor and Head, Environmental Engineering
>>>>> Civil and Environmental Engineering
>>>>> Utah Water Research Laboratory
>>>>> 8200 Old Main Hill
>>>>> Logan, UT  84322-8200
>>>>> 435 797 3229 - voice
>>>>> 435 797 1363 - fax
>>>>> david.stevens at usu.edu
>>>>>
>>>>> ______________________________________________
>>>>> R-help at r-project.org mailing list
>>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>>> PLEASE do read the posting guide
>>>>> http://www.R-project.org/posting-guide.html
>>>>> and provide commented, minimal, self-contained, reproducible code.
>>>>
>>>>
>>>> --
>>>> Gregory (Greg) L. Snow Ph.D.
>>>> 538280 at gmail.com
>>>>
>>>> ______________________________________________
>>>> R-help at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting guide
>>>> http://www.R-project.org/posting-guide.html
>>>> and provide commented, minimal, self-contained, reproducible code.
>>
>> --
>> David K Stevens, P.E., Ph.D.
>> Professor and Head, Environmental Engineering
>> Civil and Environmental Engineering
>> Utah Water Research Laboratory
>> 8200 Old Main Hill
>> Logan, UT  84322-8200
>> 435 797 3229 - voice
>> 435 797 1363 - fax
>> david.stevens at usu.edu
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>

-- 
David K Stevens, P.E., Ph.D.
Professor and Head, Environmental Engineering
Civil and Environmental Engineering
Utah Water Research Laboratory
8200 Old Main Hill
Logan, UT  84322-8200
435 797 3229 - voice
435 797 1363 - fax
david.stevens at usu.edu