R help - Feb 2009 - Problem with retrieving updated variables after attach()

Hello,

suppose I have a data frame:
> mat
 id age
1 NA  NA
2 NA  NA
3 NA  NA
4 NA  NA
5 NA  NA

Then I attach the data frame:
> attach(mat)
I assign some new values...
> id <<- sample(100,5)
> age <<- rnorm(5,mean=30)
Then I want to create a new data frame from the variables id and age which still
are attached to position 2 of the R environment...
> new_mat <- data.frame(ls(pos=2)) # I want to rescue ALL variables that
were created by attach(mat)
> new_mat
 ls.pos...2.
1         age
2          id

But this leads to a bogus object... how can I rescue the updated id and age
values into new_mat?


Regards,

David

-- 
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Hi


r-help-bounces at r-project.org napsal dne 10.02.2009 12:02:53:
> Hello,
> 
> suppose I have a data frame:
> 
> > mat
>  id age
> 1 NA  NA
> 2 NA  NA
> 3 NA  NA
> 4 NA  NA
> 5 NA  NA
> 
> Then I attach the data frame:
> 
> > attach(mat)
Look into docs what attach does. If you do not understand environments use 
attach with great care

The database is not actually attached. Rather, a new environment is 
created on the search path and the elements of a list (including columns 
of a data frame) or objects in a save file or an environment are copied 
into the new environment. If you use <<- or assign to assign to an 
attached database, you only alter the attached copy, not the original 
object. (Normal assignment will place a modified version in the user's 
workspace: see the examples.) For this reason attach can lead to 
confusion. 

> 
> I assign some new values...
> 
> > id <<- sample(100,5)
> > age <<- rnorm(5,mean=30)
> 
> Then I want to create a new data frame from the variables id and age 
which 
> still are attached to position 2 of the R environment...
> 
> > new_mat <- data.frame(ls(pos=2)) # I want to rescue ALL variables
that
were 
> created by attach(mat)
> 
> > new_mat
>  ls.pos...2.
> 1         age
> 2          id
> 
> But this leads to a bogus object... how can I rescue the updated id and 
age 
> values into new_mat?
What about not using attach and transform mat directly
> mat$age <- rnorm(5,mean=30)
> mat$id <- sample(100,5)
> mat
  id      age
1 24 29.17842
2 88 31.22606
3 32 30.81540
4  5 29.31528
5 11 29.32775

Regards
Petr

> 
> 
> Regards,
> 
> David
> 
> -- 
> Jetzt 1 Monat kostenlos! GMX FreeDSL - Telefonanschluss + DSL 
> f?r nur 17,95 Euro/mtl.!* http://dsl.gmx.de/?ac=OM.AD.PD003K11308T4569a
> 
> ______________________________________________
> 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.

attach provides a copy of rather than aliases to the variables within a
data frame.

d = data.frame(x=0)
attach(d)

x
# 0, from the attached copy of d

x = 1
x
# 1, from the global anvironment
d$x
# 0, from d

x <<- 2
x
# 1, from the global environment
d$x
# 0, from d
get('x', pos=2)
# 2, from the attached copy of d

rm(x)
x
# 2, from the attached copy of d

vQ  



David Croll wrote:
> Hello,
>
> suppose I have a data frame:
>
>   
>> mat
>>     
>  id age
> 1 NA  NA
> 2 NA  NA
> 3 NA  NA
> 4 NA  NA
> 5 NA  NA
>
> Then I attach the data frame:
>
>   
>> attach(mat)
>>     
>
> I assign some new values...
>
>   
>> id <<- sample(100,5)
>> age <<- rnorm(5,mean=30)
>>     
>
> Then I want to create a new data frame from the variables id and age which
still are attached to position 2 of the R environment...
>
>   
>> new_mat <- data.frame(ls(pos=2)) # I want to rescue ALL variables
that were created by attach(mat)
>>     
>
>   
>> new_mat
>>     
>  ls.pos...2.
> 1         age
> 2          id
>
> But this leads to a bogus object... how can I rescue the updated id and age
values into new_mat?
>