[R] grabbing from elements of a list without a loop

[David Winsemius]

On Feb 12, 2013, at 3:01 PM, Dimitri Liakhovitski wrote:

> Arun, do you know why the first line works and the second doesn't?
> newlist<-mapply(`[`,mylist,list(1,2,0),SIMPLIFY=FALSE) # it works
> neededcolumns<-c(1,2,0)                   # it does not work
> newlist<-mapply(`[`,mylist,list(neededcolumns),SIMPLIFY=FALSE)

> list(c(1,2,3))
[[1]]
[1] 1 2 3

> list(1,2,3)
[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
[1] 3


-- 
David.
> Dimitri
> On Tue, Feb 12, 2013 at 5:08 PM, arun <smartpink111 at yahoo.com> wrote:
> 
>> Hi,
>> 
>> 
>> mapply(`[`,mylist,list(1,2,0),SIMPLIFY=FALSE)
>> #[[1]]
>> #  a
>> #1 1
>> #2 2
>> 
>> #[[2]]
>> # b
>> #1 5
>> #2 6
>> 
>> #[[3]]
>> #data frame with 0 columns and 2 rows
>> 
>> 
>> A.K.
>> 
>> ----- Original Message -----
>> From: Dimitri Liakhovitski <dimitri.liakhovitski at gmail.com>
>> To: r-help <r-help at r-project.org>
>> Cc:
>> Sent: Tuesday, February 12, 2013 4:33 PM
>> Subject: [R] grabbing from elements of a list without a loop
>> 
>> Hello!
>> 
>> # I have a list with several data frames:
>> mylist<-list(data.frame(a=1:2,b=2:3),
>>           data.frame(a=3:4,b=5:6),data.frame(a=7:8,b=9:10))
>> (mylist)
>> 
>> # I want to grab only one specific column from each list element
>> neededcolumns<-c(1,2,0)  # number of the column I need from each element of
>> the list
>> 
>> # Below, I am doing it using a loop:
>> newlist<-NULL
>> for(i in 1:length(mylist) ) {
>>  newlist[[i]]<-mylist[[i]] [neededcolumns[i]]
>> }
>> newlist<-do.call(cbind,newlist)
>> (newlist)
>> 
>> I was wondering if there is any way to avoid the loop above and make it
>> faster.
>> In reality, I have a much longer list, each of my data frames is much
>> larger and I have to do it MANY-MANY times.
>> Thanks a lot!
>> 
>> Dimitri Liakhovitski
>> gfk.com <http://marketfusionanalytics.com/>
>> 
>>    [[alternative HTML version deleted]]
>> 
>> ______________________________________________
>> 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<http://www.r-project.org/posting-guide.html>
>> and provide commented, minimal, self-contained, reproducible code.
>> 
>> 
> 
> 
> -- 
> Dimitri Liakhovitski
> gfk.com <http://marketfusionanalytics.com/>
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> 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 Winsemius
Alameda, CA, USA