R help - Nov 2007 - how to extract the elements of a list of vectors in a fixed position?

Hi,
How is it possible to extract athe elements of a list of vectors in a fixed
position? suppose that I have a list of 2-element vectors, how can I extract the
2nd element of all vectors in the list? Can it be done with indexing and not by
element name?

Thanks 

carol

So in this example, I want to extract 2 and 4
v = list (c(1,2), c(3, 4))
 
> v
[[1]]
[1] 1 2

[[2]]
[1] 3 4



       
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try this:

v <-  list(c(1,2), c(3,4), 5, c(-3:3))
sapply(v, "[", i = 2)


I hope it helps.

Best,
Dimitris


----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://med.kuleuven.be/biostat/
      http://www.student.kuleuven.be/~m0390867/dimitris.htm


Quoting carol white <wht_crl at yahoo.com>:
> Hi,
> How is it possible to extract athe elements of a list of vectors in   
> a fixed position? suppose that I have a list of 2-element vectors,   
> how can I extract the 2nd element of all vectors in the list? Can it  
>  be done with indexing and not by element name?
>
> Thanks
>
> carol
>
> So in this example, I want to extract 2 and 4
> v = list (c(1,2), c(3, 4))
>
>> v
> [[1]]
> [1] 1 2
>
> [[2]]
> [1] 3 4
>
>
>
>
> ---------------------------------
> Never miss a thing.   Make Yahoo your homepage.
> 	[[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.
>
>


Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm

On 16/11/2007, at 7:05 AM, carol white wrote:
> Hi,
> How is it possible to extract athe elements of a list of vectors in  
> a fixed position? suppose that I have a list of 2-element vectors,  
> how can I extract the 2nd element of all vectors in the list? Can  
> it be done with indexing and not by element name?
>
> Thanks
>
> carol
>
> So in this example, I want to extract 2 and 4
> v = list (c(1,2), c(3, 4))
>
>> v
> [[1]]
> [1] 1 2
>
> [[2]]
> [1] 3 4
>
	lapply(v,function(x){x[2]})

or

	unlist(lapply(v,function(x){x[2]}))

or (I can't resist)

	unlist(lapply(v,function(x,i){x[i]},i=2)) # For more flexibility.


		cheers,

			Rolf Turner

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