[R] Write columns from within a list to a matrix?

[Benilton Carvalho]

oh! and if you want to be less ad-hoc:

sapply(test, function(x) x[,2])

b

On Jul 22, 2007, at 3:50 PM, Benilton Carvalho wrote:

> test <- lapply(1:3, function(i) cbind(runif(15), rnorm(15,2)))
> sapply(test, "[", 16:30)
>
> b
>
> On Jul 22, 2007, at 3:39 PM, jrg66 at comcast.net wrote:
>
>> Hello,
>>
>> I think I have a mental block when it comes to working with  
>> lists.  lapply and sapply appear to do some magical things, but I  
>> can't seem to master their usage.
>>
>> As an example, I would like to convert a column within a list to a  
>> matrix, with the list element corresponding to the new matrix column.
>>
>> #Here is a simplified example: .
>> test=vector("list", 3)
>> for (i in 1:3){ test[[i]]=cbind(runif(15), rnorm(15,2)) }  #create  
>> example list (I'm sure there is a better way to do this too).
>>
>> #Now, I wan to get the second column back out, converting it from  
>> a list to a matrix.  This works, but gets confusing/inefficient  
>> when I have multiple complex lists I am trying to manage.
>>
>> savecol2=matrix(0,15,0)
>> for (i in 1:3){
>> savecol2=cbind(savecol2, test[[i]][,1])
>> }
>>
>> #Something like??:  (of course this doesn't work)
>> savecol2=sapply(test, "[[", function(x) x[2,])
>>
>> Thank you!
>>
>> Jeff
>>
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>