R help - Jul 2007 - Write columns from within a list to a matrix?

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

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
>
> ______________________________________________
> R-help at stat.math.ethz.ch 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.

Very close... Actually it's more like

savecol2=sapply(test, function(x) x[,1])

to get the same matrix as you showed in your for-loop (did you actually want
the first or second column?).

"when I have multiple complex lists I am trying to manage"...
for this, you can try mapply() which goes something like
mapply(function(x,y) #...function body...#,
       x=list1,y=list2)


--- 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
> 
> ______________________________________________
> R-help at stat.math.ethz.ch 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.
>