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

Stephen Tucker brown_emu at yahoo.com
Sun Jul 22 22:10:19 CEST 2007

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...#,

--- 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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