[R] sampling rows from a list

[Justin Haynes]

## recreating your data
mydata<-list(matrix(1:9, nrow=3, byrow=T),
                  matrix(10:15, nrow=2, byrow=T),
                  matrix(16:30, nrow=5, byrow=T))

## get the shortest matrix in your list
n <- min(unlist(lapply(mydata, nrow)))

## subset the list into random samples of length n
out <- lapply(mydata, function(x, n) x[sample(1:nrow(x), n),], n=n)
## this  structure is still a list though...

## converting directly to an array:
out.array <- array(unlist(out), dim=c(dim(out[[1]]), length(out)))

not totally sure about what structure you're wanting in the last step,
so if i missed i apologize...

Hope that helps,

Justin


On Mon, Apr 2, 2012 at 11:24 AM, Bcampbell99 <BrianD.Campbell at ec.gc.ca> wrote:
> Hi:
>
> I'm sure this seems like a rudimentary question, but I am not well versed
> with R syntax for lists.  I have a ragged array from which I've removed
> records (entire rows) with missing data.  The functions I used to remove the
> missing cases resulted in the generation of an R list class object, that
> looks something like this;
>
> mydata
> [[1]]
>     [,1] [,2] [,3]
> [1,]    1    2    3
> [2,]    4    5    6
> [3,]    7    8    9
>
> [[2]]
>     [,1] [,2] [,3]
> [1,]   10   11   12
> [2,]   13   14   15
>
> [[3]]
>     [,1] [,2] [,3]
> [1,]   16   17   18
> [2,]   19   20   21
> [3,]   22   23   24
> [4,]   25   26   27
> [5,]   28   29   30
>
> Part1
> What I would like to do is draw an equal number of random row samples
> from[[1]],[[2]] and [[3]] (to preserve the structure of [,1][,2],[,3].
>
> Part2
> Then I would like to cocerce the list object into something like an array.
>
> Help scripting out part 1 or 2 would be much appreciated.
>
> Brian Campbell
>
>
>
>
> --
> View this message in context: http://r.789695.n4.nabble.com/sampling-rows-from-a-list-tp4526831p4526831.html
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>
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