Re: [R] make apply() return a list

Tony Plate Mon, 01 Nov 2004 11:11:47 -0800

for()-loops aren't so bad. Look inside the code of apply() and see what it uses! The important thing is that you use vectorized functions to manipulate vectors. It's often fine to use for-loops to manipulate the rows or columns of a matrix, but once you've extracted a row or a column, then use a vectorized function to manipulate that data.

In any case, one way to get apply() to return a list is to wrap the result from the subfunction inside a list, e.g.:

> x <- apply(matrix(1:6,2), 1, function(x) list((c(mean=mean(x), sd=sd(x)))))
> x
[[1]]
[[1]][[1]]
mean   sd
   3    2


[[2]]
[[2]][[1]]
mean   sd
   4    2


> # to remove the extra level of listing here, do:
> lapply(x, "[[", 1)
[[1]]
mean   sd
   3    2

[[2]]
mean   sd
   4    2

At Monday 11:37 AM 11/1/2004, Arne Henningsen wrote:

Hi,

I have a dataframe (say myData) and want to get a list (say myList) that
contains a matrix for each row of the dataframe myData. These matrices are
calculated based on the corresponding row of myData. Using a for()-loop to do
this is very slow. Thus, I tried to use apply(). However, afaik apply() does
only return a list if the matrices have different dimensions, while my
matrices have all the same dimension. To get a list I could change the
dimension of one matrix artificially and restore it after apply():

This a (very much) simplified example of what I did:
> myData <- data.frame( a = c( 1,2,3 ), b = c( 4,5,6 ) )
> myFunction <- function( values ) {
+    myMatrix <- matrix( values, 2, 2 )
+    if( all( values == myData[ 1, ] ) ) {
+       myMatrix <- cbind( myMatrix, rep( 0, 2 ) )
+    }
+    return( myMatrix )
+ }
> myList <- apply( myData, 1, myFunction )
> myList[[ 1 ]] <- myList[[ 1 ]][ 1:2, 1:2 ]
> myList
$"1"
     [,1] [,2]
[1,]    1    1
[2,]    4    4

$"2"
     [,1] [,2]
[1,]    2    2
[2,]    5    5

$"3"
     [,1] [,2]
[1,]    3    3
[2,]    6    6

This exactly does what I want and really speeds up the calculation, but I
wonder if there is an easier way to make apply() return a list.

Thanks for your help,
Arne

--
Arne Henningsen
Department of Agricultural Economics
University of Kiel
Olshausenstr. 40
D-24098 Kiel (Germany)
Tel: +49-431-880 4445
Fax: +49-431-880 1397
[EMAIL PROTECTED]
http://www.uni-kiel.de/agrarpol/ahenningsen/

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Re: [R] make apply() return a list

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