make apply() return a list

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

Hi Arne,

I'm not sure how much faster this will be over using `for' but you can try:

lapply(seq(nrow(myData)), function(i) myFunction(myData[i, ]))

--sundar