[R] make apply() return a list

[Gabor Grothendieck]

Arne Henningsen <ahenningsen <at> email.uni-kiel.de> writes:

: 
: 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 ]

Try lapplying over the columns of the transpose:

lapply(as.data.frame(t(myData)), matrix, 2, 2)