[R] make apply() return a list

[Gabor Grothendieck]

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

: 
: Hi,
: 
: thank you very much Sundar, Patrick, Tony, Mahub and Gabor for your helpful 
: answers! All your examples work great. They are all more straightforeward 
: than my example and much faster than the for-loop. 
: These are the average elapsed times (in seconds) returned by system.time()
[3] 
: (applied to my real function and my real data):
: 
: my original for-loop: 
: 5.55 
: 
: the example I presented in my previous email (using apply):
: 2.35 
: 
: example suggested by Tony (using apply):
: 2.34 
: 
: example suggested by Gabor (using lapply):
: 2.50 
: 
: examples suggested by Sundar and Mahub (using lapply):
: 2.68 
: 


Perhaps any comparison should also include simplicity.  This is
somewhat subjective but just to objectify it I have reworked
each solution to compactify it as much as I could and then
calculated the number of characters in each solution using wc:

AH - 293 characters
TP - 70 characters
ML - 62 characters
GG - 48 characters

The versions I used are below.  

---

# data
myData <- data.frame( a = c( 1,2,3 ), b = c( 4,5,6 ) )

# AH
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

# TP
lapply(apply(myData, 1, function(x) list(matrix(x, 2, 2))), "[[", 1)

# ML
lapply(1:nrow(myData), function(i) matrix(myData[i,], 2, 2))

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