[R] make apply() return a list

[Gabor Grothendieck]

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

: 
: On Tuesday 02 November 2004 15:29, Gabor Grothendieck wrote:
: > 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.  
: 
: Yes, you are totally right!
: 
: > 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
: 
: Thank you for wotking this out. I was also thinking about simplicity when I 
: compared the different suggestions. Finally I took Tonys suggestion although 
: the code is a bit longer than the others in _your_ comparison. The reason 
was 
: only partially the speed, but compared to ML and GG this code preserves the 
: (col)names of the dataframe, which I need in the real "myFunction". 
: Circumventing this small problem in ML's and GG's suggestions would make my 
: code (a bit) longer than the code based on TP's suggestion.

Note that neither the problem statement nor your example code
showing what the solution should look like (nor my compactified versions 
of the responses) exhibit colnames.

: 
: Best wishes,
: Arne
: 
: > 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)