How to efficiently grow large data in R - r

Question

The product of one simulation is a large data.frame, with fixed columns and rows. I ran several hundreds of simulations, with each result stored in a separate RData file (for efficient reading).
Now I want to gather all those files together and create statistics for each field of this data.frame into the "cells" structure which is basically a list of vectors with . This is how I do it:
#colscount, rowscount - number of columns and rows from each simulation
#simcount - number of simulation.
#colnames - names of columns of simulation's data frame.
#simfilenames - vector with filenames with each simulation
cells<-as.list(rep(NA, colscount))
for(i in 1:colscount)
{
cells[[i]]<-as.list(rep(NA,rowscount))
for(j in 1:rows)
{
cells[[i]][[j]]<-rep(NA,simcount)
}
}
names(cells)<-colnames
addcells<-function(simnr)
# This function reads and appends simdata to "simnr" position in each cell in the "cells" structure
{
simdata<readRDS(simfilenames[[simnr]])
for(i in 1:colscount)
{
for(j in 1:rowscount)
{
if (!is.na(simdata[j,i]))
{
cells[[i]][[j]][simnr]<-simdata[j,i]
}
}
}
}
library(plyr)
a_ply(1:simcount,1,addcells)
The problem is, that this the
> system.time(dane<-readRDS(path.cat(args$rdatapath,pliki[[simnr]]))$dane)
user system elapsed
0.088 0.004 0.093
While
? system.time(addcells(1))
user system elapsed
147.328 0.296 147.644
I would expect both commands to have comparable execution times (or at least the latter be max 10 x slower). I guess I am doing something very inefficient there, but what? The whole cells data structure is rather big, it takes around 1GB of memory.
I need to transpose data in this way, because later I do many descriptive statistics on the results (like computing means, sd, quantiles, and maybe histograms), so it is important, that the data for each cell is stored as a (single-dimensional) vector.
Here is profiling output:
> summaryRprof('/tmp/temp/rprof.out')
$by.self
self.time self.pct total.time total.pct
"[.data.frame" 71.98 47.20 129.52 84.93
"names" 11.98 7.86 11.98 7.86
"length" 10.84 7.11 10.84 7.11
"addcells" 10.66 6.99 151.52 99.36
".subset" 10.62 6.96 10.62 6.96
"[" 9.68 6.35 139.20 91.28
"match" 6.06 3.97 11.36 7.45
"sys.call" 4.68 3.07 4.68 3.07
"%in%" 4.50 2.95 15.86 10.40
"all" 4.28 2.81 4.28 2.81
"==" 2.34 1.53 2.34 1.53
".subset2" 1.28 0.84 1.28 0.84
"is.na" 1.06 0.70 1.06 0.70
"nargs" 0.62 0.41 0.62 0.41
"gc" 0.54 0.35 0.54 0.35
"!" 0.42 0.28 0.42 0.28
"dim" 0.34 0.22 0.34 0.22
".Call" 0.12 0.08 0.12 0.08
"readRDS" 0.10 0.07 0.12 0.08
"cat" 0.10 0.07 0.10 0.07
"readLines" 0.04 0.03 0.04 0.03
"strsplit" 0.04 0.03 0.04 0.03
"addParaBreaks" 0.02 0.01 0.04 0.03
It looks that indexing the list structure takes a lot of time. But I can't make it array, because not all cells are numeric, and R doesn't easily support hash map...