[R] rbind() overwriting data.frame()

[ONKELINX, Thierry]

This will give a matrix with 0 rows.

data.frame(matrix(nrow = 0,  ncol = 22, dimnames = list(NULL,
LETTERS[1:22])))

But you should avoid growing dataframes is the final dataframe is going
to be large. You are very likely to get memory problems. It is much to
better to create a large enough dataframe and then overwrite the rows.
And it is faster too...

> nrows <- 2000
> ncols <- 22
> system.time({
+ tmp <- data.frame(matrix(nrow = 0,  ncol = ncols))
+ for(i in seq_len(nrows)){
+ tmp <- rbind(tmp, rnorm(ncols))
+ }
+ })
   user  system elapsed 
   7.83    0.02    7.86 
> system.time({
+ tmp <- data.frame(matrix(nrow = nrows,  ncol = ncols))
+ for(i in seq_len(nrows)){
+ tmp[i, ] <- rnorm(ncols)
+ }
+ })
   user  system elapsed 
   3.75    0.00    3.76 

#In this case an apply construction was even faster

> system.time({
+ tmp <- t(sapply(seq_len(nrows), function(i){
+ rnorm(ncols)
+ }))
+ })
   user  system elapsed 
   0.02    0.00    0.02 




------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek
team Biometrie & Kwaliteitszorg
Gaverstraat 4
9500 Geraardsbergen
Belgium

Research Institute for Nature and Forest
team Biometrics & Quality Assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium

tel. + 32 54/436 185
Thierry.Onkelinx op inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey
  

> -----Oorspronkelijk bericht-----
> Van: r-help-bounces op r-project.org 
> [mailto:r-help-bounces op r-project.org] Namens rajesh j
> Verzonden: maandag 6 september 2010 16:57
> Aan: r-help op r-project.org
> Onderwerp: [R] rbind() overwriting data.frame()
> 
> Hi,
> 
> first off, I wanna ask how do I declare a data.frame of 0 
> rows and n columns?
> 
> Coming to my problem,
> 
> I have a data.frame of 22 columns by dynamic rows which I 
> insert using rbind. The total number of rows could go upto 
> 2,00,000. The problem is that after about 800 or 900 get 
> inserted rbind starts overwriting the data.frame and I end up 
> with a total of 800-900 rows. What is up with that?
> The 22 columns are all strings each having about 10 characters
> --
> Rajesh.J
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help op r-project.org mailing list
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> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 

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