random sampling inside a dataset

Hi again to everybody,

I incountered the following error when I try to make a sample inside a
dataset.
My code is:

data_ostrya <- sample(ostrya,200, replace=F)
Error in `[.data.frame`(x, .Internal(sample(length(x), size, replace,  :
   cannot take a sample larger than the population when 'replace = FALSE'

Why it does not work?
The whole dataset is composed of 536 rows and I just want to sample
randomly 200 of them...
The sample() works on vectors, not dataframes.  Since dataframes are 
lists containing the columns, it was trying to sample columns, not rows.

This would give you a sample of rows:

ostrya[sample(1:536, 200, replace=FALSE),]

Duncan Murdoch
Thank you in advance,

Gian

On 13 September 2012 14:01, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:

On 12-09-13 7:43 AM, Gian Maria Niccol?? Benucci wrote:

Thank you Duncan,

I got the result of sampling, but it gave me only the row numbers. Is it
possible to have the entire row with variables and other information?
Because I need to re-sample inside my matrix the whole rows in reason to
have 20 samples (i.e., rows) each year.
Thank you for your invaluable help!

Use those row numbers to index the dataframe or matrix, e.g.

a[rows,]

Duncan Murdoch

Gian

On 13 September 2012 13:32, Duncan Murdoch <murdoch.duncan at gmail.com>
wrote:

  On 12-09-13 7:18 AM, Gian Maria Niccol?? Benucci wrote:
  Thank you very much for your help,
I was wondering if is possible to sample randomly specifying to select
in
a
particular group of data inside the matrix, for example only within the
whole samples collected in 2011 I would randomly choose 20 random
samples...

You need two steps:  find the rows that meet your condition, then sample
from those.  For example,

rows <- which( a$year == 2011 )
sample(rows, 20)

There is one thing to watch out for:  if you have a condition that only
matches one row, you will get unexpected results here, because the sample
will be taken from 1:rows.  See the examples in ?sample for the
workaround
that uses sample.int.

Duncan Murdoch

  Thanks a again,

Gian

On 13 September 2012 12:26, anna freni sterrantino <annafreni at yahoo.it

wrote:

   Hello Gian,

sure sample function
will do it for your sampling.

a=as.data.frame(matrix(1:20,4)****)

sample(rownames(a),2)

see ?sample for more details.
Hope it helps

Cheers

Anna

Anna Freni Sterrantino
Department of Statistics
University of Bologna, Italy
via Belle Arti 41, 40124 BO.
     ------------------------------
*Da:* Gian Maria Niccol?? Benucci <gian.benucci at gmail.com>
*A:* r-help at r-project.org
*Inviato:* Gioved?? 13 Settembre 2012 10:42
*Oggetto:* [R] random sampling inside a dataset

Hi all,

I am wondering if do exist a function in R that allow me to sample or
choose randomly the rows (i.e., samples) inside a given matrix.
Thank you very much in advance.
Cheers,

--
Gian

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random sampling inside a dataset

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