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...

Thank you in advance,

Gian




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

> On 12-09-13 7:43 AM, Gian Maria Niccolò 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@gmail.com>
>> wrote:
>>
>>  On 12-09-13 7:18 AM, Gian Maria Niccolň 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@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 Niccolň Benucci <gian.benucci@gmail.com>
>>>>> *A:* r-help@r-project.org
>>>>> *Inviato:* Giovedě 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
>>>>>
>>>>>       [[alternative HTML version deleted]]
>>>>>
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>>>>> >
>>>>>
>>>>> PLEASE do read the posting guide
>>>>> http://www.R-project.org/****posting-guide.html<http://www.R-project.org/**posting-guide.html>
>>>>> <http://www.**R-project.org/posting-guide.**html<http://www.R-project.org/posting-guide.html>
>>>>> >
>>>>> <http://www.**r-project.org/**posting-guide.**html<http://r-project.org/posting-guide.**html>
>>>>> <http://**www.r-project.org/posting-**guide.html<http://www.r-project.org/posting-guide.html>
>>>>> >
>>>>>
>>>>>
>>>>>>
>>>>> and provide commented, minimal, self-contained, reproducible code.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>> This body part will be downloaded on demand.
>>>>
>>>>
>>>>
>>>
>>
>>
>

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