[R] random sampling inside a dataset

[Duncan Murdoch]

On 12-09-13 7:18 AM, Gian Maria Niccolò 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 Niccolò Benucci <gian.benucci at gmail.com>
>> *A:* r-help at r-project.org
>> *Inviato:* Giovedì 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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