Hello, If the dataset 'data_ostrya' is kept in a data.frame, then length(data_ostrya) is equal to the number of columns, not the number of rows. This is because a data.frame is a special type of list and length(some_list) is the number of its elements (obvious). if you want to sample rows, try sampling row numbers:
row_ostrya <- sample(nrow(ostrya),200, replace=F) Hope this helps, Rui Barradas Em 04-10-2012 11:06, Gian Maria Niccolò Benucci escreveu: > 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.dun...@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.dun...@gmail.com> >>> wrote: >>> >>> On 12-09-13 7:18 AM, Gian Maria NiccolÅ^ 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 <annafr...@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 NiccolÅ^ Benucci <gian.benu...@gmail.com> >>>>>> *A:* r-help@r-project.org >>>>>> *Inviato:* GiovedÄ> 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]] >>>>>> >>>>>> ______________________________****________________ >>>>>> R-help@r-project.org mailing list >>>>>> https://stat.ethz.ch/mailman/****listinfo/r-help<https://stat.ethz.ch/mailman/**listinfo/r-help> >>>>>> <https://stat.**ethz.ch/mailman/listinfo/r-**help<https://stat.ethz.ch/mailman/listinfo/r-help> >>>>>> 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. >>>>> >>>>> >>>>> >>> > [[alternative HTML version deleted]] > > > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. [[alternative HTML version deleted]]
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