[R] why does sample(x, n) give the same n items in every separate runs?

Charles C. Berry cberry at tajo.ucsd.edu
Fri Oct 31 02:07:09 CET 2008

Run help.request().

In particular you need to attend to this part:

Have you written example code that is
  - minimal
  - reproducible
  - self-contained
  - commented
using data that is either
  - constructed by the code
  - loaded by data()
  - reproduced using dump("mydata", file = "")
have you checked this code in a fresh R session (invoking R with the 
--vanilla option if possible) and is this code copied to the clipboard? (y/n)


Once you get this far and can honestly type 'y', if you have not found 
your error, you are ready to post a query.



On Thu, 30 Oct 2008, Shuhua Zhan wrote:

> Hello R users,
> I have gene expression data of two groups of genes (large and small). Gene expression intensities of those genes are classified into 1 to 10 levels. What I want is to make a random set of genes that have the same levels as the small group from large group using sample().
> I used smallvec to hold the number of genes in each levels (1 to 10) for small group, largevec for large group. I ordered the gene expression data frame of large group (largedf) by the levels and randomly chose the genes with same level as small group. Using the code below I can get the random set of genes from lagre group with same levels for small group. But I got the same set of genes when I run the code in every other runs in Linux. This gives me a doubt in my result.
> smallvec<-c(8,12,9,6,13,20,16,11,8,5) ## the No. of genes in levels 1 to 10
> largevec<-c(400,300,550,600,210,420,380,600,450,500)
> generdm<-c() ## a random set of genes
> for( i in 1:length(smallvec)){
>    generdm<-c(generdm,sample(rownames(largedf)[sum(largevec[0:(i-1)],1):sum(largevec[0:i])],smallvec[i])) ## rownames(largedf) gives gene names ordered by levels in large group
> }
> Could you please help me out?
> Thanks a lot!!
> Josh
> ______________________________________________
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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.

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901

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