[R] Sample function and prob argument

[le Gleut, Ronan]

Dear R-help mailing list,

 

First of all, many many thanks for your great work on the R project!

 

I have a very small issue regarding the sample function. Depending if we
specify values for the prob argument, we don't get the same result for a
random sampling with replacement and with equal probabilities. See the
attached R code for a minimal example with the R version 3.6.0.

 

With a previous R version (3.5.x), the result was just a permutation
between the possible realizations. They are now totally different with the
latest R version.

 

I understand that if we specify or not the prob argument, two different
internal functions are used: .Internal(sample()) or .Internal(sample2()).
Indeed, the algorithm used to draw a sample may not be the same if by
default we assume equal probabilities (without the prob argument) or if
the user defines himself the probabilities (even if they are equal).

 

I found this post on stackoverflow which explains the reasons of this
difference (answer by Matthew Lundberg):

https://stackoverflow.com/questions/23316729/r-sample-probabilities-defaul
t-is-equal-weight-why-does-specifying-equal-weigh

 

I was wondering whether the solution proposed by PatrickT could solve this
issue? He proposed to have something like if(all.equal(prob, prob,
tolerance = .Machine$double.eps) prob = NULL inside the sample.int routine
in order to replicate prob=NULL with prob=rep(1, length(x)).

 

Thanks you in advance for your response.

 

Best regards,

Ronan Le Gleut

 


 
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