After the determination of the sample size of each Strata, this function built with 'data.table' package samples units.

GetSampleO-SRS.R

require(data.table)
mysample=alloc_opti_data.table()
get_echantillon_data.table=function(
                         dataset=yourDataset,
                         alea=1435,
                         set.alea=TRUE,
                         strate="strate2013",
                         eff=mysample[,list(strate2013,nh=round(nh))]) 
{
  if (set.alea==TRUE) {
     set.seed(alea)
  }
  #convert to datatable for speed:
  dataset=data.table(dataset)
  setkey(dataset,strate)
  #to dataframe:
  eff=data.frame(eff)
  #echantillon=sample(which(ese_op2013_reste[,strate]==eff[1,2]),eff[1,1],replace=F)
  idx=sample( row.names(dataset[J(eff[1,strate])]),eff[1,"nh"],replace=F)
  for ( j in 2:nrow(eff) ) {
    set.seed(alea+j)
    idx=c(idx,
          sample( row.names(dataset[J(eff[j,strate])]),eff[j,"nh"],replace=F)                 
          )
  }
  write.table(echantillon,"echantillon.csv",sep=";")
  dataset=data.frame(dataset)
  return ( dataset[as.numeric(idx),] )
  
}