[Rd] Strange Behavior in RNG

[Rui Barradas]

Às 01:45 de 17/08/2024, Jiefei Wang escreveu:
> Hi,
> 
> I just observed a strange behavior in R. The rnorm function does not
> give me the numbers with a given length. I think it is somehow related
> to the internal representation of double-type numbers but I am not
> sure if this is supposed to happen. Below is a reproducible example
> 
> ```
> ## Create a list, we will only take the forth value, which is 0.6
> nList <- seq(0,1,0.2)
> n <- nList[4]
> n
> # [1] 0.6
> length(rnorm(1000*n))
> # [1] 600
> length(rnorm(1000-1000*n))
> # [1] 399 <--- What happened here?
> length(rnorm(1000-1000*0.6))
> # [1] 400
> 1000-1000*n
> # [1] 400 <- this looks good to me...
> 1000-1000*0.6
> # [1] 400
> identical(n, 0.6)
> # [1] FALSE
> .Internal(inspect(n))
> # @0x00000217c75d79d0 14 REALSXP g0c1 [REF(1)] (len=1, tl=0) 0.6
> .Internal(inspect(0.6))
> # @0x00000217c791e0c8 14 REALSXP g0c1 [REF(2)] (len=1, tl=0) 0.6
> ```
> 
> As you can see, length(rnorm(1000-1000*n)) does not really give me the
> result I want. This is somewhat surprising because it is hard to
> imagine that a manually-typed 0.6 can behave differently than 0.6 from
> a sequence. Furthermore, 0.6 is the only problematic number from
> `nList`. The rest numbers work fine. I can guess it is due to the
> rounding mechanism, but I think this should be treated as a bug: if
> the print function can show the result of 1000-1000*n correctly, it
> will be strange that rnorm behaves differently. Below is my session
> info
> 
> R version 4.3.0 (2023-04-21 ucrt)
> Platform: x86_64-w64-mingw32/x64 (64-bit)
> Running under: Windows 10 x64 (build 19045)
> 
> Matrix products: default
> 
> locale:
> [1] LC_COLLATE=English_United States.utf8
> [2] LC_CTYPE=English_United States.utf8
> [3] LC_MONETARY=English_United States.utf8
> [4] LC_NUMERIC=C
> [5] LC_TIME=English_United States.utf8
> 
> time zone: America/Chicago
> tzcode source: internal
> 
> ______________________________________________
> R-devel using r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
Hello,

This is R FAQ 7.31.
In fact, the sequences

seq(0, 1, 0.1)
seq(0, 1, 0.2)

should probably be a FAQ 7.31 example.
If you print the numbers with more decimals you will see why the error.



# generate the list
nList <- seq(0,1,0.2)
# compare the list with manually typed numbers
nList != c(0, 0.2, 0.4, 0.6, 0.8, 1)
#> [1] FALSE FALSE FALSE  TRUE FALSE FALSE

# note the value of 0.6
print(nList, digits = 16L)
#> [1] 0.0000000000000000 0.2000000000000000 0.4000000000000000 
0.6000000000000001
#> [5] 0.8000000000000000 1.0000000000000000


Hope this helps,

Rui Barradas



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