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@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
--
Este e-mail foi analisado pelo software antivírus AVG para verificar a presença
de vírus.
www.avg.com
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
