From w||||@mwdun|@p @end|ng |rom gm@||@com  Sun Jul 10 22:23:22 2022
From: w||||@mwdun|@p @end|ng |rom gm@||@com (Bill Dunlap)
Date: Sun, 10 Jul 2022 13:23:22 -0700
Subject: [Rd] Floating point issue
In-Reply-To: <CAEKh8uhNTF3RczavV-jh82tK9e=iDr2tzys=wGSk6ZD5e2tndg@mail.gmail.com>
References: <CAEKh8uhNTF3RczavV-jh82tK9e=iDr2tzys=wGSk6ZD5e2tndg@mail.gmail.com>
Message-ID: <CAHqSRuTN7iU2ngvr2JoEiqZCrogY8GL3ssKY2vvUZstWCydzKw@mail.gmail.com>

The following function, 'bitC' from ?numToBits, displays the bits in a
double precision number, separated into the sign bit, the 11 exponent bits,
and the 52 bits in the mantissa.  I've shown the results with your numbers
from R-2.4.0 on my Windows 11 Lenovo laptop: what do you get?

> bitC <- function(x) noquote(vapply(as.double(x), function(x) { # split
one double
+     b <- substr(as.character(rev(numToBits(x))), 2L, 2L)
+     paste0(c(b[1L], " ", b[2:12], " | ", b[13:64]), collapse = "")
+   }, ""))
> bitC(10^25)
# [1] 0 10001010010 | 0000100010110010101000101100001010000000001010010001
> bitC(10000000000000000905969664)
# [1] 0 10001010010 | 0000100010110010101000101100001010000000001010010001
> bitC(10000000000000000905969664 - 10^25)
# [1] 0 00000000000 | 0000000000000000000000000000000000000000000000000000
> bitC(1e25)
# [1] 0 10001010010 | 0000100010110010101000101100001010000000001010010001

-Bill

On Sun, Jul 10, 2022 at 7:00 AM Antoine Fabri <antoine.fabri at gmail.com>
wrote:

> Dear r-devel,
>
> For some numbers, the printed value is not equivalent to the input :
>
> options(scipen = 999)
> ## GOOD
> 1e24
> #> [1]  999999999999999983222784
> 1e24 == 999999999999999983222784
> #> [1] TRUE
>
> ## BAD
> 1e25
> #> [1] 10000000000000000905969664
> 1e25 == 10000000000000000905969664
> #> [1] FALSE
>
> ## STILL BAD
> 10000000000000000905969664
> #> [1] 10000000000000003053453312
>
> ## GOOD AGAIN
> 10000000000000003053453312
> #> [1] 10000000000000003053453312
>
> # Additionally
> 10000000000000000000000000 == 1e25
> #> [1] FALSE
>
> Are these bugs ?
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

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[Rd] Floating point issue
Bill Dunlap