[Rd] identical(0, -0)

[Martin Maechler]

>>>>> "DM" == Duncan Murdoch <murdoch at stats.uwo.ca>
>>>>>     on Mon, 10 Aug 2009 11:51:53 -0400 writes:

    DM> For people who want to play with these, here are some functions that let 
    DM> you get or set the "payload" value in a NaN.  NaN and NA, Inf and -Inf 
    DM> are stored quite similarly; these functions don't distinguish which of 
    DM> those you're working with.  Regular finite values give NA for the 
    DM> payload value, and elements of x are unchanged if you try to set their 
    DM> payload to NA.

    DM> By the way, this also shows that R *can* distinguish different NaN 
    DM> values, but you need some byte-level manipulations.

yes;  very nice code, indeed!

I propose a version of the  showBytes()  utility should be added 
either as an example e.g. in  writeBin() or even an exported
function in package 'utils'

 [.........]

    > Example:

    >> x <- c(NA, NaN, 0, 1, Inf)
    >> NaNpayload(x)
    > [1]  0.5 -0.5   NA   NA  0.0

Interestingly, on 64-bit, I get a slightly different answer above, 
(when all the following code gives exactly the same results,
 and of course, that was your main point !), namely
4.338752e-13 instead of 0.5  for 'NA', 
see below.

.. and your nice tools also let me detect an even simpler way
to get *two* versions of NA, and NaN, each :  
Conclusion:  Both  NaN  and  NA (well NA_real_) have a sign, too !

NaNpayload(NA_real_)
##[1] 4.338752e-13
NaNpayload(-NA_real_)
##[1] -4.338752e-13   ## !! different

str(NApm <- c(1[2], -1[2]))
t(sapply(NApm, showBytes))
## [1,]   a2   07   00   00   00   00   f0*   7f
## [2,]   a2   07   00   00   00   00   f0*   ff

## or summarizing things :

## Or, "in summary" -- Duncan's original example slightly extended:
x <- c(NaN, -NaN, NA, -NA_real_, 0, 0.1, Inf, -Inf)
x
names(x) <- format(x)
sapply(x, showBytes)
##       NaN  NaN   NA   NA  0.0  0.1  Inf -Inf
## [1,]   00   00   a2   a2   00   9a   00   00
## [2,]   00   00   07   07   00   99   00   00
## [3,]   00   00   00   00   00   99   00   00
## [4,]   00   00   00   00   00   99   00   00
## [5,]   00   00   00   00   00   99   00   00
## [6,]   00   00   00   00   00   99   00   00
## [7,]   f8   f8   f8*  f8*  00   b9   f0   f0
## [8,]   ff   7f   7f   ff   00   3f   7f   ff

## (*) NOTE: the  'f0*' or 'f8*' above  are
## ---       'f8' on 32-bit,  'f0' on 64-bit



    >> NaNpayload(x) <- -0.4
    >> x
    > [1] NaN NaN NaN NaN NaN
    >> y <- x
    >> NaNpayload(y) <- 0.6
    >> y
    > [1] NaN NaN NaN NaN NaN
    >> NaNpayload(x)
    > [1] -0.4 -0.4 -0.4 -0.4 -0.4
    >> NaNpayload(y)
    > [1] 0.6 0.6 0.6 0.6 0.6
    >> identical(x, y)
    > [1] TRUE

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