Hi,
as far as I remember, all data gets converted into double during pdl
construction before cast to the desired type. Apparently, unpdl does the
same. It's a known issue for a long time, AFAIK. The same even happens
when you assign to a longlong pdl. If I remember, nobody dared touch the
core hard enough to fix that one, yet.
Ingo
On 08/21/2015 12:55 PM, kmx wrote:
> Hi,
>
> while playing with LongLong piddles representing timestamps I have run
> into the following troubles with precision loss when passing 64bit
> signed int values between perl and PDL (both directions).
>
> Let's have 64bit number 253402300799999999 (BTW it is a number of
> microseconds since UNIX epoch beginning corresponding to
> 9999-12-31T23:59:59.999999Z) - it occupies "only" 58bits, so we are
> not even approaching 63bit edge which might become tricky when dealing
> with 64bit signed integers.
>
> Demonstration code:
>
> use PDL;
> use Config;
> my $perl_var = 253402300799999999;
> my $pdl_var = longlong(253402300799999999);
> print "PDL::VERSION: ", $PDL::VERSION, "\n";
> print "ivsize : ", $Config{ivsize}, "\n";
> print "use64bitint : ", $Config{use64bitint}, "\n";
> print "archname : ", $Config{archname}, "\n";
> print "perl 1: ", $perl_var, "\n";
> print "perl 2: ", ($perl_var % 1000), "\n";
> print "pdl : ", $pdl_var->info, "\n";
> print "pdl 1: ", $pdl_var, "\n";
> print "pdl 2: ", ($pdl_var % 1000), "\n";
> print "pdl 3: ", $pdl_var->unpdl->[0], "\n";
>
> Output:
>
> PDL::VERSION: 2.011
> ivsize : 8
> use64bitint : define
> archname : MSWin32-x86-multi-thread-64int
> perl 1: 253402300799999999
> perl 2: 999
> pdl : PDL: LongLong D []
> pdl 1: 253402300800000000
> pdl 2: 0
> pdl 3: 2.534023008e+017
>
>
> From my point of view there are 2 issues:
>
> 1/ A completely legal / not even extremely large / 64bit integer value
> is OK on perl side, but when turned into a piddle the value changes
> (253402300799999999 > 253402300800000000). This may be a bug.
>
> 2/ When exporting a LongLong piddle via unpdl the value (64bit signed
> int) is converted into double (perl's NV) even when the perl is
> compiled with 64bit integers. Maybe unpdl() is not the best way for
> extracting piddle value into a perl array (but I really like this
> function :).
>
> Thanks in advance for any hints.
>
> --
> kmx
>
>
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>
>
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