Re: [perl #53784] 64-bit Integers -- inexact division gives odd resultwhen is large

On 2008–05–06, at 19:58, Chris Hall (via RT) wrote:

Good news -- from the point of view of repeatability: I get the same  
results as you when running your test program on a 64-bit blead perl  
(patchlevel 33783).

> The first section shows the effect.  Where the integer division is  
> exact we
> get the right integer result.  Where it is not exact, the result is  
> floated,
> with hopeless loss of significance !


Bad news from the point of view of understanding the phenomenon: I get  
the same results as you when I put -Minteger on the 64-bit perl  
command line; that is, when arithmetic operations are carried out with  
machine integers and fractional parts are truncated. Same if I put - 
Mbigint, replacing all machine arithmetic with arbitrary precision  
integer operations.

These modules appear to be behaving themselves:

domo:64-bit_perl-current$ ./perl -lwe '$x=2**60 + 3; print $x/2 - 2**59'
0
domo:64-bit_perl-current$ ./perl -Ilib -Minteger -lwe '$x=2**60 + 3;  
print $x/2 - 2**59'
1
domo:64-bit_perl-current$ ./perl -Ilib -Mbigint -lwe '$x=2**60 + 3;  
print $x/2 - 2**59'
1

Can you say why I should be getting the same results from your test  
program in all three cases?
-- 
Dominic Dunlop

• [perl #53784] 64-bit Integers -- inexact division gives odd result when is large by Chris Hall
• Re: [perl #53784] 64-bit Integers -- inexact division gives odd resultwhen is large by Dominic Dunlop
• Re: [perl #53784] 64-bit Integers -- inexact division gives odd result when is large by Chris Hall