Re^2: bignum usage?

[syphilis]

off-by-one error

It seems you've hit some precision limit - which is avoided if you use bigint.
Try placing an extra number at the beginning of $x, and you'll soon see what I mean.
For every extra number you prepend to the start of $x, you get an extra "0" at the end of $y:

use strict;
use warnings;
use feature 'say';

use bignum;#num;
#  (1) #use bignum downgrade => "Math::BigInt", upgrade => "Math::BigI
+nt";
## (2) #use bigint;

say $^V;
my $x = 999999999921778071482940061661655974875633165533182;
say $x;
my $y = $x / 2;
say $y;

__END__

For me, outputs:
D:\pscrpt>perl try.pl
v5.39.5
999999999921778071482940061661655974875633165533182
499999999960889035741470030830827987437800000000000

Cheers,
Rob

[Anonymous Monk]

Thanks, looks like it's fallback (so called "div_scale", defaults to 40) value that was applied (and now I have a deja-vu to have already investigated this a few years back). Either of

use bignum p => 0;
bignum-> precision( 0 );
bignum-> div_scale( 999 );

(or method calls on an instance) will help. That still doesn't feel right:

use strict;
use warnings;
use feature 'say';
use Data::Dump;

use bignum; #use bigint;

say my $x = 999999999921778071482940061661655974875633165533182;
dd $x;

dd $x-> precision;
dd $x-> accuracy;
dd $x-> div_scale;
dd (Math::BigInt-> precision);
dd (Math::BigInt-> accuracy);

say my $y = $x / 1;

__END__

999999999921778071482940061661655974875633165533182
bless({
  sign  => "+",
  value => bless([165533182, 974875633, 61661655, 71482940, 999921778,
+ 999999], "Math::BigInt::Calc"),
}, "Math::BigInt")
undef
undef
40
undef
undef
999999999921778071482940061661655974875600000000000

But with bigint instead:

999999999921778071482940061661655974875633165533182
bless({
  sign  => "+",
  value => bless([165533182, 974875633, 61661655, 71482940, 999921778,
+ 999999], "Math::BigInt::Calc"),
}, "Math::BigInt")
undef
undef
40
undef
undef
999999999921778071482940061661655974875633165533182

i.e. object of the same class, with the same properties, same global parameters, but result is different. Same (per deja-vu) conclusion: avoid using bignum.

[syphilis]

... avoid using bignum.

That's the rule that I live by.

To be fair, however, bignum is quite possibly easier to use than I think.
One just has to be aware of all the caveats ;-)

UPDATE:
i.e. object of the same class, with the same properties, same global parameters, but result is different.

I think the difference occurs because the "bignum" division is done by Math::BigFloat::bdiv, whereas the "bigint" division is done by Math::BigInt::bdiv.

Cheers,
Rob