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Re^4: Variables are automatically rounded off in perl

by syphilis (Archbishop)
on Feb 11, 2019 at 07:38 UTC ( #1229754=note: print w/replies, xml ) Need Help??


in reply to Re^3: Variables are automatically rounded off in perl
in thread Variables are automatically rounded off in perl

Hi,

# In JavaScript, you can store 3335.9999999999995 which has 17 significant digits

Perl will actually store that number correctly, as this indicates:
C:\>perl -le "print 'ok' if 3335.9999999999995 > 3335.999999999999;" ok
The problem is that when perl prints these numbers out it first rounds them to 15 digits - hence perl prints out the same figure for each of the 2 values featured in the above one-liner, even though perl knows that the 2 values are different:
C:\>perl -le "print 3335.9999999999995; print 3335.999999999999;" 3336 3336
# In JavaScript, if you try to add 3335.9999999999995 + 0.0000000000001, it is equal to 3335.9999999999995

Same goes for perl:
C:>perl -le "print 'ok' if 3335.9999999999995 + 0.0000000000001 == 333 +5.9999999999995;" ok
Again, the problem is perl's commitment to outputting an approximation:
C:\>perl -le "print 3335.9999999999995;" 3336
# "print" won't display the last significant digit

Yes - annoying, isn't it. Simplest way to see what a floating point value actually is, is to either:
printf "%a", $float; or printf "%.16e", $float;
If you want to go to the trouble of installing Math::MPFR (which requires gmp and mpfr C libraries), I've just added an nvtoa() function which will return a string representation of an NV using as few digits as are necessary. (The nvtoa function requires mpfr-4.0.0 or later.)
For example:
C:\>perl -MMath::MPFR=":mpfr" -le "print nvtoa(2 ** -1074);" 5e-324 C:\>perl -MMath::MPFR=":mpfr" -le "print nvtoa(sqrt 2.0);" 1.4142135623730951
Works with __float128 and long double builds, too - though Math-MPFR-4.09 (latest CPAN release) is somewhat slower than it ought to be for these nvtypes when abs(exponent) > about 500.
(This has been addressed in the current github version.)

If you're using perl-5.28.x or earlier, then you also need to be aware that perl often assigns slightly incorrect values. (This is fixed in perl-5.29.4 and later, so long as $Config{d_strtod} is defined.)
If you want to be assured that a value is assigned correctly on perl-5.28 and earlier, simplest way might be (untested) to assign that value as a string provided to POSIX::strtod.
That is do:
use POSIX qw(strtod); $x = strtod('1234e-5');
instead of :
$x = 1234e-5;
Or, you can also use Math::MPFR:
use Math::MPFR qw(:mpfr); $x = atonv('1234e-5');
# So, in order to get the same result you would get in JavaScript, you would call FMOD() function (see below) instead of using the % (mod) operator

I think this could be just 2 different languages making different choices regarding behaviour of the modulus operator when applied to fractional values.
Someone else might be able to provide more definitive advice about that.

Cheers
Rob

Replies are listed 'Best First'.
Re^5: Variables are automatically rounded off in perl
by syphilis (Archbishop) on Feb 11, 2019 at 11:23 UTC
    Someone else might be able to provide more definitive advice about that

    As regards the operation $m % $n, the perlop documentation ( see perdoc perlop ) states:

    <quote>
    If the operands $m and $n are floating point values and the absolute value of $n (that is "abs($n)") is less than "(UV_MAX + 1)", only the integer portion of $m and $n will be used in the operation
    (Note: here "UV_MAX" means the maximum of the unsigned integer type).
    </quote>

    So perl's calculation of 3335.99999999995 % 3330 ( == 5) is being done as documented.

    Cheers,
    Not "someone else"

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