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Re: Bad math (tweak)

by tye (Cardinal)
on Oct 31, 2011 at 17:22 UTC ( #934926=note: print w/ replies, xml ) Need Help??


in reply to Bad math

You might want to request that your build of Perl get a different number of digits declared as "worth worrying about". Your addition only removes a single digit from the magnitude of the numbers involved. That should not, IMHO, be enough to trigger the showing of such noise.

It would probably be better for your version of Perl to be built to consider 13 digits (or maybe 14) as worth reporting instead of 15.

It looks like modern C compilers define DBL_DIG and Perl defaults to using that when converting a double into a string. Standard "double" has 52 bits of mantissa which gives 15.65 digits of accuracy (not 15.95 like wikipedia seems to claim, including the sign bit in the calculation for unknown reasons). My C compiler defines 15 for DBL_DIG, which makes sense. But Perl should not be caring about so many digits. We need only about 1/2 of a digit of error for "9999999" or "0000001" noise to start showing up.

I would want at least 1.5 digits of slush before such noise results so I would hope that Perl would default to using DBL_DIG-1 (DBL_DIG-2 seems even better) not DBL_DIG (for NV_DIG when NV is a "double").

- tye        


Comment on Re: Bad math (tweak)
Re^2: Bad math (tweak)
by ikegami (Pope) on Dec 01, 2011 at 23:39 UTC
    It's not the sign bit that's included, it's the implied leading bit (0 when e==0, 1 when e!=0).

      Oh, right. Thanks! So that makes it 0.95 digits of "room". (Still clearly not enough, IMHO.)

      - tye        

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