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Re: Re: Using (s)printf()

by Hofmator (Curate)
on Aug 31, 2001 at 16:20 UTC ( #109363=note: print w/replies, xml ) Need Help??

in reply to Re: Using (s)printf()
in thread Using (s)printf()

  • The first one (9.3333 -> 9.34) is a typo in the original.
  • The second and third one are easily explained by the following
    printf("%.15f", 9.55); # prints 9.550000000000001 printf("%.15f", 43.55); # prints 43.549999999999997
    This means that due to the limitations of the internal representation, the literal 9.55 is in fact 9.550000000000001 which rounds to 9.56. The same goes for 43.55 but there the representation is slightly smaller than 43.55 so a rounding down occurs.

-- Hofmator

Replies are listed 'Best First'.
Re: Re: Re: Using (s)printf()
by Anonymous Monk on Sep 01, 2001 at 07:29 UTC
    Actually, this isn't quite the whole story. Most everyone follows the IEEE convention of "round towards nearest or even."
    Examples (rounding all of these to the one's place):

    2.51 becomes 3 (this is the 'nearest' rule, which always comes first)
    2.49 becomes 2 (again, 'nearest')

    However, what happens if you have 2.50 ? Which way do you round it... 'tis no nearer to 2 than to 3. The IEEE standard says if there is a tie, round to the even number.

    2.50 becomes 2
    3.50 becomes 4

    You have to pick up or down... this method is consistent and thus tends to make your errors (statistically) smaller.

      Actually, this isn't quite the whole story. Most everyone follows the IEEE convention of "round towards nearest or even."
      I'm aware of that - but it wasn't applicable in this case. Btw, (s)printf doesn't seem to get it right:
      printf "%.15f", 0.5; # prints 0.500000000000000 printf "%.0f", 0.5; # prints 1 and should print 0
      and 0.5 can be represented exactly ...

      -- Hofmator

Re: Re: Re: Using (s)printf()
by Zteven (Initiate) on Sep 02, 2001 at 15:08 UTC
    Thank you for your explanation. How do I circumvent this problem? I have a little perl-program that pulls these numbers from a file and it has to round them correctly. Isn't there some way to store a number exactly?

      What if you want to chop a number but not round it? printf just *wants* to round, no option to tell it not to :(

      I needed something that would chop the extra digits (price calculataions) without rounding.

      quick fox was to compare the sprintf result with the original and if it was larger remove 0.01 - yuk!...

      I ended up using split and substr... - not pretty either

      There must be a better way!

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