Is this a bug?

Discussion in 'Ruby' started by Gerry Jenkins, Sep 20, 2010.

  1. for following version:
    ruby 1.9.1p129 (2009-05-12 revision 23412) [i386-darwin9.7.0]


    irb(main):002:0> sprintf("%.1f",-1.05)
    => "-1.1"
    irb(main):003:0> sprintf("%.1f",-32.05)
    => "-32.0"

    note the inconsistent rounding.

    -1.05 rounds to -1.1
    and
    -32.50 rounds to -32.0

    from sprintf
     
    Gerry Jenkins, Sep 20, 2010
    #1
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  2. note the inconsistent rounding.

    For me it looks like you are rounding -32.05, not -32.50.

    Regards, Rimantas
     
    Rimantas Liubertas, Sep 20, 2010
    #2
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  3. Yes, I intend to round -32.05 to the nearest 1 decimal place not to a
    whole number. sprintf is suppose to round to the precision you specify.
     
    Gerry Jenkins, Sep 20, 2010
    #3
  4. OK also inconsistent:

    irb(main):007:0> sprintf("%.0f",-1.5)
    => "-2"
    irb(main):008:0> sprintf("%.0f",-10.5)
    => "-10"

    -1.5 rounds down to -2
    -10.5 rounds up to -10
     
    Gerry Jenkins, Sep 20, 2010
    #4
  5. fails even positive numbers:

    sprintf("%.0f",10.5)
    => "10"

    irb(main):013:0> sprintf("%.0f",11.5)
    => "12"
     
    Gerry Jenkins, Sep 20, 2010
    #5
  6. Gerry Jenkins

    Ben Giddings Guest

    =
    http://en.wikipedia.org/wiki/Floating_point#Representable_numbers.2C_conve=
    rsion_and_rounding

    "Rounding is used when the exact result of a floating-point operation =
    (or a conversion to floating-point format) would need more digits than =
    there are digits in the significand. There are several different =
    rounding schemes (or rounding modes). Historically, truncation was the =
    typical approach. Since the introduction of IEEE 754, the default method =
    (round to nearest, ties to even, sometimes called Banker's Rounding) is =
    more commonly used."

    As for the original issue, how numbers ending in 0.05 are rounded for =
    display with 1 digit after the decimal:

    It does seem "inconsistent", but it isn't wrong. Both 32.1 and 32.0 are =
    equally distant from 32.05. Internally, floating point numbers are =
    represented as binary digits, so 32.05 isn't *exactly* 32.05, but the =
    closest value that can be represented in binary, which will be ever so =
    slightly higher or lower.

    Try looking at the numbers with more decimal points and you'll see what =
    I mean:

    irb(main):043:0> sprintf("%.1f",-29.05)
    =3D> "-29.1"
    irb(main):044:0> sprintf("%.1f",-30.05)
    =3D> "-30.1"
    irb(main):045:0> sprintf("%.1f",-31.05)
    =3D> "-31.1"
    irb(main):046:0> sprintf("%.1f",-32.05)
    =3D> "-32.0"
    irb(main):047:0> sprintf("%.1f",-33.05)
    =3D> "-33.0"
    irb(main):048:0> sprintf("%.1f",-34.05)
    =3D> "-34.0"
    irb(main):049:0> sprintf("%.20f",-29.05)
    =3D> "-29.05000000000000071054"
    irb(main):050:0> sprintf("%.20f",-30.05)
    =3D> "-30.05000000000000071054"
    irb(main):051:0> sprintf("%.20f",-31.05)
    =3D> "-31.05000000000000071054"
    irb(main):052:0> sprintf("%.20f",-32.05)
    =3D> "-32.04999999999999715783"
    irb(main):053:0> sprintf("%.20f",-33.05)
    =3D> "-33.04999999999999715783"
    irb(main):054:0> sprintf("%.20f",-34.05)
    =3D> "-34.04999999999999715783"

    So what it's rounding isn't the exact number you're typing in, but the =
    internal representation of that number in binary floating point format.

    There are ways of tweaking the math so that the rounding seems more =
    consistent, but no matter what, the values you enter won't be precisely =
    equal to that value when you're using floating point numbers.

    If you're not careful about rounding floating point digits, you can end =
    up with something like this:

    http://img.thedailywtf.com/images/200902/errord/DSC00669.JPG

    Ben
     
    Ben Giddings, Sep 20, 2010
    #6
  7. Thank you Ben,

    I did discover that there are various ways of rounding just before your
    reply at http://en.wikipedia.org/wiki/Rounding

    I am writing a ruby web based unit testing for my java students to
    submit their programs to.

    My real problem is that sprintf in ruby behaves differently in this
    regard to how java rounds.

    Oh well, I fixed it by creating unit test data that was within the
    precision of the final java output, so that rounding was not used in the
    unit testing.
     
    Gerry Jenkins, Sep 20, 2010
    #7
  8. Since you are coming from a Java background I guess you are aware that
    JUnit's TestCase uses a delta for equals comparisons of float and double:

    http://www.junit.org/junit/javadoc/3.8.1/junit/framework/Assert.html
    The proper solution IMHO would be a comparison of numeric values (i.e.
    not of strings) that takes the delta into account much the same way as
    it's done in JUnit because there is also named mismatch between the
    binary internal representation and the decimal external representation
    that we humans love to use.

    Cheers

    robert
     
    Robert Klemme, Sep 21, 2010
    #8
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