Re: accuracy problem in calculation

Discussion in 'Python' started by Chris Angelico, Nov 8, 2012.

  1. On Fri, Nov 9, 2012 at 4:05 AM, Debashish Saha <> wrote:
    > (1500000000+1.00067968)-(1500000000+1.00067961)
    > Out[102]: 2.384185791015625e-07
    >
    > 1.00067968-(1.00067961)
    > Out[103]: 7.000000001866624e-08
    >
    > above i am showing the two different results,though the two outputs
    > should be same if we do it in copy(the lass one is acceptable value).
    > so my question is how to increase the accuracy(windows7(32bit)
    > ,python2.7.2)


    Welcome to floating point. You're working with very large and very
    small numbers, and you _will_ lose accuracy.

    There are a few options. It's possible that a 64-bit build of Python
    will give you more accuracy, but better would be to separate your huge
    numbers from your tiny ones and work with them separately.
    Alternatively, switch to the Decimal or Fraction types, but be aware
    that your script will probably run a lot slower.

    >>> from decimal import Decimal
    >>> (Decimal("1500000000")+Decimal("1.00067968"))-(Decimal("1500000000")+Decimal("1.00067961"))

    Decimal('7E-8')
    >>> Decimal("1.00067968")-Decimal("1.00067961")

    Decimal('7E-8')

    Unless something's tying you to Python 2, consider moving to Python 3.
    You may find that, on Python 3.3, you can switch to Decimal without
    losing too much performance.

    ChrisA
     
    Chris Angelico, Nov 8, 2012
    #1
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  2. On 2012-11-08, Chris Angelico <> wrote:
    > On Fri, Nov 9, 2012 at 4:05 AM, Debashish Saha <> wrote:


    >> (1500000000+1.00067968)-(1500000000+1.00067961)
    >> Out[102]: 2.384185791015625e-07
    >>
    >> 1.00067968-(1.00067961)
    >> Out[103]: 7.000000001866624e-08
    >>
    >> above i am showing the two different results,though the two outputs
    >> should be same if we do it in copy (the lass one is acceptable value).


    Then do it the way you did the last one.

    Seriously, that's the answer they teach you in numerical analysis
    classes.

    >> so my question is how to increase the accuracy(windows7(32bit)
    >> ,python2.7.2)

    >
    > Welcome to floating point. You're working with very large and very
    > small numbers, and you _will_ lose accuracy.
    >
    > There are a few options. It's possible that a 64-bit build of Python
    > will give you more accuracy,


    Pretty doubtful. 64-bit and 32-bit builds on all common OSes and
    hardware are both going to use 64-bit IEEE floating point.

    > but better would be to separate your huge numbers from your tiny ones
    > and work with them separately.


    > Alternatively, switch to the Decimal or Fraction types, but be aware
    > that your script will probably run a lot slower.


    Or admit to yourself that the measurements that produce your input
    data just aren't that accurate anyway and forget about it. :)

    --
    Grant Edwards grant.b.edwards Yow! Bo Derek ruined
    at my life!
    gmail.com
     
    Grant Edwards, Nov 8, 2012
    #2
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