[Tutor] Decimals 'not equal to themselves' (e.g. 0.2 equals 0.200000001)
Thomas Pani
thomas.pani at gmail.com
Sun Aug 3 17:32:27 CEST 2008
CNiall wrote:
> I want to make a simple script that calculates the n-th root of a given
> number (e.g. 4th root of 625--obviously five, but it's just an example
> :P), and because there is no nth-root function in Python I will do this
> with something like x**(1/n).
>
Side note: of course there are python built-in ways to do that. You just
named one yourself:
In [6]: 625**(1.0/4)
Out[6]: 5.0
also:
In [9]: pow(625, 1.0/4)
Out[9]: 5.0
> However, with some, but not all, decimals, they do not seem to 'equal
> themselves'.
>
> As you can see, the last two decimals are very slightly inaccurate.
> However, it appears that when n in 1/n is a power of two, the decimal
> does not get 'thrown off'. How might I make Python recognise 0.2 as 0.2
> and not 0.20000000000000001?
>
You just can't store 0.1 as a binary floating point.
You might want to read:
http://www.network-theory.co.uk/docs/pytut/FloatingPointArithmeticIssuesandLimitations.html
http://www.network-theory.co.uk/docs/pytut/RepresentationError.html
The decimal module provides decimal floating point arithmetic:
http://docs.python.org/lib/module-decimal.html
like in:
In [1]: 0.2 * 2
Out[1]: 0.40000000000000002
In [2]: from decimal import Decimal
In [3]: Decimal('0.2') * 2
Out[3]: Decimal("0.4")
thomas
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