Significant figures calculation

[Steven D'Aprano]

On Fri, 24 Jun 2011 13:05:41 -0700, Harold wrote:

> Hi,
> 
> I am looking for an easy way to do significant figure calculations in
> python (which I want to use with a class that does unit calculations).
> Significant figure calculations should have the semantics explained,
> e.g., here:
> http://chemistry.about.com/od/mathsciencefundamentals/a/sigfigures.htm
> 
> My hope was that the decimal module would provide this functionality,
> but:
> 
>>>> print Decimal('32.01') + Decimal(5.325) + Decimal('12')
> 49.335     # instead of 49
>>>> print Decimal('25.624') / Decimal('25')
> 1.02496   # instead of 1.0
>>>> print Decimal('1.2') == Decimal('1.23')
> False      # actually not sure how the semantics should be
> 
> I tried to modify the DecimalContext (e.g. getcontext().prec = 2) but
> that did not lead to the correct behavior.

Really? It works for me.


>>> import decimal
>>> D = decimal.Decimal
>>> decimal.getcontext().prec = 2
>>>
>>> D('32.01') + D('5.325') + D('12')
Decimal('49')
>>>
>>> D('25.624') / D('25')
Decimal('1.0')

The only thing you have to watch out for is this:

>>> D('1.2') == D('1.23')  # no rounding
False
>>> D('1.2') == +D('1.23')  # force rounding
True



-- 
Steven