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Nov 4th, 2007

noob: Decimal Comparison in Python

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How do you compare two decimal numbers in python?

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jobs

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Junior Poster in Training

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since Jan 2007

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Nov 4th, 2007

Re: noob: Decimal Comparison in Python

I assume you mean floating point numbers. The way they are represented in most computer languages you can have small roundoff errors in the last bit. You have to compare them in narrow range, example:

python Syntax (Toggle Plain Text)
pi1 = 355/113.0
print pi1  # 3.14159292035
 
pi2 = 3.14159
delta = 0.00003
# compare within narrow range
if (pi2 + delta) > pi1 > (pi2 - delta):
    print 'close enough'
pi1 = 355/113.0
print pi1  # 3.14159292035

pi2 = 3.14159
delta = 0.00003
# compare within narrow range
if (pi2 + delta) > pi1 > (pi2 - delta):
    print 'close enough'

Python also has high accuracy module decimal avoiding round off errors.

bumsfeld

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Nearly a Posting Virtuoso

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Nov 4th, 2007

Re: noob: Decimal Comparison in Python

I've resorted to creating a function:

Python Syntax (Toggle Plain Text)
def fuzzyequals(a,b,delta = 0.00001):
    return -delta < a-b < delta
def fuzzyequals(a,b,delta = 0.00001):
    return -delta < a-b < delta

Jeff

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Practically a Master Poster

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Nov 6th, 2007

Re: noob: Decimal Comparison in Python

If you mean decimal, not float, I don't see the need for anything special. E.g.,

Python Syntax (Toggle Plain Text)
>>> import decimal
>>> x1 = decimal.Decimal("4.567567567")
>>> x2 = decimal.Decimal("4.56756757")
>>> if x1==x2:
...     print "OK!"
...
>>> if x1<x2:
...     print "OK!"
...
OK!
>>>
>>> import decimal
>>> x1 = decimal.Decimal("4.567567567")
>>> x2 = decimal.Decimal("4.56756757")
>>> if x1==x2:
...     print "OK!"
...
>>> if x1<x2:
...     print "OK!"
...
OK!
>>>

BearofNH

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Posting Whiz

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