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noob: Decimal Comparison in Python

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Join Date: Jan 2007

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noob: Decimal Comparison in Python

Nov 4th, 2007

How do you compare two decimal numbers in python?

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Re: noob: Decimal Comparison in Python

Nov 4th, 2007

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:

 Help with Code Tags 
 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.

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Re: noob: Decimal Comparison in Python

Nov 4th, 2007

I've resorted to creating a function:

 Help with Code Tags 
 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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Re: noob: Decimal Comparison in Python

Nov 6th, 2007

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

 Help with Code Tags 
 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!
>>>