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Python bug??? Or just a stupid question?

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Join Date: Oct 2007

Posts: 2

Reputation: tesak is an unknown quantity at this point

Solved Threads: 0

tesak

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Newbie Poster

Python bug??? Or just a stupid question?

Oct 5th, 2007

Hello,
I am a newbie in Python and maybe this is a stupid question, but when I run this code I receive a very strange output.

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
status = 0
for i in range(10):
    status = status + 0.1        
 
    if status == 0.1:
        print status
    elif status == 0.2:
        print status
    elif status == 0.3:
        status
    elif status == 0.4:
        print status
    elif status == 0.5:
        print status
    elif status == 0.6:
        print status
    elif status == 0.7:
        print status
    elif status == 0.8:
        print status
    elif status == 0.9:
        print status
    elif status == 1.0:
        print status
status = 0
for i in range(10):
    status = status + 0.1        
        
    if status == 0.1:
        print status
    elif status == 0.2:
        print status
    elif status == 0.3:
        status
    elif status == 0.4:
        print status
    elif status == 0.5:
        print status
    elif status == 0.6:
        print status
    elif status == 0.7:
        print status
    elif status == 0.8:
        print status
    elif status == 0.9:
        print status
    elif status == 1.0:
        print status

At first I supposed that I will receive something like this:
>>>
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
>>>

But finally I was really surprised. Output looks like this:
>>>
0.1
0.2
0.4
0.5
0.6
0.7
>>>

Could somebody tells me what I am doing wrong?

•

Join Date: Oct 2004

Posts: 4,075

Reputation: vegaseat is just really nice

Solved Threads: 939

vegaseat

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DaniWeb's Hypocrite

Re: Python bug??? Or just a stupid question?

Oct 5th, 2007

This happens with many computer languages. You are trying to compare floating point values directly. Your 0.8 might be more like 0.799999999999999 and the compare flunks!

You can make this work by giving it a range you can compare within ...

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
if 0.79999 < status < 0.80001:
    ... do something`
if 0.79999 < status < 0.80001:
    ... do something`

Last edited by vegaseat; Oct 5th, 2007 at 10:22 pm.

May 'the Google' be with you!

•

Join Date: Dec 2006

Posts: 1,045

Reputation: woooee is a jewel in the rough

Solved Threads: 294

woooee

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Veteran Poster

Re: Python bug??? Or just a stupid question?

Oct 5th, 2007

Look at the line following
elif status == 0.3:
And why do you have if/elif? Why not just print the value?

•

Join Date: Oct 2007

Posts: 2

Reputation: tesak is an unknown quantity at this point

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tesak

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Newbie Poster

Re: Python bug??? Or just a stupid question?

Oct 6th, 2007

Thanks for an answer. It was just an example which really surprise me. But is there a way how to compare two float numbers without unpredictable results?

•

Join Date: Jul 2006

Posts: 608

Reputation: jrcagle is on a distinguished road

Solved Threads: 150

jrcagle

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Practically a Master Poster

Re: Python bug??? Or just a stupid question?

Oct 6th, 2007

Well, there are actually three issues in the code:

(1) is a matter of efficiency, which woooee already raised.

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
status = 0
for i in range(10):
    status = status + 0.1        
    print status
status = 0
for i in range(10):
    status = status + 0.1        
    print status

does what you wanted to do, and is cleaner to read. One payoff of this approach is that...

(2) ...in your original code, you didn't get 0.3 in the output because you left out the print statement. The lines

Help with Code Tags

Python Syntax (Toggle Plain Text)

elif status == 0.3:
    status

have the effect of evaluating, but not printing, status. And then throwing away the result, because you aren't doing anything with it.

(3) Finally, comparing floating points is a common source of bugs. Even if we worked with BCD, so that we didn't have to work about base-2 to base-10 errors, still and all, our decimal approximations would still lead to imprecision. For example, the test π == 3.14159265358979 is False.

Vega gives a very good approach to this by selecting a range. My own approach is to define a function that is in my code base:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
def fuzzyequals(a,b,epsilon = 0.0001):
   return abs(a-b) < epsilon
def fuzzyequals(a,b,epsilon = 0.0001):
   return abs(a-b) < epsilon

This function returns True if a is within the "halo" of b; that is, if b-epsilon < a < b+epsilon.

You can then use it like this:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
...
if fuzzyequals(status,0.2):      # or fuzzyequals(status, 0.2, 0.01)
   print status
...
...
if fuzzyequals(status,0.2):      # or fuzzyequals(status, 0.2, 0.01)
   print status
...