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Confusing math problem

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  Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 19:33 +0000
    Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 15:25 -0500
      Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 22:39 +0000
        Re: Confusing math problem Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-21 22:53 +0000
          Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 23:41 +0000
            Re: Confusing math problem Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-22 00:04 +0000
            Re: Confusing math problem Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-21 17:19 -0700
      Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 23:39 +0000
    Re: Confusing math problem Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-21 13:42 -0700
    Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 07:46 +1100
      Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 22:44 +0000
        Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 11:29 +1100
        Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 21:19 -0500
    Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 15:49 -0500
    Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 08:23 +1100
      Re: Confusing math problem Peter Pearson <ppearson@nowhere.invalid> - 2013-02-21 21:59 +0000
        Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 09:11 +1100
        Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 17:33 -0500
        Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 10:15 +1100
      Re: Confusing math problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-22 09:16 +0000
        Re: Confusing math problem Serhiy Storchaka <storchaka@gmail.com> - 2013-02-22 13:48 +0200
    Re: Confusing math problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-22 09:27 +0000

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#39552

On 22.02.13 11:16, Steven D'Aprano wrote:
> On Fri, 22 Feb 2013 08:23:27 +1100, Chris Angelico wrote:
>> and you can cast out 1's in binary to find out if it's a
>> multiple of 1, too.
>
> O_o
>
> I wanna see the numbers that aren't a multiple of 1.

What "to be a multiple of" means? If A is a multiple of B means A % B == 
0, then 0.5 isn't a multiple of 1.

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#39529

On Thu, 21 Feb 2013 19:33:32 +0000, Schizoid Man wrote:

> Hi there,
> 
> I run the following code in Python 3.3.0 (on a Windows 7 machine) and
> Python 2.7.3 on a Mac and I get two different results:

Others have already explained that math.pow and the ** exponentiation 
operator are subtly different. However I wish to discuss your code:


> result1 = []
> result2 = []
> for a in range(2,101):
>     for b in range(2,101):
>         result1.append(math.pow(a,b))
>         result2.append(a**b)
> result1 = list(set(result1))
> result2 = list(set(result2))
> print (len(result1))
> print (len(result2))

This is more simply written as:

result1 = set()
result2 = set()
for a in range(2, 101):
    for b in range(2, 101):
        result1.add(a**b)
        result2.add(math.pow(a, b))

print(len(result1))
print(len(result2))

No need for the pointless conversion from list to set to list again, if 
all you want is the number of unique values.

More interesting is to gather the pairs of values that differ:

results = []
for a in range(2, 101):
    for b in range(2, 101):
        if a**b != math.pow(a, b): results.append((a, b))

You will see that integer exponentiation and floating point 
exponentiation are more frequently different than not.

(8105 of the calculations differ, 1696 of them are the same.)



-- 
Steven

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