Confusing math problem
Dave Angel
davea at davea.name
Thu Feb 21 21:25:23 CET 2013
On 02/21/2013 02:33 PM, 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:
>
> 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))
>
> On the Windows box, I get 9183 for on both lines. However, on the Mac I
> get 9220 and 9183. Why this difference? Is there some sort of precision
> subtlety I'm missing between ** and math.pow()?
>
> Thank you.
I assumed this was some difference between Python 2.x and 3.x.
However, on my 2.7.3 on Linux, I also get 9183 both times.
However, there is an important inaccuracy in math.pow, because it uses
floats to do the work. If you have very large integers, that means some
of them won't be correct. The following are some examples for 2.7.3 on
Linux:
a b math.pow(a,b) a**b
3 34 1.66771816997e+16 16677181699666569
3 35 5.0031545099e+16 50031545098999707
...
5 23 1.19209289551e+16 11920928955078125
The built-in pow, on the other hand, seems to get identical answers for
all these cases. So use pow() instead of math.pow()
One other test:
diff = set(map(int, result1)).symmetric_difference(set(result2))
if diff:
print diff
print len(diff)
shows me a diff set of 15656 members. One such member:
13552527156068805425093160010874271392822265625000000000000000000000000000000000000000000000000000000000000000000L
Notice how using floats truncated lots of the digits in the value?
--
DaveA
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