Confusing math problem

[Dave Angel]

On 02/21/2013 03:25 PM, Dave Angel wrote:
>
>>    <snip>
>>
>   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?

Sorry, I just rechecked, and that value is correct for 50**66 power.

However, if I do:

print 3**60, "\n", int(math.pow(3,60)), "\n", pow(3,60)


I get:

42391158275216203514294433201
42391158275216203520420085760
42391158275216203514294433201


and the middle one is the one that's wrong.  You can tell by casting out 
9's.  The middle one gets 1 instead of zero, showing that it's NOT 
divisible by 3.

-- 
DaveA