[Tutor] puzzled by Python 3's print()

[Eike Welk]

Hello Richard!

On Thursday July 1 2010 15:11:21 Richard D. Moores wrote:
> Thanks to yours and others responses, I've learned some things I
> didn't know, but remember, I'm starting with long ints such as

Also note that in Python 3 the "/" (division) operator returns a floating 
point number when you divide integers. This is one of the changes that Python 
3 introduces.

As you are using long integers (and you were previously writing about prime 
numbers) the precision of floating point numbers might not be enough for your 
purposes.

Therefore you should probably use the integer division operator: "//"


The following (edited) snippet from IPython demonstrates "//" and the loss of 
precision when using "/":


...
Python 2.6.2 (r262:71600, Mar 29 2010, 15:30:01)
Type "copyright", "credits" or "license" for more information.

IPython 0.10 -- An enhanced Interactive Python.
...

In [1]: from __future__ import  division

In [2]: a = 1000000000000000000000000000000000000000000000000000000002

In [3]: a
Out[3]: 1000000000000000000000000000000000000000000000000000000002L

In [4]: a//2
Out[4]: 500000000000000000000000000000000000000000000000000000001L

In [5]: a/2
Out[5]: 4.9999999999999994e+56

In [6]: long(a/2)
Out[6]: 499999999999999937061060126016582882140297920412594995200L


Eike.