[Tutor] Find out if a number is even or not

[Bill Mill]

Just for fun, I wrote each of the suggestions into a file and timed
how long it took them to test each number from 0 to 100 for evenness.
If you don't want to read the code and results, I'll just tell you
that the bitwise method is the fastest, and the 'Odd' or 'Even' method
is the slowest. This makes sense as the bitwise operator has a simple
hardware operation, while the latter method has to create a string and
possibly do an extra boolean operation.

For 100,000 runs from 0 to 100, the difference is about 1.6 seconds.

If you're interested, the code and results follow:

import timeit

def isEven1(n): return not n&1
def isEven2(n): return n % 2 == 0
def isEven3(n): return n % 2 and 'Odd' or 'Even'

def testfunc(predicate):
    for i in range(100): predicate(i)

def testtimes():
    for function in ('isEven1', 'isEven2', 'isEven3'):
        t = timeit.Timer('testfunc(%s)' % (function), \
            'from __main__ import %s, testfunc' % function)
        print "%s: %f" % (function, t.timeit(100000))

if __name__ == "__main__": testtimes()

##############
# Results in:

isEven1: 8.174000
isEven2: 8.839000
isEven3: 9.899000

Peace
Bill Mill
bill.mill at gmail.com

On Sat, 16 Oct 2004 13:45:11 -0400, Lloyd Hugh Allen
<chandrakirti at gmail.com> wrote:
> I'm going to say more about mine.
> 
> We know that computers like binary better, because it is easier to
> tell the difference between "on" and "off" than for a circuit to try
> to measure an analog level of voltage on a circuit.
> 
> So to the computer, 6 looks like 110, which is (going from right to
> left) 0*(2**0) + 1*(2**1) + 1*(2**2) == 0 + 2 + 4. Counting in binary
> would look like
> 
> 1  1
> 2  10
> 3  11
> 4  100
> 5  101
> 6  110
> 7  111
> 8  1000
> 
> we notice that the odd numbers have a "one's" digit, in binary, of one.
> 
> The bitwise and operator, &, compares the binary digits of its two
> arguments. For instance, 4&5 notes that the only digit that is on in
> both binary expansions is the one in the fours place. Therefore, 4&5
> == 4. Similarly, 1 & odd == 1 and 1 & even == 0.
> 
> Python 2.3 has booleans. I like them, and the original question was
> phrased as, "I want to know when a number is even."
> 
> So num & 1 will give a nonzero result for odds and a result of zero
> for evens; since not zero is True and not (nonzero) is False, taking
> not (num & 1) gives the "even-ness" of an integer.
> 
> As a happy sidenote, bitwise and gets angry when a float is thrown at
> it. I don't want to guess as to whether a float is even or odd; my
> function throws an exception.
> 
> On Fri, 15 Oct 2004 23:09:45 -0400, Lloyd Hugh Allen
> 
> 
> <chandrakirti at gmail.com> wrote:
> > There's also the bitwise & operation
> >
> > def isEvenQ(num):
> >     return not (num & 1)
> >
> > don't know if it's actually faster though.
> >
> >
> >
> > On Fri, 15 Oct 2004 12:00:18 +0100, nick at javacat.f2s.com
> > <nick at javacat.f2s.com> wrote:
> > > Hi group,
> > >
> > > Im sure this is simple to do, but Ive just been looking through the python docs
> > > and the archive of this mailing list and cant find what Im after.
> > >
> > > I just want to know if a number is even or not.
> > >
> > > I've had a look at the math package but that doesnt seem to have anything this
> > > simple in it.
> > >
> > > I thought something like
> > >
> > > >>> if 10 div 2 == 0 : print 'even'
> > > >>> else : print 'odd'
> > >
> > > but obviously that doesnt work.
> > >
> > > Sorry for such a basic question, but I really cant find out how to do this.
> > >
> > > Kind regards
> > > Nick.
> > >
> > > -------------------------------------------------
> > > Everyone should have http://www.freedom2surf.net/
> > > _______________________________________________
> > > Tutor maillist  -  Tutor at python.org
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> > >
> >
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