[Tutor] Binary search question

Dave Angel davea at ieee.org
Sun Apr 25 23:10:02 CEST 2010

Lie Ryan wrote:
> On 04/24/10 23:39, Robert Berman wrote:
>>> -----Original Message-----
>>> From: tutor-bounces+bermanrl=cfl.rr.com at python.org [mailto:tutor-
>>> bounces+bermanrl=cfl.rr.com at python.org] On Behalf Of Alan Gauld
>>> Sent: Friday, April 23, 2010 7:41 PM
>>> To: tutor at python.org
>>> Subject: Re: [Tutor] Binary search question
>>> "Emile van Sebille" <emile at fenx.com> wrote
>>>>    BIG SNIP
>>> And even at 10000000 entries, the list creation slowed right
>>> down - about 10 seconds, but the searches even for "-5" were
>>> still around a second.
>>> So 'in' looks pretty effective to me!
>> Now that is most impressive.
> But that is with the assumption that comparison is very cheap. If you're
> searching inside an object with more complex comparison, say, 0.01
> second per comparison, then with a list of 10 000 000 items, with 'in'
> you will need on *average* 5 000 000 comparisons which is 50 000 seconds
> compared to *worst-case* 24 comparisons with bisect which is 0.24 seconds.
> Now, I say that's 208333 times difference, most impressive indeed.

The ratio doesn't change with a slow comparison, only the magnitude.

And if you have ten million objects that are complex enough to take .01 
secs per comparison, chances are it took a day or two to load them up 
into your list.  Most likely you won't be using a list anyway, but a 
database, so you don't have to reload them each time you start the program.

It's easy to come up with situations in which each of these solutions is 
better than the other.


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