# For loop searching takes too long!

Chris Colbert sccolbert at gmail.com
Fri Jan 29 02:20:22 CET 2010

```if you're open to other libraries, this could be done extremely fast in
numpy.

On my machine summing that whole array takes 75ms.

On Thu, Jan 28, 2010 at 8:00 PM, Steven D'Aprano <
steve at remove-this-cybersource.com.au> wrote:

> On Thu, 28 Jan 2010 15:52:14 -0800, elsa wrote:
>
> > Now, what I need to do is randomly choose one myList[i], however the
> > distribution of my random choice needs to be proportional to the values
> > of myList[i][0]. So, for this list above, I'd have a much higher chance
> > of choosing myList[0] than myList[1].
> >
> > Here is how I'm doing it at the moment:
> >
> > def chooseI(myList):
> >       mySum=0
> >       choice = random.choice(range(1,sum([i[0] for i in myList])+1))
> >       for i in range(len(myList)):
> >               mySum+=myList[i][0]
> >               if mySum>=choice:
> >                       return i
> >                       break
>
> This isn't related to your problem, but you don't need the break after
> the return -- the return will leave the loop and the function, and the
> break will never be reached.
>
> You could probably speed the code up a little by changing all the calls
> to range into xrange. range actually generates a list of integers, which
> is time consuming, while xrange is a lazy generator which just produces
> each integer one at a time.  (You can ignore this if you are using Python
> 3.0 or 3.1.)
>
> Another small optimization you could use is to use a generator expression
> instead of a list comprehension inside the call to sum. That should
> generate the sum lazily, instead of calculating a giant list first and
>
> But I'd try something like this:
>
> # Untested.
> def chooseI(myList):
>    total = sum(i[0] for i in myList)
>    mySum = 0
>    choice = random.randrange(total)
>    for i in myList:
>        mySum += i[0]
>         if mySum >= choice:
>            return i
>
>
>
> --
> Steven
> --
> http://mail.python.org/mailman/listinfo/python-list
>
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