Velocity Reviews

Velocity Reviews (http://www.velocityreviews.com/forums/index.php)
-   Python (http://www.velocityreviews.com/forums/f43-python.html)
-   -   List loops (http://www.velocityreviews.com/forums/t543031-list-loops.html)

Tommy Grav 10-09-2007 02:30 PM

List loops
 
Hi everyone,

I have a list of objects where I have want to do two loops.
I want to loop over the list and inside this loop, work on all
the elements of the list after the one being handled in the outer
loop. I can of course do this with indexes:

>>> alist = range(3)
>>> for i in xrange(len(alist)):

.... for j in xrange(i+1,len(alist)):
.... print i,j,alist[i],alist[j]
....
0 1 0 1
0 2 0 2
1 2 1 2
>>>



Is there a way to do this without using indexes?

Cheers
Tommy




John Machin 10-09-2007 09:53 PM

Re: List loops
 
On 10/10/2007 12:30 AM, Tommy Grav wrote:
> Hi everyone,
>
> I have a list of objects where I have want to do two loops.
> I want to loop over the list and inside this loop, work on all
> the elements of the list after the one being handled in the outer
> loop. I can of course do this with indexes:
>
> >>> alist = range(3)


If you REALLY want this to work ONLY on lists of the form range(n), then
say so. Otherwise give a generic example e.g. alist = ['foo', 42,
3.14159] so that the channel is not clogged with red-herring responses :-)

> >>> for i in xrange(len(alist)):

> ... for j in xrange(i+1,len(alist)):
> ... print i,j,alist[i],alist[j]
> ...
> 0 1 0 1
> 0 2 0 2
> 1 2 1 2
> >>>

>
>
> Is there a way to do this without using indexes?
>


Probably not efficiently, if your list size is huge and you want to
avoid making copies of sub-lists e.g. alist[i+1:].

A somewhat more efficient version of your code:
n = len(any_old_list)
for i in xrange(n-1):
# n-1 avoids possible problems if you need to use i here
for j in xrange(i+1, n):
# etc

How big is n likely to be?


All times are GMT. The time now is 09:33 AM.

Powered by vBulletin®. Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO ©2010, Crawlability, Inc.