List loops

[John Machin]

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?