set and dict iteration

Aaron Brady castironpi at gmail.com
Fri Aug 17 19:47:16 CEST 2012


On Thursday, August 16, 2012 6:07:40 PM UTC-5, Ian wrote:
> On Thu, Aug 16, 2012 at 4:55 PM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> 
> > On Thu, Aug 16, 2012 at 12:00 PM, Aaron Brady <castironpi at gmail.com> wrote:
> 
> >> The inconsistency is, if we remove an element from a set and add another during iteration, the new element might appear later in the iteration, and might not, depending on the hash code; therefore comparing the size of the set between iterations isn't adequate.  Example:
> 
> >
> 
> > It can be more than just the new element.  For example, here the
> 
> > entire set is repeated (Python 3.2):
> 
> >
> 
> >>>> s = set(range(8, 13))
> 
> >>>> it = iter(s)
> 
> >>>> from itertools import islice
> 
> >>>> list(islice(it, 5))  # avoid exhausting the iterator
> 
> > [8, 9, 10, 11, 12]
> 
> >>>> s.add(13)
> 
> >>>> s.remove(13)
> 
> >>>> list(it)
> 
> > [8, 9, 10, 11, 12]
> 
> >
> 
> > This occurs because the addition of the sixth item triggers a resize
> 
> > of the underlying hash table, and the existing items, which were
> 
> > originally in slots 0-4, are now in slots 8-12.
> 
> 
> 
> Another curious example:
> 
> 
> 
> >>> s = set(range(8, 48, 8))
> 
> >>> s
> 
> {8, 16, 40, 24, 32}
> 
> >>> it = iter(s)
> 
> >>> from itertools import islice
> 
> >>> list(islice(it, 4))
> 
> [8, 16, 40, 24]
> 
> >>> s.add(48)
> 
> >>> s.remove(48)
> 
> >>> list(it)
> 
> [8, 16, 40, 24]
> 
> 
> 
> Hey, what happened to 32?

Good examples.  The former occurs without the 'islice' as well.

s= set( range( 8, 13 ) ) 
it= iter( s ) 
print( [ next( it ) for _ in range( 5 ) ] )
s.add( 13 ) 
s.remove( 13 ) 
print( [ next( it ) for _ in range( 5 ) ] )

Output:

[8, 9, 10, 11, 12]
[8, 9, 10, 11, 12]


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