Iterating through set

Thomas Rachel nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915 at spamschutz.glglgl.de
Thu Jul 17 15:22:56 CEST 2014


Am 15.07.2014 02:10 schrieb LJ:
> Hi All.
>
> I'm coding a Dynamic Programming algorithm to solve a network flow problem. At some point in the algorithm I have to iterate through a set of nodes, while adding and/or removing elements, until the set is empty. I know a regular set() object does not work in a case like this, so I wonder if anyone knows of an efficient pythonic way to handle this.
>
> Thanks in advance!
>

This sounds like you want to avoid processing of an item as soon as it 
is removed.

Then I'd suggest the following:

add = set()
remove = set()

while myset or add:
     for item in myset:
         if item not in remove:
             # process
     myset -= remove
     myset += add
     remove.clear()
     add.clear()

Adding happens via add.add(item), removing via remove.add(item).

If there is additionally the need to take care in which order to apply 
add/remove, or if it can happen that item X is added and removed in the 
same loop run, it gets a bit more complicated.

Then adding would be like

if item in remove:
     remove.remove(item)
elif item not in myset and item not in add:
     add.add(item)

and removing like

if item in add:
     add.remove(item)
elif item in myset:
     remove.add(item)


HTH,

Thomas



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