On Mar 8, 3:20 pm, Roel Schroeven <[EMAIL PROTECTED]> wrote: > malkarouri schreef: > > > > > Hi everyone, > > > I have an algorithm in which I need to use a loop over a queue on > > which I push values within the loop, sort of: > > > while not(q.empty()): > > x = q.get() > > #process x to get zero or more y's > > #for each y: > > q.put(y) > > > The easiest thing I can do is use a list as a queue and a normal for > > loop: > > > q = [a, b, c] > > > for x in q: > > #process x to get zero or more y's > > q.append(y) > > > It makes me feel kind of uncomfortable, though it seems to work. The > > question is: is it guaranteed to work, or does Python expect that you > > wouldn't change the list in the loop? > > Changing a loop while iterating over it is to be avoided, if possible. > In any case, a deque is more efficient for this kind of use. I'd use it > like this: > > from collections import deque > > q = deque([a, b, c]) > while q: > x = q.popleft() > # ... > q.append(y) > > -- > The saddest aspect of life right now is that science gathers knowledge > faster than society gathers wisdom. > -- Isaac Asimov > > Roel Schroeven
Thanks for your response. My same feeling, avoid loop variable, but no explicit reason. Thanks for reminding me of the deque, which I never used before. Alas, in terms of efficiency - which I need - I don't really need to pop the value on the list/deque. This additional step takes time enough to slow the loop a lot. So its not ideal here. Still, why avoid changing loop variable? Does python treat looping over a list differently from looping over an iterator, where it doesn't know if the iterator future changes while loop running? Regards, Muhammad Alkarouri -- http://mail.python.org/mailman/listinfo/python-list
