On 4/26/2010 10:19 PM, Dave Angel wrote:
Note also that if you insert or delete from the list while you're
looping, you can get undefined results.
The results are not undefined. They m,ight be unexpected, but that is
due to an invalid assumption.
The behavior is explained in section 7.3 of The Python Language
Reference. Note especially:
"""
There is a subtlety when the sequence is being modified by the loop
(this can only occur for mutable sequences, i.e. lists). An internal
counter is used to keep track of which item is used next, and this is
incremented on each iteration. When this counter has reached the length
of the sequence the loop terminates. This means that if the suite
deletes the current (or a previous) item from the sequence, the next
item will be skipped (since it gets the index of the current item which
has already been treated). Likewise, if the suite inserts an item in the
sequence before the current item, the current item will be treated again
the next time through the loop. This can lead to nasty bugs that can be
avoided by making a temporary copy using a slice of the whole sequence,
e.g.,
for x in a[:]:
if x < 0: a.remove(x)
"""
--
Bob Gailer
919-636-4239
Chapel Hill NC
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