[Python-Dev] {}.first[key,value,item] and {}.pop (was Re: {}.getitem())

Tim Peters tim.one@home.com
Thu, 30 Nov 2000 17:46:52 -0500

> ...
> One more concern: if you repeatedly remove the *first* item, the hash
> table will start looking lobsided.  Since we don't resize the hash
> table on deletes, maybe picking an item at random (but not using an
> expensive random generator!) would be better.

Which is the reason SETL doesn't specify *which* set item is removed:  if
you always start looking at "the front" of a dict that's being consumed, the
dict fills with turds without shrinking, you skip over them again and again,
and consuming the entire dict is still quadratic time.

Unfortunately, while using a random start point is almost always quicker
than that, the expected time for consuming the whole dict remains quadratic.

The clearest way around that is to save a per-dict search finger, recording
where the last search left off.  Start from its current value.  Failure if
it wraps around.  This is linear time in non-pathological cases (a
pathological case is one in which it isn't linear time <wink>).