On Jun 9, 2005, at 07:45, Kevin Reeder wrote:
> I'm having trouble with this code which is meant to run a time
> comparison between two similar functions. The first module is
> makezeros.py
>
> def lots_of_appends():
> zeros = []
> for i in range(10000):
> zeros.append(0)
>
> def one_multiply():
> zeros = [0] * 10000
Note that your functions don't do the same thing. The first one
builds the list properly, but the second builds a list out of
references to the same object -- that is, the 10000 zeros in your
resulting second list are actually the same object.
As long as you're working with immutable types (numbers,
strings) you won't notice there's a problem, but consider the following:
>>> a = [[0]] * 10
>>> a[0].append(1)
>>> a
[[0, 1], [0, 1], [0, 1], [0, 1], [0, 1], [0, 1], [0, 1], [0, 1], [0,
1], [0, 1]]
[0] is a list -- therefore, a mutable object: you can apply
modifications to it. So, the list is actually a list of ten
references to the same object -- [0]. When you run one of its methods
that modifies it, since all references point to it, all members of
the list seem to be modified as well -- but they're actually one
single object that's present 10 times in the list.
-- Max
maxnoel_fr at yahoo dot fr -- ICQ #85274019
"Look at you hacker... A pathetic creature of meat and bone, panting
and sweating as you run through my corridors... How can you challenge
a perfect, immortal machine?"
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