Deleting the first element of a list
bruce_deletethis_dawson at cygnus-software.com
Thu Oct 3 06:26:58 CEST 2002
Peter Hansen wrote:
> Erik Max Francis wrote:
>> JB wrote:
>> quantitative information about where the bottlenecks truly are. This
>> concept is especially powerful in a high-level language like Python; the
>> very choice of using Python means that you're willing to trade raw speed
>> for flexibility. If you're overconcerned with squeezing every last bit
>> of CPU power right from the start, then Python (or any other high-level
>> language) was probably not a good choice of languages.
> Well stated, although a note about changing algorithms generally
> providing the most significant speedups might be in order. Even
> with Python, picking a better algorithm can produce huge gains.
> (This does not, of course, apply to a small idiomatic choice such
> as this thread was about.)
Highly applicable to this thread. Deleting the first item of a Python
list or C++ vector is an O(n) operation - proportional to the number of
items in the list. If you do that for every item in the list it is
O(n^2) - proportional to the square.
The thing about O(n^2), and any complexity higher than that, is that
there are many values of N that are small enough for N items to easily
fit into memory, but large enough to make the program run for days and
days and days and days.
O(n^2) is bad, unless you know that n will always be small.
So, if performance is a problem, the OP should consider reworking the
code. For instance, if the original code was:
# Do Something with the first element
Then it should be much better to go:
# Do Something with the last element
This changes it from O(n^2) to O(n), which could make it run thousands
of times faster.
Premature optimization is the root of all evil, but O(n^2) is pretty
darned evil too!
Of course, the OP's code may not be so easily reworked as my simple example.
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