On 29May2017 01:17, Alan Gauld <alan.ga...@yahoo.co.uk> wrote:
On 29/05/17 00:12, Alex Kleider wrote:
Would
if Month in {'January', '1'}:
be even better? (regarding efficiency perhaps? Trivial point, I know,
but just wondering.)
If in doubt try it out and profile/time it.
But I don't think it will make much difference since ultimately
it still has to test each value (although a hashing algorithm
may be involved that works on partial matches...) But if in
doubt...
Hi Alex,
As written it should be a bit slower: to construct a set each member get tested
for presence. The cost is in making the set, not in searching it.
_However_, supposing your program were doing this a lot. You might well have a
global (or, better, long lived shared object) containing a set that has already
been constructed. Then:
if Month in the_set:
is very fast; constant time. Whereas as you would expect, checking a list is
linear with the size of the list.
So, using a list:
seen = []
for item in some_item_generator():
if item in seen:
continue
seen.append(item)
... do stuff with item, which is new ...
The cost of the "if" goes up linearly as you add more items.
Using a set:
seen = {}
for item in some_item_generator():
if item in seen:
continue
seen.add(item)
... do stuff with item, which is new ...
The second version will be much more effiient as the "seen" set grows; the
lookup time on the set is essentially O(1) (constant time).
But for an ad hoc 2 element list as in your original example the difference
will be pretty small; making the 2 element set _should_ be slightly more
expensive, and isn't the common idiom (==> less readable). Personally I use:
if value in ('a', 'b', 'c'):
BTW, in Python we tend to use named like "Fred" for classes (or factories), and
"fred" for regular variables. And "FRED" for things that would be constants in
other languages. Eg:
MAX_THINGS = 16
class Foo:
....
def FooBah(x):
return Foo(x, style="bah")
for fred in ....:
Cheers,
Cameron Simpson <c...@zip.com.au>
_______________________________________________
Tutor maillist - Tutor@python.org
To unsubscribe or change subscription options:
https://mail.python.org/mailman/listinfo/tutor
