fast way to filter a set?

[Fortepianissimo]

I know I can do things like

s=Set(range(1,11))
s=Set(filter(lambda x:x%2==0,s))

But this seems a bit slow since filter returns a list which then must
be converted back to a set. Any tips? Thanks!

[Nicola Mingotti]

fo*************@yahoo.com.tw (Fortepianissimo) writes:

I know I can do things like

s=Set(range(1,11))
s=Set(filter(lambda x:x%2==0,s))

But this seems a bit slow since filter returns a list which then must
be converted back to a set. Any tips? Thanks!

Can you ?
I tried python2.3 and 2.2 and it replies
: "NameError: name 'Set' is not defined" .

There is a PEP but it seems it's steel 'under consideration'.
http://www.python.org/peps/pep-0218.html

May be , if this proposal is accepted you
one day could write :
{ x for x in S if x%2 == 0 } # S be a set .

bye.

[Skip Montanaro]

fortepianissimo> I know I can do things like
fortepianissimo> s=Set(range(1,11))
fortepianissimo> s=Set(filter(lambda x:x%2==0,s))

fortepianissimo> But this seems a bit slow since filter returns a list
fortepianissimo> which then must be converted back to a set. Any tips?

The only thing which comes to mind is:

s = sets.Set(range(1,11))
s Set([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) keys = list(s)
dummy = [s.discard(x) for x in keys if x%2]
s

Set([2, 4, 6, 8, 10])

You still create a list, but don't create a second set.

Skip

[Peter Otten]

Nicola Mingotti wrote:

fo*************@yahoo.com.tw (Fortepianissimo) writes:

I know I can do things like

s=Set(range(1,11))
s=Set(filter(lambda x:x%2==0,s))

But this seems a bit slow since filter returns a list which then must
be converted back to a set. Any tips? Thanks!

Can you ?
I tried python2.3 and 2.2 and it replies
: "NameError: name 'Set' is not defined" .

In 2.3 it works if you do

from sets import Set

first.

Peter

[Peter Otten]

Fortepianissimo wrote:

I know I can do things like

s=Set(range(1,11))
s=Set(filter(lambda x:x%2==0,s))

But this seems a bit slow since filter returns a list which then must
be converted back to a set. Any tips? Thanks!

The Set constructor accepts any iterable, so you can do it all with
iterators instead of temporary lists:

from sets import Set
from itertools import ifilter

s = Set(range(1, 11))
print Set(ifilter(lambda x: x % 2 == 0, s))

Peter