Nils Bruin wrote: > On Dec 12, 4:24 pm, Jason Grout <jason-s...@creativetrax.com> wrote: >> [f(x) for x in [1..10] if f(x)>0] > > This is actually bad style. It means that f gets evaluated twice for > all the values that end up in the list.
Good point. > The magma language solves this with modified semantics for the "where" > clause. One would write: > > [ v : x in [1..10] | v gt 0 where v:=f(x)] > > The point being that for every iteration of x, the where clause > supplies a variable local to the condition which survives into the > value computing part. > Does python have a similar construct? I guess a nested construction > with iterators might come close: > > [v for v in ( f(x) for x in [1..10] ) if v > 0] > > but I don't know what the overhead is in that construction. > There's apparently a lot more overhead than I would have imagined: sage: %timeit [v for v in (x for x in range(10) ) if v > 0] 100000 loops, best of 3: 5.4 盜 per loop sage: %timeit [v for v in range(10) if v>0] 100000 loops, best of 3: 2.98 盜 per loop Interestingly, the builtin filter function isn't much faster than the slow version above: sage: %timeit filter(lambda x: x>0, range(10)) 100000 loops, best of 3: 5.3 盜 per loop and a hand-coded filter is about the same speed: 1 def fltr(lst): 2 result=[] 3 for x in lst: 4 v = x 5 if v>0: 6 result.append(v) 7 return result sage: %timeit fltr(range(10)) 100000 loops, best of 3: 5.18 盜 per loop Thanks, Jason -- Jason Grout -- To post to this group, send an email to sage-devel@googlegroups.com To unsubscribe from this group, send an email to sage-devel+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-devel URL: http://www.sagemath.org
