On 06/01/2013 11:58 PM, Jim Mooney wrote:
It's a little unclear to me where generators are more efficient.
I'm astounded nobody has asked you what version of Python this is for.
in Python 2.x, the range(x,y) function produces the whole list, and then
the expression around it converts that to a generator. That alone would
make it waste tons of memory (and time) for large y.
So I'll ASSUME you're using 3.x, or that you actually are using xrange
below.
Below
I'm creating a list of non-even squares. I figured Python would be
smart enough to see I only wanted a small slice of a large generated
list, but as I increased y, Py got slower and eventually died of a
memory error. If I don't make it into a list I get a type error. So
where are generators more efficient? Or more specifically, how would i
make this efficient and use the generator to only pick out the small
2:10 slice I really want, but leave the capability of the entire list
if I wanted to slice and dice it in different ways further on?
def uneven_squares(x,y):
squarelist = (c**2 for c in range(x,y) if c%2 != 0)
return squarelist #returning a generator
print(list(uneven_squares(10,10000))[2:10]) #slows as y gets bigger, then dies
So the real question is how to slice a generator without first
converting it to a list.
That's what iterator.islice() is for.
Try (untested):
import iterator
def uneven_squares(x,y):
....
print( iterator.islice(uneven_squares(10, 10000), 2, 10) )
--
DaveA
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