What is a list compression in Python?

Kit wkfung.eric at gmail.com
Tue Jan 19 01:41:49 CET 2010


Cool! Thank you very much. You mean this instead right?

print [x*x for x in xrange(1,11,2)]

Kit


On 1月19日, 上午8時36分, Gary Herron <gher... at islandtraining.com> wrote:
> Kit wrote:
> > Thank you so much guys.
>
> > Just out of curiosity: can I do something like this to "square all
> > even numbers in the range 1-10"?
> > print [x^2 for x in range (1,11) if x % 2 == 0]
>
> > Or is there a better way of doing it? Thanks for the help, and I am
> > really appreciate your help.
>
> That's a fine way to do it, although:
>
>   xrange is slightly more efficient than range for large
>   sets of values.  (This isn't such a large set for it to matter.)
>
>   You can get range and xrange to count by two's, eliminating
>   the need for the "if" portion in this case.
>
>   The exponent operator is ** not ^, and  x*x is more efficient than x**2.
>
> So.
>
> print [x**2 for x in xrange(1,11,2)]
>
> Gary Herron
>
>
>
> > Kit.
>
> > On 1月19日, 上午12時30分, Steven D'Aprano <st... at REMOVE-THIS-
> > cybersource.com.au> wrote:
>
> >> On Mon, 18 Jan 2010 08:07:41 -0800, Kit wrote:
>
> >>> Hello Everyone, I am not sure if I have posted this question in a
> >>> correct board. Can anyone please teach me:
>
> >>> What is a list compression in Python?
>
> >> Google "python list comprehension".
>
> >> If Google is broken for you, try Yahoo, or any other search engine.
>
> >>> Would you mind give me some list compression examples?
>
> >> Instead of this:
>
> >> L = []
> >> for x in range(10):
> >>     L.append(x**2)
>
> >> you can write:
>
> >> L = [x**2 for x in range(10)]
>
> >> Instead of this example:
>
> >> L = []
> >> for x in range(10):
> >>     if x % 2 == 0:
> >>         L.append(x**2)
>
> >> you can write:
>
> >> L = [x**2 for x in range(10) if x % 2 == 0]
>
> >> --
> >> Steven




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