On 04/05/2012 08:39 PM, Greg Christian wrote:
> I am just wondering if anyone can explain how the return statement in this
> function is working (the code is from activestate.com)? Where does x come
> from – it is not initialized anywhere else and then just appears in the
> return statement. Any help would be appreciated.
>
>
> def primes(n):
> """Prime number generator up to n - (generates a list)"""
> ## {{{ http://code.activestate.com/recipes/366178/ (r5)
> if n == 2: return [2]
> elif n < 2: return []
> s = range(3, n + 1, 2)
> mroot = n ** 0.5
> half = (n + 1)/2 - 1
> i = 0
> m = 3
> while m <= mroot:
> if s[i]:
> j = (m * m - 3)/2
> s[j] = 0
> while j < half:
> s[j] = 0
> j += m
> i = i + 1
> m = 2 * i + 3
> return [2]+[x for x in s if x]
>
The expression [x for x in s if x] is called a list comprehension, and
it defines x as it needs it. The results of that expression is a list,
which is concatenated to the end of the list [2], and the combined list
is returned.
For example, try the one-liner:
print [i for i in xrange(5)]
--
DaveA
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