Stefan Behnel, 15.06.2010 14:23:
superpollo, 15.06.2010 13:49:
goal (from e.c.m.): evaluate
1^2+2^2+3^2-4^2-5^2+6^2+7^2+8^2-9^2-10^2+...-2010^2, where each three
consecutive + must be followed by two - (^ meaning ** in this context)
my solution:
>>> s = 0
>>> for i in range(1, 2011):
... s += i**2
... if not (i+1)%5:
... s -= 2*i**2
... if not i%5:
... s -= 2*i**2
Pretty ugly, if you ask me. What about this:
s = 0
for i in range(1, 2011):
if i%5 in (1,2,3):
s += i**2
else:
s -= i**2
Here's the obvious one-liner:
s = sum(i**2 if i%5 in (1,2,3) else -i**2
for i in range(1, 2011))
Runs in ~600 usecs for me according to timeit - pretty fast.
Just for the record, this Cython code runs in ~15 usecs for me:
def makesum():
return <int>sum(i**2 if i%5 in (1,2,3) else -i**2
for i in range(1, 2011))
using the latest Cython 0.13pre. The "<int>" cast is required to drop the
sum() into efficient C code. Without it, the code runs in 55 usecs.
Stefan
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