[Numpy-discussion] Is sum() slow?

[Mads Ipsen]

Hi,

Sorry if this is a double post - unsure if it made it the first time:

Here are some timings that puzzle me a bit. Sum the two rows of a 2xn
matrix, where n is some large number

python -m timeit -s "from numpy import array,sum,reshape; x =
array([1.5]*1000); x = reshape(x,(2,500))" "x.sum(0)"
10000 loops, best of 3: 36.2 usec per loop

python -m timeit -s "from numpy import array,sum,reshape; x =
array([1.5]*1000); x = reshape(x,(2,500))" "x[0] + x[1]"
100000 loops, best of 3: 5.35 usec per loop

The two calculations are completely equivalent (at least from the
numerical results point of view). But using the built-in sum() function is
app. 6 times slower.

Just for the reference - using Numeric gives

python -m timeit -s "from Numeric import array,sum,reshape; x =
array([1.5]*1000); x = reshape(x,(2,500))" "sum(x)"
100000 loops, best of 3: 7.08 usec per loop

Any suggestions to why this might be - or am I doing something wrong here?

Thanks // Mads

PS. Thanks to everybody that responded to my previous posting on problems
and questions regarding rint() and related coding issues. Your replies
were very helpful!