[SciPy-user] Python loops too slow
alex
argriffi@ncsu....
Wed Apr 8 08:06:51 CDT 2009
Ross Williamson wrote:
> Hi All
>
> I'm trying to convert some IDL code into Python and am coming across the
> incredibly slow for loop issue in python. Now I detest IDL but is there
> any fancy way to make the following code not use for loops? Sorry if
> this is obvious.
>
> def make_ellgrid(ngrid, reso):
>
> result = zeros([ngrid, ngrid])
>
> for i in arange((ngrid / 2)+1):
> for j in arange((ngrid / 2)+1):
> result[j,i] = 2. * pi * sqrt(i ** 2. + j ** 2) / reso / (ngrid*1.0)
>
> for i in xrange(ngrid):
> result[i,ngrid / 2+1:] = result[i,1:(ngrid / 2)][::-1]
> for i in xrange(ngrid):
> result[ngrid / 2+1:,i] = result[1:(ngrid / 2),i][::-1]
>
> return result
>
> Cheers
>
> Ross
By factoring out a constant and using hypot you might be able to speed
up the inner loop without doing anything fancy. Maybe something like:
c = ...
for i in ...:
for j in ...:
result[j,i] = hypot(j, i)
result = c * result
From here it is easy to vectorize hypot. To get started, you can see that:
>>> a = np.array([[1, 1], [2, 2]])
>>> np.hypot(a, a.T)
array([[ 1.41421356, 2.23606798],
[ 2.23606798, 2.82842712]])
This should be like:
for i in (1,2):
for j in (1,2):
result[j,i] = hypot(j,i)
Now all you need to do is make the 'a' matrix where a[i,j] is equal to
i, and I'll bet you know how to do that fast (I don't because I don't
really know numpy).
Alex
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