[Numpy-discussion] Need to eliminate a nested loop

[eat]

Hi,

On Wed, May 11, 2011 at 8:28 AM, Elfnor <elfnor at gmail.com> wrote:

>
> Hi
>
> The following code produces the desired result but has a slow triple loop
> iterating over the matrix multiplication.
>
> I'm sure it can be eliminated with a neat indexing trick but I can't figure
> out how.
>
> Any suggestions please?
> -----------------------------
> import numpy
> #define domain of function
> x = numpy.linspace(-5,5,64)
> y = numpy.linspace(-5,5,64)
> z = numpy.linspace(-5,5,64)
>
> #calculate f at each point in domain
> a = 5.0
> b = 3.0
> c = 2.0
> #ellipsoid
> E = numpy.array([[1/a**2,   0   ,   0  ,  0  ],
>                [   0   ,1/b**2 ,   0  ,  0  ],
>                [   0   ,   0   ,1/c**2,  0  ],
>                [   0   ,   0   ,   0  , -1  ]])
>
> f = numpy.zeros((x.size,y.size,z.size))
>
> for i,xi in enumerate(x):
>    for j,yj in enumerate(y):
>        for k,zk in enumerate(z):
>            X = numpy.array([xi,yj,zk,1])
>            f[i,j,k] = numpy.dot(numpy.dot(X.T,E),X)
> -----------------------------------
>
Something like this:
n= 64
u= np.linspace(-5, 5, n)[None, :]
u0= u.repeat(n** 2)[None, :]

u1= u.repeat(n)[None, :].repeat(n, axis= 0).reshape(1, -1)

u2= u.repeat(n** 2, axis= 0).reshape(1, -1)

U= np.r_[u0, u1, u2, np.ones((1, n** 3))]

f= (np.dot(E, U)* U).sum(0).reshape(n, n, n)


Regards,eat

>
> Thanks Eleanor
> --
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>
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