[Numpy-discussion] transform an array of points efficiently?
Kurt Smith
kwmsmith at gmail.com
Thu Jul 9 23:45:08 EDT 2009
On Thu, Jul 9, 2009 at 9:36 PM, Chris Colbert<sccolbert at gmail.com> wrote:
> no, because dot(x,y) != dot(y,x)
>
Try this, then:
In [24]: x = np.random.rand(10,4)
In [25]: y = np.random.rand(4,4)
In [26]: result = np.dot(y,x.T).T
In [39]: for i, res in enumerate(result):
....: assert np.allclose(res, np.dot(y,x[i,:]))
....:
....:
In [40]:
>>>> x = np.random.rand(3,4)
>>>> y = np.random.rand(4,4)
>>>> np.dot(x, y)
> array([[ 1.67624043, 1.66719374, 1.72465017, 1.20372021],
> [ 0.70046162, 0.60187869, 0.73094349, 0.4604766 ],
> [ 0.78707401, 1.01959666, 0.61617829, 0.43147398]])
>>>> np.dot(y, x[0,:
> ... ])
> array([ 1.44627767, 1.09332339, 2.66001046, 1.13972652])
>>>> np.dot(y, x[1,:])
> array([ 0.27854715, 0.56261048, 0.7793413 , 0.44260709])
>>>> np.dot(y, x[2,:])
> array([ 0.70468211, 0.42843143, 1.34022702, 0.53021987])
>>>>
>
>
> hence I need xnew = [Transform]*[xold]
>
> and not [xold]*[Transform]
>
>
>
>
> On Thu, Jul 9, 2009 at 10:22 PM, Keith Goodman<kwgoodman at gmail.com> wrote:
>> On Thu, Jul 9, 2009 at 7:08 PM, Chris Colbert<sccolbert at gmail.com> wrote:
>>> say i have an Nx4 array of points and I want to dot every [n, :] 1x4
>>> slice with a 4x4 matrix.
>>>
>>> Currently I am using apply_along_axis in the following manner:
>>>
>>> def func(slice, mat):
>>> return np.dot(mat, slice)
>>>
>>> np.apply_along_axis(func, arr, 1, mat)
>>>
>>> Is there a more efficient way of doing this that doesn't require a
>>> python function for each slice?
>>
>> I'm sure I'm missing an important point, but can't you solve the whole
>> problem with one dot:
>>
>>>> x = np.random.rand(3,4)
>>>> y = np.random.rand(4,4)
>>
>>>> np.dot(x, y)
>>
>> array([[ 0.86488057, 0.23456114, 0.91592677, 0.89798689],
>> [ 1.24197754, 0.39907686, 1.45453141, 1.13645076],
>> [ 1.41419289, 0.81818818, 1.09768428, 1.32719635]])
>>
>>>> np.dot(x[0,:], y)
>> array([ 0.86488057, 0.23456114, 0.91592677, 0.89798689])
>>>> np.dot(x[1,:], y)
>> array([ 1.24197754, 0.39907686, 1.45453141, 1.13645076])
>>>> np.dot(x[2,:], y)
>> array([ 1.41419289, 0.81818818, 1.09768428, 1.32719635])
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