# [Numpy-discussion] transform an array of points efficiently?

Chris Colbert sccolbert@gmail....
Thu Jul 9 22:51:24 CDT 2009

```hey,

great man! thanks!

I had thought that it may have been possible with a single dot, but
how to do it escaped me.

Thanks again!

Chris

On Thu, Jul 9, 2009 at 11:45 PM, Kurt Smith<kwmsmith@gmail.com> wrote:
> On Thu, Jul 9, 2009 at 9:36 PM, Chris Colbert<sccolbert@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@gmail.com> wrote:
>>> On Thu, Jul 9, 2009 at 7:08 PM, Chris Colbert<sccolbert@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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```