# [Numpy-discussion] repeat an array without allocation

Eelco Hoogendoorn hoogendoorn.eelco at gmail.com
Mon May 5 05:43:46 EDT 2014

```If b is indeed big I don't see a problem with the python loop, elegance
aside; but Cython will not beat it on that front.

On Mon, May 5, 2014 at 9:34 AM, srean <srean.list at gmail.com> wrote:

> Great ! thanks. I should have seen that.
>
> Is there any way array multiplication (as opposed to matrix
> multiplication) can be sped up without forming A and (A * b) explicitly.
>
> A = np.repeat(x, [4, 2, 1, 3], axis = 0)    # A.shape == 10,10
> c = sum(b * A, axis = 1)                        # b.shape == 10,10
>
> In my actual setting b is pretty big, so I would like to avoid creating
> another array the same size. I would also like to avoid a Python loop.
>
> st = 0
> for (i,rep) in enumerate([4, 2, 1, 3]):
>      end = st + rep
>      c[st : end] = np.dot(b[st : end, :], a[i,:])
>      st  = end
>
> Is Cython the only way ?
>
>
> On Mon, May 5, 2014 at 1:20 AM, Jaime Fernández del Río <
> jaime.frio at gmail.com> wrote:
>
>> On Sun, May 4, 2014 at 9:34 PM, srean <srean.list at gmail.com> wrote:
>>
>>> Hi all,
>>>
>>>   is there an efficient way to do the following without allocating A
>>> where
>>>
>>>  A = np.repeat(x, [4, 2, 1, 3], axis=0)
>>>  c = A.dot(b)    # b.shape
>>>
>>
>> If x is a 2D array you can call repeat **after** dot, not before, which
>> will save you some memory and a few operations:
>>
>> >>> a = np.random.rand(4, 5)
>> >>> b = np.random.rand(5, 6)
>> >>> np.allclose(np.repeat(a, [4, 2, 1, 3], axis=0).dot(b),
>> ...             np.repeat(a.dot(b), [4, 2, 1, 3], axis=0))
>> True
>>
>> Similarly, if x is a 1D array, you can sum the corresponding items of b
>> before calling dot:
>>
>> >>> a = np.random.rand(4)
>> >>> b = np.random.rand(10)
>> >>> idx = np.concatenate(([0], np.cumsum([4,2,1,3])[:-1]))
>> >>> np.allclose(np.dot(np.repeat(a, [4,2,1,3] ,axis=0), b),
>> ... )
>> True
>>
>> Jaime
>>
>> --
>> (\__/)
>> ( O.o)
>> ( > <) Este es Conejo. Copia a Conejo en tu firma y ayúdale en sus planes
>> de dominación mundial.
>>
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>>
>
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