On Tue, 27 Feb 2018 07:36:31 -0700, Ian Kelly <ian.g.ke...@gmail.com> wrote:
> On Tue, Feb 27, 2018 at 4:08 AM, Peter Otten <__pete...@web.de> wrote: >> Seb wrote: >>> On Tue, 27 Feb 2018 12:25:30 +1300, >>> Gregory Ewing <greg.ew...@canterbury.ac.nz> wrote: >>>> Seb wrote: >>>>> I was wondering is whether there's a faster way of multiplying >>>>> each row (1x3) of a matrix by another matrix (3x3), compared to >>>>> looping through the matrix row by row as shown in the code. >>>> Just multiply the two matrices together. >>>> If A is an nx3 matrix and B is a 3x3 matrix, then C = A @ B is an >>>> nx3 matrix where C[i] = A[i] @ B. >>>> (This is a property of matrix multiplication in general, nothing >>>> special about numpy.) >>> I think that's only true if B is the same for every row in A. In >>> the code I posted, B varies by row of A. >> Yeah, you would have to substitute the N 3x3 matrices with an Nx3x3 >> tensor, though I don't know if numpy provides an op such that >> Nx3 op Nx3x3 --> desired result >> or >> op(Nx3, Nx3x3) --> desired result > Nx1x3 @ Nx3x3 ought to do it, with the result being Nx1x3. That's right. I just tried this manipulation by replacing the last block of code in my example, from the line above `for` loop with: ---<--------------------cut here---------------start------------------->--- # Alternative using `np.matmul` uvw_alt = uvw.reshape((uvw.shape[0], 1, uvw.shape[1])) bmats = np.asarray(map(randint_mat, maxint)) uvw_rots_alt = np.matmul(uvw_alt, bmats).squeeze() ---<--------------------cut here---------------end--------------------->--- Interestingly, the time savings from IPython are not spectacular: %run -t -N100 loop_approach.py IPython CPU timings (estimated): Total runs performed: 100 Times : Total Per run User : 0.28 s, 0.00 s. System : 0.00 s, 0.00 s. Wall time: 0.28 s. %run -t -N100 matmul_approach.py IPython CPU timings (estimated): Total runs performed: 100 Times : Total Per run User : 0.17 s, 0.00 s. System : 0.00 s, 0.00 s. Wall time: 0.18 s. -- Seb -- https://mail.python.org/mailman/listinfo/python-list
