Re: [Numpy-discussion] How to get rid of the loop?
2009/11/7 Stas K : > Thank you, Josef > It is exactly what I want: > > ar[:,None]**2 + ar**2 > > Do you know something about performance of this? In my real program ar have > ~ 10k elements, and expression for v more complicated (it has some > trigonometric functions) The construction of ar[:,None] (which I prefer to spell ar[:,np.newaxis]) is cheap, since it just constructs a "view" into ar. Computing ar**2 and ar[:,None] require squaring each element of ar, but this is done in a C loop. (It's done twice in this expression, so this isn't as efficient as it might be). Only when it comes time to do the addition do you do n**2 operations. For very large n, this can be a crushing memory burden, and if you only need these values for an intermediate calculation it sometimes turns out that it's better to loop over one of the dimensions. But this expression is probably about as efficient as you can hope for, given what it does. If you need to do some more complicated calculation, the main thing to be careful of is that you do as much calculation as possible on the separate arrays, while they're still only n elements and not n**2. Anne > On 07.11.2009, at 21:57, josef.p...@gmail.com wrote: > > On Sat, Nov 7, 2009 at 1:51 PM, Stas K wrote: > > Can I get rid of the loop in this example? And what is the fastest way > > to get v in the example? > > ar = array([1,2,3]) > > for a in ar: > > for b in ar: > > v = a**2+b**2 > > ar[:,None]**2 + ar**2 > > array([[ 2, 5, 10], > [ 5, 8, 13], > [10, 13, 18]]) > > I think, for this case there is also directly a function in numpy > hypot which should also work. > > Josef > > ___ > > NumPy-Discussion mailing list > > NumPy-Discussion@scipy.org > > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > ___ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > > ___ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] How to get rid of the loop?
Thank you, Josef It is exactly what I want: ar[:,None]**2 + ar**2 Do you know something about performance of this? In my real program ar have ~ 10k elements, and expression for v more complicated (it has some trigonometric functions) On 07.11.2009, at 21:57, josef.p...@gmail.com wrote: On Sat, Nov 7, 2009 at 1:51 PM, Stas K wrote: Can I get rid of the loop in this example? And what is the fastest way to get v in the example? ar = array([1,2,3]) for a in ar: for b in ar: v = a**2+b**2 ar[:,None]**2 + ar**2 array([[ 2, 5, 10], [ 5, 8, 13], [10, 13, 18]]) I think, for this case there is also directly a function in numpy hypot which should also work. Josef ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] How to get rid of the loop?
On 11/7/2009 1:51 PM, Stas K wrote: > Can I get rid of the loop in this example? And what is the fastest way > to get v in the example? > > ar = array([1,2,3]) > for a in ar: > for b in ar: > v = a**2+b**2 >>> a2 = a*a >>> np.add.outer(a2,a2) array([[ 2, 5, 10], [ 5, 8, 13], [10, 13, 18]]) hth, Alan Isaac ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] How to get rid of the loop?
On Sat, Nov 7, 2009 at 1:51 PM, Stas K wrote: > Can I get rid of the loop in this example? And what is the fastest way > to get v in the example? > > ar = array([1,2,3]) > for a in ar: > for b in ar: > v = a**2+b**2 >>> ar[:,None]**2 + ar**2 array([[ 2, 5, 10], [ 5, 8, 13], [10, 13, 18]]) I think, for this case there is also directly a function in numpy hypot which should also work. Josef > ___ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > ___ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
