Mailman 3 December 2015 - NumPy-Discussion

mixed mode arithmetic
by Neal Becker July 11, 2023

July 11, 2023

I've been browsing the numpy source. I'm wondering about mixed-mode arithmetic on arrays. I believe the way numpy handles this is that it never does mixed arithmetic, but instead converts arrays to a common type. Arguably, that might be efficient for a mix of say, double and float. Maybe not. But for a mix of complex and a scalar type (say, CDouble * Double), it's clearly suboptimal in efficiency. So, do I understand this correctly? If so, is that something we should improve?

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Invalid value encoutered : how to prevent numpy.where to do this?
by Eric Emsellem Feb. 18, 2023

Feb. 18, 2023

Dear all, I have a code using lots of "numpy.where" to make some constrained calculations as in: data = arange(10) result = np.where(data == 0, 0., 1./data) # or data1 = arange(10) data2 = arange(10)+1.0 result = np.where(data1 > data2, np.sqrt(data1-data2), np.sqrt(data2-data2)) which then produces warnings like: /usr/bin/ipython:1: RuntimeWarning: invalid value encountered in sqrt or for the first example: /usr/bin/ipython:1: RuntimeWarning: divide by zero encountered in divide How do I avoid these messages to appear? I know that I could in principle use numpy.seterr. However, I do NOT want to remove these warnings for other potential divide/multiply/sqrt etc errors. Only when I am using a "where", to in fact avoid such warnings! Note that the warnings only happen once, but since I am going to release that code, I would like to avoid the user to get such messages which are irrelevant here (because I am testing, with the where, when NOT to divide by zero or take a sqrt of a negative number). thanks! Eric

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Re: [Numpy-discussion] example reading binary Fortran file
by Neil Martinsen-Burrell July 22, 2021

July 22, 2021

David Froger <david.froger.info <at> gmail.com> writes: > Hy,My question is about reading Fortran binary file (oh no this question > again...) I've posted this before, but I finally got it cleaned up for the Cookbook. For this purpose I use a subclass of file that has methods for reading unformatted Fortran data. See http://www.scipy.org/Cookbook/FortranIO/FortranFile. I'd gladly see this in numpy or scipy somewhere, but I'm not sure where it belongs. > program makeArray > implicit none > integer,parameter:: nx=10,ny=20 > real(4),dimension(nx,ny):: ux,uy,p > integer :: i,j > open(11,file='uxuyp.bin',form='unformatted') > do i = 1,nx > do j = 1,ny > ux(i,j) = real(i*j) > uy(i,j) = real(i)/real(j) > p (i,j) = real(i) + real(j) > enddo > enddo > write(11) ux,uy > write(11) p > close(11) > end program makeArray When I run the above program compiled with gfortran on my Intel Mac, I can read it back with:: >>> import numpy as np >>> from fortranfile import FortranFile >>> f=FortranFile('uxuyp.bin', endian='<') >>> uxuy = f.readReals(prec='f') # 'f' for default reals >>> len(uxuy) 400 >>> ux = np.array(uxuy[:200]).reshape((20,10)).T >>> uy = np.array(uxuy[200:]).reshape((20,10)).T >>> p = f.readReals('f').reshape((20,10)).T >>> ux array([[ 1., 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20.], [ 2., 4., 6., 8., 10., 12., 14., 16., 18., 20., 22., 24., 26., 28., 30., 32., 34., 36., 38., 40.], [ 3., 6., 9., 12., 15., 18., 21., 24., 27., 30., 33., 36., 39., 42., 45., 48., 51., 54., 57., 60.], [ 4., 8., 12., 16., 20., 24., 28., 32., 36., 40., 44., 48., 52., 56., 60., 64., 68., 72., 76., 80.], [ 5., 10., 15., 20., 25., 30., 35., 40., 45., 50., 55., 60., 65., 70., 75., 80., 85., 90., 95., 100.], [ 6., 12., 18., 24., 30., 36., 42., 48., 54., 60., 66., 72., 78., 84., 90., 96., 102., 108., 114., 120.], [ 7.,Proxy-Connection: keep-alive Cache-Control: max-age=0 14., 21., 28., 35., 42., 49., 56., 63., 70., 77., 84., 91., 98., 105., 112., 119., 126., 133., 140.], [ 8., 16., 24., 32., 40., 48., 56., 64., 72., 80., 88., 96., 104., 112., 120., 128., 136., 144., 152., 160.], [ 9., 18., 27., 36., 45., 54., 63., 72., 81., 90., 99., 108., 117., 126., 135., 144., 153., 162., 171., 180.], [ 10., 20., 30., 40., 50., 60., 70., 80., 90., 100., 110., 120., 130., 140., 150., 160., 170., 180., 190., 200.]]) >>> uy array([[ 1. , 0.5 , 0.33333334, 0.25 , 0.2 , 0.16666667, 0.14285715, 0.125 , 0.11111111, 0.1 , 0.09090909, 0.08333334, 0.07692308, 0.07142857, 0.06666667, 0.0625 , 0.05882353, 0.05555556, 0.05263158, 0.05 ], [ 2. , 1. , 0.66666669, 0.5 , 0.40000001, 0.33333334, 0.2857143 , 0.25 , 0.22222222, 0.2 , 0.18181819, 0.16666667, 0.15384616, 0.14285715, 0.13333334, 0.125 , 0.11764706, 0.11111111, 0.10526316, 0.1 ], [ 3. , 1.5 , 1. , 0.75 , 0.60000002, 0.5 , 0.42857143, 0.375 , 0.33333334, 0.30000001, 0.27272728, 0.25 , 0.23076923, 0.21428572, 0.2 , 0.1875 , 0.17647059, 0.16666667, 0.15789473, 0.15000001], [ 4. , 2. , 1.33333337, 1. , 0.80000001, 0.66666669, 0.5714286 , 0.5 , 0.44444445, 0.40000001, 0.36363637, 0.33333334, 0.30769232, 0.2857143 , 0.26666668, 0.25 , 0.23529412, 0.22222222, 0.21052632, 0.2 ], [ 5. , 2.5 , 1.66666663, 1.25 , 1. , 0.83333331, 0.71428573, 0.625 , 0.55555558, 0.5 , 0.45454547, 0.41666666, 0.38461539, 0.35714287, 0.33333334, 0.3125 , 0.29411766, 0.27777779, 0.2631579 , 0.25 ], [ 6. , 3. , 2. , 1.5 , 1.20000005, 1. , 0.85714287, 0.75 , 0.66666669, 0.60000002, 0.54545456, 0.5 , 0.46153846, 0.42857143, 0.40000001, 0.375 , 0.35294119, 0.33333334, 0.31578946, 0.30000001], [ 7. , 3.5 , 2.33333325, 1.75 , 1.39999998, 1.16666663, 1. , 0.875 , 0.77777779, 0.69999999, 0.63636363, 0.58333331, 0.53846157, 0.5 , 0.46666667, 0.4375 , 0.41176471, 0.3888889 , 0.36842105, 0.34999999], [ 8. , 4. , 2.66666675, 2. , 1.60000002, 1.33333337, 1.14285719, 1. , 0.8888889 , 0.80000001, 0.72727275, 0.66666669, 0.61538464, 0.5714286 , 0.53333336, 0.5 , 0.47058824, 0.44444445, 0.42105263, 0.40000001], [ 9. , 4.5 , 3. , 2.25 , 1.79999995, 1.5 , 1.28571427, 1.125 , 1. , 0.89999998, 0.81818181, 0.75 , 0.69230771, 0.64285713, 0.60000002, 0.5625 , 0.52941179, 0.5 , 0.47368422, 0.44999999], [ 10. , 5. , 3.33333325, 2.5 , 2. , 1.66666663, 1.42857146, 1.25 , 1.11111116, 1. , 0.90909094, 0.83333331, 0.76923078, 0.71428573, 0.66666669, 0.625 , 0.58823532, 0.55555558, 0.52631581, 0.5 ]]) >>> p array([[ 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21.], [ 3., 4., 5., 6., 7., 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22.], [ 4., 5., 6., 7., 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23.], [ 5., 6., 7., 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23., 24.], [ 6., 7., 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23., 24., 25.], [ 7., 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23., 24., 25., 26.], [ 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23., 24., 25., 26., 27.], [ 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23., 24., 25., 26., 27., 28.], [ 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23., 24., 25., 26., 27., 28., 29.], [ 11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23., 24., 25., 26., 27., 28., 29., 30.]]) Note that you have to provide the shape information for ux and uy because fortran writes them together as a stream of 400 numbers. -Neil

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C-API: multidimensional array indexing?
by Johann Bauer March 31, 2016

March 31, 2016

Dear experts, is there a C-API function for numpy which implements Python's multidimensional indexing? Say, I have a 2d-array PyArrayObject * M; and an index int i; how do I extract the i-th row or column M[i,:] respectively M[:,i]? I am looking for a function which gives again a PyArrayObject * and which is a view to M (no copied data; the result should be another PyArrayObject whose data and strides points to the correct memory portion of M). I searched the API documentation, Google and mailing lists for quite a long time but didn't find anything. Can you help me? Thanks, Johann

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FeatureRequest: support for array construction from iterators
by Stephan Sahm Feb. 18, 2016

Feb. 18, 2016

[ this request /discussion refers to numpy issue #5863 https://github.com/numpy/numpy/pull/5863#issuecomment-159738368 ] Dear all, As far as I can think, the expected functionality of np.array(...) would be np.array(list(...)) or something even nicer. Therefore, I like to request a generator/iterator support for np.array(...) as far as list(...) supports it. A more detailed reasoning behind this follows now. In general it seems possible to identify iterators/generators as needed for this purpose: - someone actually implemented this feature already (see #5863 <https://mail.google.com/mail/u/0/%E2%80%8Bhttps://github.com/numpy/numpy/pu…> ) - there is ``type.GeneratorType`` and ``collections.abc.Iterator`` for ``isinstance(...)`` check - numpy can destinguish them already from all other types which get well translated into a numpy array Given this, I think the general argument goes roughly like the following: PROS (effect maybe 10% of numpy user or more): - more intuitive overall behaviour, array(...) = array(list(...)) roughly - python3 compatibility (see e.g. #5951 <https://github.com/numpy/numpy/issues/5951>) - compatibility with analog ``__builtin__`` functions (see e.g. #5756 <https://github.com/numpy/numpy/issues/5756>) - all the above make numpy easier to use in an interactive style (e.g. ipython --pylab) (computation not that important, however coding time well) CONS (effect less than 0.1% numpy user I would guess): - might break existing code which in total, at least for me at this stage, speaks in favour of merging the already existing featurebranch (see #5863 <https://mail.google.com/mail/u/0/%E2%80%8Bhttps://github.com/numpy/numpy/pu…> ) or something similar into numpy master . Discussion, please! cheers, Stephan

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Generalize hstack/vstack --> stack; Block matrices like in matlab
by Stefan Otte Jan. 9, 2016

Jan. 9, 2016

Hey, quite often I work with block matrices. Matlab offers the convenient notation [ a b; c d ] to stack matrices. The numpy equivalent is kinda clumsy: vstack([hstack([a,b]), hstack([c,d])]) I wrote the little function `stack` that does exactly that: stack([[a, b], [c, d]]) In my case `stack` replaced `hstack` and `vstack` almost completely. If you're interested in including it in numpy I created a pull request [1]. I'm looking forward to getting some feedback! Best, Stefan [1] https://github.com/numpy/numpy/pull/5057

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Proposal for a new function: np.moveaxis
by Stephan Hoyer Jan. 2, 2016

Jan. 2, 2016

I've put up a pull request implementing a new function, np.moveaxis, as an alternative to np.transpose and np.rollaxis: https://github.com/numpy/numpy/pull/6630 This functionality has been discussed (even the exact function name) several times over the years, but it never made it into a pull request. The most pressing issue is that the behavior of np.rollaxis is not intuitive to most users: https://mail.scipy.org/pipermail/numpy-discussion/2010-September/052882.html https://github.com/numpy/numpy/issues/2039 http://stackoverflow.com/questions/29891583/reason-why-numpy-rollaxis-is-so… In this pull request, I also allow the source and destination axes to be sequences as well as scalars. This does not add much complexity to the code, solves some additional use cases and makes np.moveaxis a proper generalization of the other axes manipulation routines (see the pull requests for details). Best of all, it already works on ndarray duck types (like masked array and dask.array), because they have already implemented transpose. I think np.moveaxis would be a useful addition to NumPy -- I've found myself writing helper functions with a subset of its functionality several times over the past few years. What do you think? Cheers, Stephan

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Numpy funding update
by Ralf Gommers Jan. 2, 2016

Jan. 2, 2016

Hi all, A quick good news message: OSDC has made a $5k contribution to NumFOCUS, which is split between support for a women in technology workshop and support for Numpy: http://www.numfocus.org/blog/osdc-donates-5k-to-support-numpy-women-in-tech This was a very nice surprise to me, and a first sign that the FSA (fiscal sponsorship agreement) we recently signed with NumFOCUS is going to yield significant benefits for Numpy. NumFOCUS is also doing a special end-of-year fundraiser. Funds donated (up to $5k) will be tripled by anonymous sponsors: http://www.numfocus.org/blog/numfocus-end-of-year-fundraising-drive-5000-ma… So think of Numpy (or your other favorite NumFOCUS-sponsored project of course) if you're considering a holiday season charitable gift! Cheers, Ralf

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what would you expect A[none] to do?
by Neal Becker Jan. 1, 2016

Jan. 1, 2016

In my case, what it does is: A.shape = (5760,) A[none] -> (1, 5760) In my case, use of none here is just a mistake. But why would you want this to be accepted at all, and how should it be interpreted?

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Dynamic array list implementation
by Nicolas P. Rougier Dec. 31, 2015

Dec. 31, 2015

I've coded a typed dynamic list based on numpy array (needed for the glumpy project). Code is available from https://github.com/rougier/numpy-list A Numpy array list is a strongly typed list whose type can be anything that can be interpreted as a numpy data type. >>> L = ArrayList( [[0], [1,2], [3,4,5], [6,7,8,9]] ) >>> print(L) [[0], [1 2], [3 4 5], [6 7 8 9]] >>> print(L.data) [0 1 2 3 4 5 6 7 8 9] You can add several items at once by specifying common or individual size: a single scalar means all items are the same size while a list of sizes is used to specify individual item sizes. >>> L = ArrayList( np.arange(10), [3,3,4]) >>> print(L) [[0 1 2], [3 4 5], [6 7 8 9]] >>> print(L.data) [0 1 2 3 4 5 6 7 8 9] You can also us typed list for storing strings with different sizes: >>> L = ArrayList(["Hello", "world", "!"]) >>> print(L[0]) 'Hello' >>> L[1] = "brave new world" >>> print(L) ['Hello', 'brave new world', '!'] Nicolas

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