> From my point of view, such function is a bit of a corner-case to be added to > numpy. And it doesn’t justify it’s naming anymore. It is not one operation > anymore. It is a cumsum and prepending 0. And it is very difficult to argue > why prepending 0 to cumsum is a part of cumsum.
That is backwards. Consider the array [x0, x1, x2]. The sum of the first 0 elements is 0. The sum of the first 1 elements is x0. The sum of the first 2 elements is x0+x1. The sum of the first 3 elements is x0+x1+x2. Hence, the array of partial sums is [0, x0, x0+x1, x0+x1+x2]. Thus, the operation [x0, x1, x2] -> [0, x0, x0+x1, x0+x1+x2] is a natural and primitive one. The current behaviour of numpy.cumsum is the composition of two basic operations, computing the partial sums and omitting the initial value: [x0, x1, x2] -> [0, x0, x0+x1, x0+x1+x2] -> [x0, x0+x1, x0+x1+x2]. > What I would rather vouch for is adding an argument to `np.diff` so that it > leaves first row unmodified. > def diff0(a, axis=-1): > """Differencing which appends first item along the axis""" > a0 = np.take(a, [0], axis=axis) > return np.concatenate([a0, np.diff(a, n=1, axis=axis)], axis=axis) > This would be more sensible from conceptual point of view. As difference can > not be made, the result is the difference from absolute origin. With > recognition that first non-origin value in a sequence is the one after it. > And if the first row is the origin in a specific case, then that origin is > correctly defined in relation to absolute origin. > Then, if origin row is needed, then it can be prepended in the beginning of a > procedure. And np.diff and np.cumsum are inverses throughout the sequential > code. > np.diff0 was one the first functions I had added to my numpy utils and been > using it instead of np.diff quite a lot. This suggestion is bad: diff0 is conceptually confused. numpy.diff changes an array of numpy.datetime64s to an array of numpy.timedelta64s, but numpy.diff0 changes an array of numpy.datetime64s to a heterogeneous array where one element is a numpy.datetime64 and the rest are numpy.timedelta64s. In general, whereas numpy.diff changes an array of positions to an array of displacements, diff0 changes an array of positions to a heterogeneous array where one element is a position and the rest are displacements. _______________________________________________ NumPy-Discussion mailing list -- numpy-discussion@python.org To unsubscribe send an email to numpy-discussion-le...@python.org https://mail.python.org/mailman3/lists/numpy-discussion.python.org/ Member address: arch...@mail-archive.com
