I've implemented such functions in Cython and packaged them into a library called numpy_illustrated <https://pypi.org/project/numpy-illustrated/>
It exposes the following functions: find(a, v) # returns the index of the first occurrence of v in a first_above(a, v) # returns the index of the first element in a that is strictly above v first_nonzero(a) # returns the index of the first nonzero element They scan the array and bail out immediately once the match is found. Have a significant performance gain if the element to be found is closer to the beginning of the array. Have roughly the same speed as alternative methods if the value is missing. The complete signatures of the functions look like this: find(a, v, rtol=1e-05, atol=1e-08, sorted=False, default=-1, raises=False) first_above(a, v, sorted=False, missing=-1, raises=False) first_nonzero(a, missing=-1, raises=False) This covers the most common use cases and does not accept Python callbacks because accepting them would nullify any speed gain one would expect from such a function. A Python callback can be implemented with Numba, but anyone who can write the callback in Numba has no need for a library that wraps it into a dedicated function. The library has a 100% test coverage. Code style 'black'. It should be easy to add functions like 'first_below' if necessary. A more detailed description of these functions can be found here <https://betterprogramming.pub/the-numpy-illustrated-library-7531a7c43ffb?sk=8dd60bfafd6d49231ac76cb148a4d16f> . Best regards, Lev Maximov On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis <dom.grigo...@gmail.com> wrote: > I juggled a bit and found pretty nice solution using numba. Which is > probably not very robust, but proves that such thing can be optimised while > retaining flexibility. Check if it works for your use cases and let me know > if anything fails or if it is slow compared to what you used. > > first_true_str = """def first_true(arr, n): result = np.full((n, > arr.shape[1]), -1, dtype=np.int32) for j in range(arr.shape[1]): k > = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] > if cond(x): result[k, j] = i k += 1 > if k >= n: break return result""" > > class FirstTrue: > CONTEXT = {'np': np} > > def __init__(self, expr): > self.expr = expr > self.expr_ast = ast.parse(expr, mode='exec').body[0].value > self.func_ast = ast.parse(first_true_str, mode='exec') > self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast > self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") > exec(self.func_cmp, self.CONTEXT) > self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name]) > > def __call__(self, arr, n=1, axis=None): > # PREPARE INPUTS > in_1d = False > if axis is None: > arr = np.ravel(arr)[:, None] > in_1d = True > elif axis == 0: > if arr.ndim == 1: > in_1d = True > arr = arr[:, None] > else: > raise ValueError('axis ~in (None, 0)') > res = self.func_nb(arr, n) > if in_1d: > res = res[:, 0] > return res > > if __name__ == '__main__': > arr = np.arange(125).reshape((5, 5, 5)) > ft = FirstTrue('np.sum(x) > 30') > print(ft(arr, n=2, axis=0)) > > [[1 0 0 0 0] > [2 1 1 1 1]] > > In [16]: %timeit ft(arr, 2, axis=0)1.31 µs ± 3.94 ns per loop (mean ± std. > dev. of 7 runs, 1,000,000 loops each) > > Regards, > DG > > On 29 Oct 2023, at 23:18, rosko37 <rosk...@gmail.com> wrote: > > An example with a 1-D array (where it is easiest to see what I mean) is > the following. I will follow Dom Grigonis's suggestion that the range not > be provided as a separate argument, as it can be just as easily "folded > into" the array by passing a slice. So it becomes just: > idx = first_true(arr, cond) > > As Dom also points out, the "cond" would likely need to be a "function > pointer" (i.e., the name of a function defined elsewhere, turning > first_true into a higher-order function), unless there's some way to pass a > parseable expression for simple cases. A few special cases like the first > zero/nonzero element could be handled with dedicated options (sort of like > matplotlib colors), but for anything beyond that it gets unwieldy fast. > > So let's say we have this: > ****************** > def cond(x): > return x>50 > > search_arr = np.exp(np.arange(0,1000)) > > print(np.first_true(search_arr, cond)) > ******************* > > This should print 4, because the element of search_arr at index 4 (i.e. > the 5th element) is e^4, which is slightly greater than 50 (while e^3 is > less than 50). It should return this *without testing the 6th through > 1000th elements of the array at all to see whether they exceed 50 or not*. > This example is rather contrived, because simply taking the natural log of > 50 and rounding up is far superior, not even *evaluating the array of > exponentials *(which my example clearly still does--and in the use cases > I've had for such a function, I can't predict the array elements like > this--they come from loaded data, the output of a simulation, etc., and are > all already in a numpy array). And in this case, since the values are > strictly increasing, search_sorted() would work as well. But it illustrates > the idea. > > > > > > On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis <dom.grigo...@gmail.com> > wrote: > >> Could you please give a concise example? I know you have provided one, >> but it is engrained deep in verbose text and has some typos in it, which >> makes hard to understand exactly what inputs should result in what output. >> >> Regards, >> DG >> >> > On 25 Oct 2023, at 22:59, rosko37 <rosk...@gmail.com> wrote: >> > >> > I know this question has been asked before, both on this list as well >> as several threads on Stack Overflow, etc. It's a common issue. I'm NOT >> asking for how to do this using existing Numpy functions (as that >> information can be found in any of those sources)--what I'm asking is >> whether Numpy would accept inclusion of a function that does this, or >> whether (possibly more likely) such a proposal has already been considered >> and rejected for some reason. >> > >> > The task is this--there's a large array and you want to find the next >> element after some index that satisfies some condition. Such elements are >> common, and the typical number of elements to be searched through is small >> relative to the size of the array. Therefore, it would greatly improve >> performance to avoid testing ALL elements against the conditional once one >> is found that returns True. However, all built-in functions that I know of >> test the entire array. >> > >> > One can obviously jury-rig some ways, like for instance create a "for" >> loop over non-overlapping slices of length slice_length and call something >> like np.where(cond) on each--that outer "for" loop is much faster than a >> loop over individual elements, and the inner loop at most will go >> slice_length-1 elements past the first "hit". However, needing to use such >> a convoluted piece of code for such a simple task seems to go against the >> Numpy spirit of having one operation being one function of the form >> func(arr)". >> > >> > A proposed function for this, let's call it "np.first_true(arr, >> start_idx, [stop_idx])" would be best implemented at the C code level, >> possibly in the same code file that defines np.where. I'm wondering if I, >> or someone else, were to write such a function, if the Numpy developers >> would consider merging it as a standard part of the codebase. It's possible >> that the idea of such a function is bad because it would violate some >> existing broadcasting or fancy indexing rules. Clearly one could make it >> possible to pass an "axis" argument to np.first_true() that would select an >> axis to search over in the case of multi-dimensional arrays, and then the >> result would be an array of indices of one fewer dimension than the >> original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would >> return [1,1,0] for cond(x): x>4. The case where no elements satisfy the >> condition would need to return a "signal value" like -1. But maybe there >> are some weird cases where there isn't a sensible return val >> ue, hence why such a function has not been added. >> > >> > -Andrew Rosko >> > _______________________________________________ >> > 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: dom.grigo...@gmail.com >> >> _______________________________________________ >> 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: rosk...@gmail.com >> > _______________________________________________ > 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: dom.grigo...@gmail.com > > > _______________________________________________ > 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: lev.maxi...@gmail.com >
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