Return or return not clear solution for Moore Neighbourhood by veky - python coding challenges

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Return or return not solution in Clear category for Moore Neighbourhood by veky

from itertools import product

def at(seq, k):
    if k in range(len(seq)): yield seq[k]

def count_neighbours(grid, i, j):    
    def around():
        for di, dj in filter(any, product({-1, 0, 1}, repeat=2)):
            for row in at(grid, i + di): yield from at(row, j + dj)
    return sum(around())

Aug. 16, 2015

Comments:

veky on Aug. 16, 2015, 6:07 p.m.
There is no `try`. ;-D

hao.daniel on June 1, 2019, 5:06 p.m.
This makes my head spin.... after 30+ minutes, I think I finally get it. Could you please add some verbiage to enlighten me? Thanks.

veky on June 2, 2019, 10:12 a.m.
It depends. Of course, it would help immensely if you said what you have the problem with. (It's really not obvious.) First, does <pre class='brush: python'>around = [] for di in {-1, 0, 1}: for dj in {-1, 0, 1}: row = grid[i + di] around.append(row[j + dj]) return sum(around) </pre> pose any problems for you? I hope not -- if I'm wrong, just say. Now, I'm just doing some things to this code to make it work correctly in all cases, and be more efficient (both in memory and in explanatory power;). First, it's obviously unnecessary to keep all the values in the neighborhood in the list at once: since we can generate them one by one and sum needs them one by one, let's switch `around` to a generator, `yield`ing instead of `append`ing. Since it is local, it doesn't need any parameters -- it can access `grid` from outer scope without problems. Second, that repetition in lines 2 and 3 can be refactored using `repeat` kwarg of `itertools.product`. And, now we can get all 3^2=9 of our offsets in one place, it's really easy to filter the `0, 0` combination out -- it is the only one that doesn't really offset any coordinate, not satisfying `any`. Third, let's finally do something about invalid indices: if `i, j` is at the edge, the brackets above would either raise `IndexError`, or wrap around zero and give bogus results. We can handle the exception and add the check of nonnegativity manually, but since we are already "in the generator monad", it is eaisest to just subsume the Maybe monad into it: yield it only if it's within limits. For the name I chose `at`, since it is the usual name <a href="http://www.cplusplus.com/reference/vector/vector/at/">in STL</a> for translation from Undefined Behaviour monad to Exception monad. :-)

hao.daniel on June 2, 2019, 3:13 p.m.
All clear now! Thanks for writing back and elaborate it in such detail. Do you ever think to open some courses in the future? Seriously, that's a best avenue to share and spread the knowledge.

veky on June 3, 2019, 5:50 a.m.
It really depends a lot on the personality of the learner, and the teacher. Courses are too rigid format for me. I have a blog, though: truepython.blogspot.com.

hao.daniel on June 3, 2019, 12:03 p.m.
Definitely. Will check out. The definition of "course" could be very loosely defined - something like "Daily Coding Problem" format. (I heard from friends -_ this is quite popular these days...)__

veky on June 12, 2019, 10:01 p.m.
Oh, I had that... even here on CiO. Search for "veky code review". :-)

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