Advent of Code - adventofcode.com

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danb in reply to @lf

yeah similar here, was filtering to get dy lines after moving down 1 rather than moving down dy lines and then counting...

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wence in reply to @lf

Very C-like, I ended up writing

def trees(grid, r, d):
    return sum(row[(i*r) % len(row)] == "#"
               for i, row in enumerate(grid[::d]))

for the find-trees function.