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POIterate over a ‘window’ of adjacent elements in Python
 

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  1. POIterate over a ‘window’ of adjacent elements in Python
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    <p>This is more a question of elegance and performance rather than “how to do at all”, so I'll just show the code:</p> <pre><code>def iterate_adjacencies(gen, fill=0, size=2, do_fill_left=True, do_fill_right=False): """ Iterates over a 'window' of `size` adjacent elements in the supploed `gen` generator, using `fill` to fill edge if `do_fill_left` is True (default), and fill the right edge (i.e. last element and `size-1` of `fill` elements as the last item) if `do_fill_right` is True. """ fill_size = size - 1 prev = [fill] * fill_size i = 1 for item in gen: # iterate over the supplied `whatever`. if not do_fill_left and i &lt; size: i += 1 else: yield prev + [item] prev = prev[1:] + [item] if do_fill_right: for i in range(fill_size): yield prev + [fill] prev = prev[1:] + [fill] </code></pre> <p>and then ask: is there already a function for that? And, if not, can you do the same thing in a better (i.e. more neat and/or more fast) way?</p> <p>Edit:</p> <p>with ideas from answers of @agf, @FogleBird, @senderle, a resulting somewhat-neat-looking piece of code is:</p> <pre><code>def window(seq, size=2, fill=0, fill_left=True, fill_right=False): """ Returns a sliding window (of width n) over data from the iterable: s -&gt; (s0,s1,...s[n-1]), (s1,s2,...,sn), ... """ ssize = size - 1 it = chain( repeat(fill, ssize * fill_left), iter(seq), repeat(fill, ssize * fill_right)) result = tuple(islice(it, size)) if len(result) == size: # `&lt;=` if okay to return seq if len(seq) &lt; size yield result for elem in it: result = result[1:] + (elem,) yield result </code></pre>
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    1. COYou might want to explain in more detail what this does with words.
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    2. COI should note that I didn't use `iter(gen)` (to fill `prev` initially when `do_fill_left` is False) because I'm not completely certain (even though still quite certain) that it will behave in the same way for any possible type of `gen`. Also, it would be possible then to chain `gen` with `[fill]*fill_size if do_fill_right` instead of last 4 lines.
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    3. COCould you provide a sample input and a samplle of the expected output please? it would be easier to understand what your function is doing.
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