Bernardo Sulzbach posted a much prettier version than mine that is a bit shorter. But his is also somewhat slower (and I believe asymptotically so as the number of equal elements in subsequence goes up). He needs to sum up a bunch of 1's repeatedly rather than do the O(1) `len()` function. For a list with 1000 run lengths of 1000 each, we get: In [53]: %timeit [(k, sum(1 for _ in g)) for k, g in groupby(lst)] 10 loops, best of 3: 66.2 ms per loop In [54]: %timeit [(k,len(l)) for k, g in groupby(lst) for l in [list(g)]] 100 loops, best of 3: 17.5 ms per loop On Sat, Jun 10, 2017 at 7:59 PM, David Mertz <mertz@gnosis.cx> wrote:
Here's a one-line version:
from itertools import groupby rle_encode = lambda it: ( (l[0],len(l)) for g in groupby(it) for l in [list(g[1])])
Since "not every one line function needs to be in the standard library" is a guiding principle of Python, and even moreso of `itertools`, probably this is a recipe in the documentation at most. Or maybe it would have a home in `more_itertools`.
On Sat, Jun 10, 2017 at 7:20 PM, Neal Fultz <nfultz@gmail.com> wrote:
Hello python-ideas,
I am very new to this, but on a different forum and after a couple conversations, I really wished Python came with run-length encoding built-in; after all, it ships with zip, which is much more complicated :)
The general idea is to be able to go back and forth between two representations of a sequence:
[1,1,1,1,2,3,4,4,3,3,3]
and
[(1, 4), (2, 1), (3, 1), (4, 2), (3, 3)]
where the first element is the data element, and the second is how many times it is repeated.
I wrote an encoder/decoder in about 20 lines ( https://github.com/nfultz/rle.py/blob/master/rle.py ) and would like to offer it for the next version; I think it might fit in nicely in the itertools module, for example. I am curious about your thoughts.
Best,
-Neal
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