How to find Run length encoding in python

Question

I have an array ar = [2,2,2,1,1,2,2,3,3,3,3]. For this array, I want to find the lengths of consecutive same numbers like:

 values: 2, 1, 2, 3
lengths: 3, 2, 2, 4

In R, this is obtained by using rle() function. Is there any existing function in python which provides required output?

Answer 1

You can do this with groupby

from itertools import groupby
ar = [2,2,2,1,1,2,2,3,3,3,3]
print([(k, sum(1 for i in g)) for k,g in groupby(ar)])
# [(2, 3), (1, 2), (2, 2), (3, 4)]

Answer 2

Here is an answer for pure numpy:

import numpy as np


def find_runs(x):
    """Find runs of consecutive items in an array."""

    # ensure array
    x = np.asanyarray(x)
    if x.ndim != 1:
        raise ValueError('only 1D array supported')
    n = x.shape[0]

    # handle empty array
    if n == 0:
        return np.array([]), np.array([]), np.array([])

    else:
        # find run starts
        loc_run_start = np.empty(n, dtype=bool)
        loc_run_start[0] = True
        np.not_equal(x[:-1], x[1:], out=loc_run_start[1:])
        run_starts = np.nonzero(loc_run_start)[0]

        # find run values
        run_values = x[loc_run_start]

        # find run lengths
        run_lengths = np.diff(np.append(run_starts, n))

        return run_values, run_starts, run_lengths

Credit goes to https://github.com/alimanfoo