# Value switcher for groups in Python/Numpy

I have a list:

``````groups = ['A', 'A', 'A', 'B', 'B', 'C', 'C', 'D']
``````

I need to map each value to have an output like this, independently from the numbers of groups and elements inside:

``````[0,0,0,1,1,0,0,1]
``````

The values in output should switch every time when the group is changing.

## Using python

With a list comprehension and the walrus operator of python 3.8+:

``````groups = ['A', 'A', 'A', 'B', 'B', 'C', 'C', 'D']

flag = 0

out = [flag if a==b else (flag:=1-flag) for a, b in zip(groups, groups[:1]+groups)]
``````

Or `itertools`:

``````from itertools import groupby, chain

out = list(chain.from_iterable([i%2]*len(list(g))
for i, (_, g) in enumerate(groupby(groups))))
``````

Output:

``````[0, 0, 0, 1, 1, 0, 0, 1]
``````

## Using pandas:

``````import pandas as pd

out = pd.factorize(groups)[0]%2
``````

Output:

``````array([0, 0, 0, 1, 1, 0, 0, 1])
``````

Or:

``````s = pd.Series(groups)
out = (s.ne(s.shift(fill_value=s[0]))
.cumsum().mod(2).tolist()
)
``````

Output:

``````[0, 0, 0, 1, 1, 0, 0, 1]
``````

## Using numpy:

``````import numpy as np

out = np.cumsum(groups != np.r_[groups[:1], groups[:-1]])%2
``````

Output:

``````array([0, 0, 0, 1, 1, 0, 0, 1])
``````