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.

### >Solution :

## 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])
```