Count distinct strings in rolling window using pandas

Question

How do I count the number of unique strings in a rolling window of a pandas dataframe?

a = pd.DataFrame(['a','b','a','a','b','c','d','e','e','e','e'])
a.rolling(3).apply(lambda x: len(np.unique(x)))

Output, same as original dataframe:

    0
0   a
1   b
2   a
3   a
4   b
5   c
6   d
7   e
8   e
9   e
10  e

Expected:

    0
0   1
1   2
2   2
3   2
4   2
5   3
6   3
7   3
8   2
9   1
10  1

Answer 1

I think you need first convert values to numeric - by factorize or by rank. Also min_periods parameter is necessary for avoid NaN in start of column:

a[0] = pd.factorize(a[0])[0]
print (a)
    0
0   0
1   1
2   0
3   0
4   1
5   2
6   3
7   4
8   4
9   4
10  4

b = a.rolling(3, min_periods=1).apply(lambda x: len(np.unique(x))).astype(int)
print (b)
    0
0   1
1   2
2   2
3   2
4   2
5   3
6   3
7   3
8   2
9   1
10  1

Or:

a[0] = a[0].rank(method='dense')
      0
0   1.0
1   2.0
2   1.0
3   1.0
4   2.0
5   3.0
6   4.0
7   5.0
8   5.0
9   5.0
10  5.0

b = a.rolling(3, min_periods=1).apply(lambda x: len(np.unique(x))).astype(int)
print (b)
    0
0   1
1   2
2   2
3   2
4   2
5   3
6   3
7   3
8   2
9   1
10  1