[Python] how to find the count of each unique element in a single column pandas dataframe

Question

I have a Pandas DataFrame with just one column. The column has some repeated values. How can I find the count of each unique element in the dataframe?

Answer 1

You can apply the groupby() function to the column and then apply the transform() function with argument 'len' to the result of the groupby() function.

Here is an example:

>>> import numpy as np
>>> import pandas as pd
>>> df = pd.DataFrame({'A':[1,1,2,2,1,2,3,2,6,5,4,4,6,4,5,6]})
>>> df
    A
0   1
1   1
2   2
3   2
4   1
5   2
6   3
7   2
8   6
9   5
10  4
11  4
12  6
13  4
14  5
15  6

>>> df['size'] = df.groupby('A')['A'].transform(len)
>>> df
    A  size
0   1     3
1   1     3
2   2     4
3   2     4
4   1     3
5   2     4
6   3     1
7   2     4
8   6     3
9   5     2
10  4     3
11  4     3
12  6     3
13  4     3
14  5     2
15  6     3

The new column 'size' shows the count of each element. If you want to covert the above dataframe to a dictionary with unique elements as keys and their counts as values, you can apply the to_dict() function to the above dataframe.

>>> {v[0]:v[1] for v in df.to_dict('split')['data']}
{1: 3, 2: 4, 3: 1, 6: 3, 5: 2, 4: 3}