python - Count of elements in lists within pandas data frame

Question

I need to get the frequency of each element in a list when the list is in a pandas data frame columns

In data:

din=pd.DataFrame({'x':[['a','b','c'],['a','e','d', 'c']]})`

              x
0     [a, b, c]
1  [a, e, d, c]

Desired Output:

   f  x
0  2  a
1  1  b
2  2  c
3  1  d
4  1  e

I can expand the list into rows and then perform a group by but this data could be large ( million plus records ) and was wondering if there is a more efficient/direct way.

Thanks

Answer 1

First flatten values of lists and then count by value_counts or size or Counter:

a = pd.Series([item for sublist in din.x for item in sublist])

Or:

a = pd.Series(np.concatenate(din.x))

---

df = a.value_counts().sort_index().rename_axis('x').reset_index(name='f')

Or:

df = a.groupby(a).size().rename_axis('x').reset_index(name='f')

---

from collections import Counter
from  itertools import chain

df = pd.Series(Counter(chain(*din.x))).sort_index().rename_axis('x').reset_index(name='f')

print (df)
   x  f
0  a  2
1  b  1
2  c  2
3  d  1
4  e  1