Finding the last position of each column in data frame by Pandas

Question

I have done some searching to answer this question, but all I can not figure out how to do this:

I have a dataset which contains 185 rows and 30 columns. Not all of the rows have value. I want to look for the position of the last value on each column and take the index of that column. I am not sure how to do this operation as when I use the code below it gives me the length of the data frame not just that column :

len(data_exam['col'])

I would appreciate for any suggestion.

Also I want to make sure that if I want to read all the columns in a loop, does my following code would be a good choice or not! :

 list=[]
 for col in data:
    function which find the length of column

Thanks.

Answer 1

IIUC, you want the value of the last non-nan in each column:

df[::-1].bfill().iloc[0]

Example:

df = pd.DataFrame({'A':[1,2,3,4,np.nan],'B':[1,np.nan,np.nan,np.nan,np.nan],'C':[1,2,3,4,5]})

     A    B  C
0  1.0  1.0  1
1  2.0  NaN  2
2  3.0  NaN  3
3  4.0  NaN  4
4  NaN  NaN  5

Output:

A    4.0
B    1.0
C    5.0
Name: 4, dtype: float64

Answer 2

df.reset_index().melt('index').dropna().groupby('variable')['index'].max()
Out[487]: 
variable
A    3
B    0
C    4
Name: index, dtype: int64