How to convert Dataframe into Series?

Question

I want to convert N columns into one series. How to do it effectively?

Input:

    0   1   2   3
0  64  98  47  58
1  80  94  81  46
2  18  43  79  84         


        

Answer 1

df.T.stack().reset_index(drop=True)

Out:

0     64
1     80
2     18
3     57
4     98
5     94
6     43
7     35
8     47
9     81
10    79
11    81
12    58
13    46
14    84
15    31
dtype: int64