Convert dataframe from wide to long - pandas

Question

I have a dataframe like this:

index    S_1 S_2 S_3 S_4
 0        1    0   0   1
 1        1    1  Nan Nan

I am trying to change it from long

Answer 1

There is pandas.wide_to_long, which is nice when the columns have stubs like that.

import pandas as pd

df.reset_index(inplace=True,drop=True)
df['id'] = df.index
df = pd.wide_to_long(df, stubnames='S_', i='id', j='num').reset_index().rename(columns={'S_':'S'})

#  id num  index    S
#0   0   1      0  1.0
#1   1   1      1  1.0
#2   0   2      0  0.0
#3   1   2      1  1.0
#4   0   3      0  0.0
#5   1   3      1  NaN
#6   0   4      0  1.0
#7   1   4      1  NaN