Transform a dataset from wide to long pandas-CodePudding

I have a little problem with the transformation from wide to long on a dataset. I tried with melt but I didn't get a good result. I hope that someone could help me. The dataset is as follow:

pd.DataFrame({'id': [0, 1, 2, 3, 4, 5],
              'type': ['a', 'b', 'c', 'd', 'e', 'f'],
              'rank': ['alpha', 'beta', 'gamma', 'epsilon', 'phi', 'ro'],
              'type.1': ['d', 'g', 'z', 'a', 'nan', 'nan'],
              'rank.1': ['phi', 'sigma', 'gamma', 'lambda', 'nan', 'nan'],
              'type.2': ['nan', 'nan', 'j', 'r', 'nan', 'nan'],
              'rank.2': ['nan', 'nan', 'eta', 'theta', 'nan', 'nan']})

And I need the dataset in this way:

pd.DataFrame({'id': [0, 0, 1, 1, 2, 2, 2, 3, 3, 3, 4, 5],
              'type': ['a', 'd', 'b', 'g', 'c', 'z', 'j', 'd', 'a', 'r', 'e', 'f'],
              'rank': ['alpha', 'phi', 'beta', 'sigma', 'gamma', 'gamma', 'eta', 'epsilon', 'lambda', 'theta', 'phi', 'ro']})

Can anyone help me with that? Thanks a lot

CodePudding user response：

Use wide_to_long:

# normalize the `type` and `rank` columns so they have the same format as others
df = df.rename(columns={'type': 'type.0', 'rank': 'rank.0'})

(pd.wide_to_long(df, stubnames=['type', 'rank'], i='id', j='var', sep='.')
   [lambda x: (x['type'] != 'nan') | (x['rank'] != 'nan')].reset_index())

    id  var type     rank
0    0    0    a    alpha
1    1    0    b     beta
2    2    0    c    gamma
3    3    0    d  epsilon
4    4    0    e      phi
5    5    0    f       ro
6    0    1    d      phi
7    1    1    g    sigma
8    2    1    z    gamma
9    3    1    a   lambda
10   2    2    j      eta
11   3    2    r    theta

You can drop the var column if not needed.