Flatten multiple columns in a dataframe to a single column

Question

I have a dataframe like this:

id    other_id_1    other_id_2    other_id_3
1     100           101           102
2     200           201           202
3              


        

Answer 1

By using pd.wide_to_long:

pd.wide_to_long(df,'other_id_',i='id',j='drop').reset_index().drop('drop',axis=1).sort_values('id')
    Out[36]: 
       id  other_id_
    0   1        100
    3   1        101
    6   1        102
    1   2        200
    4   2        201
    7   2        202
    2   3        300
    5   3        301
    8   3        302

or unstack

df.set_index('id').unstack().reset_index().drop('level_0',1).rename(columns={0:'other_id'})

Out[43]: 
   id  other_id
0   1       100
1   2       200
2   3       300
3   1       101
4   2       201
5   3       301
6   1       102
7   2       202
8   3       302

Answer 2

If id isn't the index, set it first:

df = df.set_index('id')

df

    other_id_1  other_id_2  other_id_3
id                                    
1          100         101         102
2          200         201         202
3          300         301         302

Now, call the pd.DataFrame constructor. You'll have to tile the index using np.repeat.

df_new = pd.DataFrame({'other_id' : df.values.reshape(-1,)}, 
                         index=np.repeat(df.index, len(df.columns)))
df_new

    other_id
id          
1        100
1        101
1        102
2        200
2        201
2        202
3        300
3        301
3        302

Answer 3

Well, if you haven't already, set id as the index:

>>> df
   id  other_id_1  other_id_2  other_id_3
0   1         100         101         102
1   2         200         201         202
2   3         300         301         302
>>> df.set_index('id', inplace=True)
>>> df
    other_id_1  other_id_2  other_id_3
id
1          100         101         102
2          200         201         202
3          300         301         302

Then, you can simply use pd.concat:

>>> df = pd.concat([df[col] for col in df])
>>> df
id
1    100
2    200
3    300
1    101
2    201
3    301
1    102
2    202
3    302
dtype: int64

And if you need the values sorted:

>>> df.sort_values()
id
1    100
1    101
1    102
2    200
2    201
2    202
3    300
3    301
3    302
dtype: int64
>>>

Answer 4

One more (or rather two):)

pd.melt(df, id_vars='id', value_vars=['other_id_1', 'other_id_2', 'other_id_3'], value_name='other_id')\
.drop('variable', 1).sort_values(by = 'id')

Option 2:

df.set_index('id').stack().reset_index(1,drop = True).reset_index()\ 
.rename(columns = {0:'other_id'})

Both ways you get

    id  other_id
0   1   100
1   1   101
2   1   102
3   2   200
4   2   201
5   2   202
6   3   300
7   3   301
8   3   302