Passing columns to rows on python pandas

Question

I have a data frame like this

 id  a  b  c
101  0  3  0
102  2  0  5
103  0  1  4

and I want something like this

 id  letter  num
101     a     0
101     b     3
101     c     0
102     a     2
102     b     0
102     c     5
103     a     0
103     b     1
103     c     4

I want to pass the column names to values of a row with their corresponding id and the result of the df.

I was trying to make it in a loop, introducing each element according to its id, but it's horrible. Is there an easy way to do this?

Answer 1

You could melt and then sort:

>>> pd.melt(df, id_vars='id', value_vars=['a','b','c'], 
            var_name='letter', value_name='num').sort_values('id')
    id letter  num
0  101      a    0
3  101      b    3
6  101      c    0
1  102      a    2
4  102      b    0
7  102      c    5
2  103      a    0
5  103      b    1
8  103      c    4

If you want to reset the index, you could always use .reset_index(drop=True) on the returned DataFrame.