How can I go from this Pandas dataframe:

```
     0    1
0  A11  A12
1  A21  A22
2  A31  A32

```

to this:

```
    0   1    2
0  r1  c1  A11
1  r1  c2  A12
2  r2  c1  A21
3  r2  c2  A22
4  r3  c1  A31
5  r3  c2  A32

```

rX is the row number (+1) of the element from the previous dataframe

cX is the column number (+1) of the element from the previous dataframe

This is the code that I was trying to write but failed:

```
# Number of samples and features
n_samples = len(df)
n_features = len(df.columns)

data = pd.DataFrame([[s for s in range(1, n_samples+1)],
                    [m for m in range(1, n_features+1)],
                    [something in here]])

```

I'm really struggling with list comprehension/functional programming. Thanks in advance!