Combine index and columns and keep value

I have a DataFrame as below

    A   B   C   
1   1   2   3    
2   4   2   5 

And I want to combine index and column into a new index while keeping the value from the previous column + index as below

    Value
1A  1  
1B  2
1C  3
2A  4
2B  2
2C  5

I know I can iterate through it with df.iterrows() and create a new DataFrame from that, but I’m working with a huge dataset and it is too inefficient. Dataset is tens of millions of observations.

>Solution :

Use DataFrame.stack with list comprehension:

df = df.stack().to_frame('Value')
df.index = [f'{a}{b}' for a, b in df.index]
print (df)
    Value
1A      1
1B      2
1C      3
2A      4
2B      2
2C      5

Or use numpy solution with ravel:

c = np.tile(df.columns, len(df))
i = np.repeat(df.index, len(df.columns))

df = pd.DataFrame({'value': df.to_numpy().ravel()}, index=[f'{a}{b}' for a, b in zip(i, c)])
print (df)
    value
1A      1
1B      2
1C      3
2A      4
2B      2
2C      5