Adding and Insert a column

I have a dataframe in where I have pulled out 1% of the dataset to create a new column and assign a value to it. Now, I would like to add that data back to the intial dataframe. How would I go about that?

Thanks

I have tried using the left join, but it seems to be adding more row than inserting it back again. Any help is appreciated.

>Solution :

You have to use index alignment:

df = pd.read_csv('test.csv')

df1 = df.sample(frac=0.05, random_state=42, replace=True)
df1["Flag"] = 'Y'

#                            HERE --v              --v
df_merged = pd.merge(df, df1, left_index=True, right_index=True, how='left')

But a more straightforward way is:

df.loc[df.sample(frac=0.05).index, 'Flag'] = 'Y'

With numpy:

df['Flag'] = np.where(df.index.isin(df.sample(frac=0.05).index), 'Y', 'N')