How Do I Add a New Column to an Existing Pandas DataFrame?

Adding a Column to an Existing DataFrame

You have an indexed DataFrame with named columns and rows that are not continuous numbers. To add a new column, 'e', to the existing DataFrame without altering the data, follow these steps:

1. Determine DataFrame Length

Estimate the length of your DataFrame using the following code:

sLength = len(df1['a'])

2. Create a Series with the Column Values

Create a Series with randomly generated values that match the length of your DataFrame:

e_values = pd.Series(np.random.randn(sLength))

3. Specify the Target Column

Indicate that you want to add the new values to the 'e' column:

df1['e'] = ...

4. Populate the New Column

Assign the Series of values to the new 'e' column using one of the following methods:

a. Current Best Method (as of 2017)

df1 = df1.assign(e=e_values.values)

b. Method Prior to 2017

df1['e'] = e_values

By following these steps, you can successfully add a new column to your DataFrame while preserving the original data.

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