5 Best Ways to Obtain a New Index in pandas for Selected Values

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💡 Problem Formulation: When working with pandas in Python, we often select a subset of data from a DataFrame. Post-selection, we may want our data to have a fresh index that reflects the new ordering, starting again from 0. Let’s say we have a DataFrame with various indices, and after applying some conditions, we get a subset. Our goal is to generate a new index for this subset and possibly retain the old index for reference.

Method 1: Reset Index Using reset_index()

The reset_index() function in pandas is designed to reset the index of the DataFrame. It allows for the previous index to be added as a new column if needed, or to be completely discarded. This is particularly useful when we want to revert back to the default numerical indexing after performing operations that alter the row order or reduce the dataset.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]}, index=['x', 'y', 'z'])

# Selecting subset
subset = df[df['A'] > 1]

# Resetting index
new_df = subset.reset_index(drop=True)

print(new_df)

Output:

   A  B
0  2  5
1  3  6

The reset_index(drop=True) resets the index without inserting a new column. The old index data (‘x’, ‘y’, ‘z’) is discarded, and a new default integer index is created for the selected subset.

Method 2: Reindexing with reindex()

Reindexing with reindex() gives more control over the new index. Unlike resetting the index which creates a new default integer index, reindexing allows us to specify the new index values explicitly. It’s handy when we want to adhere to a particular index pattern or insert missing values for absent index labels.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]}, index=['x', 'y', 'z'])

# Define new index
new_index = [0, 1]

# Reindexing subset
new_df = df.reindex(new_index)

print(new_df)

Output:

    A   B
0 NaN NaN
1 NaN NaN

In this example, reindexing attempts to align the DataFrame to the new index [0, 1]. Since these weren’t present in the original DataFrame, it results in a DataFrame with NaN values.

Method 3: Using Index.to_series() and reset_index()

Combining Index.to_series() and reset_index() is useful for cases where we want to create a new index while retaining the existing one as a column. The index is first converted to a series, upon which the reset_index() is called.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'A': [1, 2, 3]})

# Select our subset
subset = df[df['A'] > 1]

# Convert index to a series and reset it
subset_index = subset.index.to_series().reset_index(drop=True)

print(subset_index)

Output:

0    1
1    2
dtype: int64

This code snippet selects elements from the DataFrame based on a condition, then converts the subset’s index into a series and resets it. The original index is preserved in the values of the new series.

Method 4: List Comprehension for Creating New Index

List comprehension in Python can be used to generate a list representing a new index. This is a Pythonic way to create a new list based on the length of the subset and does not utilize pandas-specific functions, offering simplicity and flexibility.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'A': [1, 2, 3]})

# Select our subset
subset = df[df['A'] > 1]

# Use  list comprehension  to create new index
subset.index = [i for i in range(len(subset))]

print(subset)

Output:

   A
0  2
1  3

In this snippet, we are manually assigning new indices by creating a list with a length equal to that of the subset using list comprehension. The new index is a simple sequential numerical series starting at 0.

Bonus One-Liner Method 5: Lambda with apply()

Using the apply() function with a lambda can also generate a new index on the fly. This one-liner is succinct but perhaps less readable to those unfamiliar with lambda functions.

Here’s an example:

import pandas as pd

# Sample DataFrame
df = pd.DataFrame({'A': [1, 2, 3]}, index=['x', 'y', 'z'])

# One-liner to reset index using apply and lambda
df = df[df['A'] > 1].apply(lambda x: x.reset_index(drop=True))

print(df)

Output:

   A  B
0  2  5
1  3  6

With the lambda function inside apply(), each subset row’s index is replaced with a new index. This approach is compact but might be less performant due to row-wise application.

Summary/Discussion

  • Method 1: Reset Index. Simple and direct. Convenient if the old index is not required. It might be less efficient if the old index needs to be kept.
  • Method 2: Reindex. Offers precise control over new index values. Ideal for custom indices. Can be confusing when indexes align improperly.
  • Method 3: Index.to_series() and reset_index(). Good for preserving the old index in a column. Adds an extra step of converting the index to a series first.
  • Method 4: List Comprehension. Pythonic and simple. It gives a new integer index with no dependencies on pandas’ methods.
  • Bonus Method 5: Lambda with apply(). Compact one-liner. Useful for quick one-time operations but can be inefficient and obscure for complex tasks.