Python Pandas - Compute indexer and mask for new index given the current index

To compute indexer and mask for new index given the current index, use the get_indexer() method in Pandas. This method returns an array of indices corresponding to the positions of the target elements in the original index.

Syntax

Index.get_indexer(target, method=None, limit=None, tolerance=None)

Parameters

The key parameters are ?

• target ? The target values to find indices for
• method ? Method to use for inexact matches (pad, backfill, nearest)
• limit ? Maximum number of consecutive NaN values to forward/backward fill
• tolerance ? Maximum distance between original and target values

Example

Let's create a Pandas index and compute indexer positions for target values ?

import pandas as pd

# Creating Pandas index
index = pd.Index([10, 20, 30, 40, 50, 60, 70])

# Display the Pandas index
print("Pandas Index...\n", index)

# Return the number of elements in the index
print("\nNumber of elements in the index...\n", index.size)

# Compute indexer and mask
# Marked by -1, as it is not in index
print("\nGet the indexes...\n", index.get_indexer([30, 40, 90, 100, 50]))

Pandas Index...
 Int64Index([10, 20, 30, 40, 50, 60, 70], dtype='int64')

Number of elements in the index...
7

Get the indexes...
[ 2  3 -1 -1  4]

How It Works

The get_indexer() method returns an array where ?

• Positive integers represent the position of found elements
• -1 indicates elements not found in the original index
• For target [30, 40, 90, 100, 50], the result is [2, 3, -1, -1, 4]

Using Different Methods

You can use different methods for handling missing values ?

import pandas as pd

index = pd.Index([10, 20, 30, 40, 50])
target = [15, 25, 35, 45]

print("Exact match (default):")
print(index.get_indexer(target))

print("\nUsing 'pad' method (forward fill):")
print(index.get_indexer(target, method='pad'))

print("\nUsing 'backfill' method:")
print(index.get_indexer(target, method='backfill'))

Exact match (default):
[-1 -1 -1 -1]

Using 'pad' method (forward fill):
[0 1 2 3]

Using 'backfill' method:
[1 2 3 4]

Conclusion

The get_indexer() method efficiently finds positions of target values in a Pandas index. Use different methods like 'pad' or 'backfill' for approximate matching when exact matches are not found.