Python Pandas - Compute indexer and find the previous index value if no exact match

To compute indexer and find the previous index value if no exact match, use the index.get_indexer() method with the method parameter set to ffill (forward fill).

Syntax

index.get_indexer(target, method='ffill')

Parameters

The key parameters are:

• target − Array-like values to find indexers for
• method − Set to 'ffill' to find the previous index value for non-exact matches

Example

Let's create a Pandas index and demonstrate how get_indexer() works with forward fill ?

import pandas as pd

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

# Display the Pandas index
print("Pandas Index...")
print(index)

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

# Compute indexer and mask using the "get_indexer"
# Find the previous index value if no exact match using the "method" parameter.
# The value is set "ffill"
print("\nGet the indexes...")
print(index.get_indexer([30, 20, 75, 80, 50, 59], method="ffill"))

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

Number of elements in the index:
7

Get the indexes...
[2 1 6 6 4 4]

How It Works

The get_indexer() method with method='ffill' works as follows:

• 30 − Exact match found at index 2
• 20 − Exact match found at index 1
• 75 − No exact match, uses previous value (70 at index 6)
• 80 − No exact match, uses previous value (70 at index 6)
• 50 − Exact match found at index 4
• 59 − No exact match, uses previous value (50 at index 4)

Conclusion

Use get_indexer() with method='ffill' to find the position of the nearest previous value when no exact match exists. This is useful for time series data and sorted indices where you need the last valid position.