Python Pandas - Get integer location for requested label and find the previous index value if no exact match

To get the integer location for a requested label and find the previous index value if no exact match, use the get_loc() method with the method parameter set to "ffill" (forward fill).

Syntax

index.get_loc(key, method='ffill')

Creating a Pandas Index

First, let's create a Pandas Index with some integer values −

import pandas as pd

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

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

Finding Exact Matches

When the label exists in the index, get_loc() returns its integer position −

import pandas as pd

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

# Get exact locations
print("Location of 20:", index.get_loc(20))
print("Location of 50:", index.get_loc(50))

Location of 20: 1
Location of 50: 4

Finding Previous Index with Forward Fill

When no exact match is found, using method="ffill" returns the location of the previous (smaller) value −

import pandas as pd

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

# Get location of previous index if no exact match
# 45 doesn't exist, so it returns location of 40 (previous value)
print("Location for 45 (ffill):", index.get_loc(45, method="ffill"))

# 35 doesn't exist, so it returns location of 30 (previous value) 
print("Location for 35 (ffill):", index.get_loc(35, method="ffill"))

Location for 45 (ffill): 3
Location for 35 (ffill): 2

Complete Example

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)

# Get integer location from the given index
print("\nDisplay integer location from given index...\n", index.get_loc(20))
print("\nDisplay integer location from given index...\n", index.get_loc(50))

# Get the location of the previous index if no exact match
# The value is set "ffill" using the "method" parameter of the get_loc()
print("\nGet the location of the previous index if no exact match...\n", index.get_loc(45, method="ffill"))

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

Number of elements in the index...
7

Display integer location from given index...
1

Display integer location from given index...
4

Get the location of the previous index if no exact match...
3

How It Works

When searching for value 45 with method="ffill":

• 45 is not in the index [10, 20, 30, 40, 50, 60, 70]
• The method finds 40 (the largest value ? 45)
• Returns position 3 (index of 40)

Conclusion

The get_loc() method with method="ffill" is useful for finding the nearest previous value's position when an exact match doesn't exist. This is particularly helpful in time series data or when you need approximate lookups.