Python | Pandas Series.str.index()

The Series.str.index() method in Pandas is used to find the lowest first occurrence index of a specified substring in each element of the Series. This method returns -1 if the substring is not found in any element of the Series.

Syntax

Series.str.index(sub, start=0, end=len(string))

• sub: A string value representing the substring to search for.
• start: (optional) An integer value representing the starting index of the search range in each element of the Series.
• end: (optional) An integer value representing the ending index of the search range in each element of the Series.

Parameters

• sub: It is a required parameter and represents the substring to be searched.
• start: It is an optional parameter that specifies the starting index of the search range.
• end: It is an optional parameter that specifies the ending index of the search range.

Return Value

The method returns a Series containing the lowest first occurrence index of the specified substring in each element. If the substring is not found, it returns -1.

Example

Let's consider a Pandas Series containing names of fruits:

import pandas as pd

data = {'fruits': ['apple', 'banana', 'orange', 'grape']}
series = pd.Series(data['fruits'])

Example 1: Find index of 'an' in each element

sub_string = 'an'
result = series.str.index(sub_string)
print(result)

Output:

0    1
1    1
2   -1
3    2
dtype: int64

Example 2: Find index of 'phon' in each element starting from index 2 to 5

start_index = 2
end_index = 5
sub_string = 'phon'
result = series.str.index(sub_string, start=start_index, end=end_index)
print(result)

Output:

0   -1
1   -1
2   -1
3   -1
dtype: int64

In example 1, the method finds the lowest first occurrence index of 'an' in each element of the Series. The result is a new Series where the first element has index 1, the second element has index 1, the third element does not have 'an' and the fourth element has index 2.

In example 2, the method tries to find 'phon' in the range from index 2 to 5 in each element. As 'phon' is not present in any of the elements, the returned Series contains -1 in all the positions.