Python – Character indices Mapping in String List

When working with string lists, you may need to map each character to the indices where it appears. Python provides an efficient solution using defaultdict from the collections module combined with enumeration and set operations.

Syntax

from collections import defaultdict

result = defaultdict(set)
for index, string in enumerate(string_list):
    for char in string.split():
        result[char].add(index + 1)

Example

Below is a demonstration that maps each character to its string positions ?

from collections import defaultdict

my_list = ['p y t h o n', 'i s', 'f u n', 't o', 'l e a r n']

print("The list is :")
print(my_list)

my_result = defaultdict(set)

for index, element in enumerate(my_list):
    for sub in element.split():
        my_result[sub].add(index + 1)

my_result = {key: list(val) for key, val in my_result.items()}

print("The result is :")
print(my_result)

The list is :
['p y t h o n', 'i s', 'f u n', 't o', 'l e a r n']
The result is :
{'p': [1], 'y': [1], 't': [1, 4], 'h': [1], 'o': [1, 4], 'n': [1, 3, 5], 'i': [2], 's': [2], 'f': [3], 'u': [3], 'l': [5], 'e': [5], 'a': [5], 'r': [5]}

How It Works

The algorithm follows these steps:

• Import defaultdict: Creates a dictionary with default set values

• Enumerate strings: Gets both index and string content using enumerate()

• Split characters: Uses split() to separate space-delimited characters

• Map indices: Adds each character's position (index + 1) to its corresponding set

• Convert to lists: Dictionary comprehension converts sets to lists for final output

Key Points

• defaultdict(set) automatically creates an empty set for new keys

• enumerate() provides both index and value during iteration

• Using sets prevents duplicate indices for the same character

• Index + 1 creates 1-based positioning instead of 0-based

Conclusion

This approach efficiently maps characters to their string positions using defaultdict and sets. The technique is particularly useful for text analysis and character frequency tracking in structured string data.