Python - Index Directory of Elements

When it is required to index directory of elements in a list, list comprehension along with set operator is used. This creates a dictionary mapping each unique element to all its index positions in the list.

Syntax

{key: [index for index, value in enumerate(list) if value == key] 
 for key in set(list)}

Example

Below is a demonstration of creating an index directory ?

my_list = [81, 36, 42, 57, 68, 12, 26, 26, 38]

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

my_result = {key: [index for index, value in enumerate(my_list) if value == key]
             for key in set(my_list)}

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

The list is :
[81, 36, 42, 57, 68, 12, 26, 26, 38]
The result is :
{36: [1], 68: [4], 38: [8], 42: [2], 12: [5], 81: [0], 57: [3], 26: [6, 7]}

How It Works

• set(my_list) creates unique elements: {81, 36, 42, 57, 68, 12, 26, 38}

• enumerate(my_list) provides (index, value) pairs for iteration

• List comprehension finds all indices where value == key

• Dictionary comprehension maps each unique element to its index list

Practical Example

Finding positions of specific grades in a student list ?

grades = ['A', 'B', 'A', 'C', 'B', 'A', 'D']

grade_positions = {grade: [i for i, g in enumerate(grades) if g == grade]
                   for grade in set(grades)}

print("Grade positions:")
for grade, positions in sorted(grade_positions.items()):
    print(f"Grade {grade}: positions {positions}")

Grade positions:
Grade A: positions [0, 2, 5]
Grade B: positions [1, 4]
Grade C: positions [3]
Grade D: positions [6]

Alternative Using defaultdict

For large lists, using defaultdict might be more efficient ?

from collections import defaultdict

my_list = [81, 36, 42, 57, 68, 12, 26, 26, 38]

index_dict = defaultdict(list)
for index, value in enumerate(my_list):
    index_dict[value].append(index)

print("Using defaultdict:")
print(dict(index_dict))

Using defaultdict:
{81: [0], 36: [1], 42: [2], 57: [3], 68: [4], 12: [5], 26: [6, 7], 38: [8]}

Conclusion

Dictionary comprehension with set() and enumerate() efficiently creates an index directory. For large datasets, consider using defaultdict for better performance as it requires only one pass through the list.