Python - Ways to find indices of value in list

Finding indices of specific values in a list is a common task in Python programming. While the index() method finds the first occurrence, there are several ways to find all indices where a particular value appears in a list.

Using enumerate()

The enumerate() function returns both index and value, making it perfect for this task ?

# initializing list
numbers = [1, 3, 4, 3, 6, 7]
print("Original list :", numbers)

# using enumerate() to find indices for 3
indices = [i for i, value in enumerate(numbers) if value == 3]
print("Indices of 3 :", indices)

Original list : [1, 3, 4, 3, 6, 7]
Indices of 3 : [1, 3]

Using List Comprehension with range()

This approach uses range() to iterate through indices and checks values at each position ?

# initializing list
numbers = [1, 3, 4, 3, 6, 7]
print("Original list :", numbers)

# using list comprehension with range()
indices = [i for i in range(len(numbers)) if numbers[i] == 3]
print("Indices of 3 :", indices)

Original list : [1, 3, 4, 3, 6, 7]
Indices of 3 : [1, 3]

Using filter() Function

The filter() function can filter indices based on a condition using a lambda function ?

# initializing list
numbers = [1, 3, 4, 3, 6, 7]
print("Original list :", numbers)

# using filter() to find indices for 3
indices = list(filter(lambda x: numbers[x] == 3, range(len(numbers))))
print("Indices of 3 :", indices)

Original list : [1, 3, 4, 3, 6, 7]
Indices of 3 : [1, 3]

Using Traditional Loop

A simple for loop approach that manually checks each element ?

# initializing list
numbers = [1, 3, 4, 3, 6, 7]
print("Original list :", numbers)

# using traditional loop to find indices for 3
indices = []
for i in range(len(numbers)):
    if numbers[i] == 3:
        indices.append(i)

print("Indices of 3 :", indices)

Original list : [1, 3, 4, 3, 6, 7]
Indices of 3 : [1, 3]

Comparison

Conclusion

The enumerate() method is generally the most Pythonic and readable approach for finding all indices of a value. Use list comprehension for simple one-liners or traditional loops when you need more complex logic during iteration.