Modify Variables In-Place

Using enumerate() to Modify Variables In-Place

enumerate() is a built-in Python function that returns both the index and the value of items in a sequence (like lists, tuples, or strings) during iteration. Its primary purpose is to keep a count of iterations. But when combined with sequences that support in-place modification (like lists), it can be a powerful tool to update values directly.

Basic Usage:

for index, value in enumerate(sequence):
    ...

Advantages:

1. Conciseness: Avoids manual index tracking. The index is provided automatically.
2. Readability: Makes it clear to a reader that both index and value are being used.
3. Direct Modification: Allows in-place updates, especially for lists.

Example:

Say you have a list of numbers, and you want to square all even numbers. Using enumerate(), you can do this in-place:

numbers = [1, 2, 3, 4, 5]

for idx, num in enumerate(numbers):
    if num % 2 == 0:
        numbers[idx] = num ** 2

After this loop, the numbers list will be updated to [1, 4, 3, 16, 5].

Caution:

• enumerate() works seamlessly with lists because they are mutable. However, for immutable sequences like strings and tuples, you can’t modify the original variable in-place.
• When using with dictionaries, it enumerates over the keys. For both keys and values, consider dict.items().

Custom Starting Index:

By default, enumeration starts at 0. However, you can provide a second argument to start from a different index:

for index, value in enumerate(sequence, start=1):
    ...

In the above code, enumeration starts at 1.