Iterating Over a List and Adding to Another List in Python

Python offers several ways to iterate over a list and add items to a new list. This is useful for filtering, transforming, or extracting specific elements.

Using a for-Loop to Iterate and Add to a List

The for-loop is one of the most common ways to iterate through a list in Python. Here’s a simple example:

original_list = [1, 2, 3, 4, 5]
new_list = []
for item in original_list:
    new_list.append(item * 2)
print(new_list)

[2, 4, 6, 8, 10]

In this example, each element from original_list is doubled and added to new_list using append().

Using List Comprehension for Compact Code

List comprehensions are a powerful way to create new lists. They are often more concise than for-loops.

original_list = [1, 2, 3, 4, 5]
new_list = [item * 2 for item in original_list]
print(new_list)

[2, 4, 6, 8, 10]

This one-line comprehension achieves the same result as the previous loop but with more readable code.

To learn more about looping and comprehensions, see Looping Through Lists to Create a Dictionary in Python.

Adding Elements Conditionally

Sometimes, you may want to add only certain elements. A conditional expression inside a list comprehension lets you filter items:

original_list = [1, 2, 3, 4, 5]
filtered_list = [item for item in original_list if item % 2 == 0]
print(filtered_list)

[2, 4]

In this example, only even numbers are added to filtered_list.

Using extend() for List Concatenation

When adding multiple elements to a list, extend() is more efficient than using append() in a loop:

new_list = []
new_list.extend([item * 2 for item in original_list])
print(new_list)

[2, 4, 6, 8, 10]

Using extend() adds all elements from the comprehension at once, improving performance with larger lists.

Conclusion

Python offers flexible ways to iterate and add items to a list. Choose from for-loops, list comprehensions, or extend() based on your needs for readability and efficiency.

For more techniques, explore Python’s documentation on list comprehensions.