Building New Lists With Loops

Building New Lists With Loops

In this tutorial, we'll create a new list from an existing sequence.

A common pattern is to start with one list and build another.

numbers = [1, 2, 3]
doubled = []

for number in numbers:
    doubled.append(number * 2)

print(doubled)

Output:

[2, 4, 6]

The Pattern

The pattern has three steps:

1. Create an empty list 2. Loop over the original list 3. Append new values to the result list

Example:

names = ["ada", "grace", "linus"]
capitalized = []

for name in names:
    capitalized.append(name.capitalize())

print(capitalized)

Filtering While Building

You can choose which items to include:

numbers = [-2, 5, 0, 8, -1]
positive_numbers = []

for number in numbers:
    if number > 0:
        positive_numbers.append(number)

print(positive_numbers)

Output:

[5, 8]

Transforming and Filtering Together

You can also change values before appending them:

words = ["ada", "", "grace"]
clean_names = []

for word in words:
    if word != "":
        clean_names.append(word.capitalize())

print(clean_names)

This skips empty strings and capitalizes the names that remain.

Common Mistake

Create the empty list before the loop, not inside it:

for number in numbers:
    doubled = []  # wrong place
    doubled.append(number * 2)

This resets the list every time.

Practice

Start with a list of words. Build a new list containing only words longer than five characters.