CyberStars

List Comprehension

By now we've written a lot of loops that create a new list from an existing one. Something like: "go through a list, do something with each element, and put the result in a new list"

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

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

print(doubled)

Output [2, 4, 6, 8, 10]

This pattern is so common that Python gives us a shortcut: list comprehension

numbers = [1, 2, 3, 4, 5]
doubled = [n * 2 for n in numbers]
print(doubled)

Output [2, 4, 6, 8, 10]. Same result, one line instead of four. The syntax is: [expression for variable in collection]

Let's break it down:

  • n * 2 is the expression, what we want to do with each element
  • for n in numbers is the loop, going through each element
  • The square brackets [] tell Python to put the results in a new list

We can also add an if to filter elements

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = [n for n in numbers if n % 2 == 0]
print(evens)

Output [2, 4, 6, 8, 10]. Only the numbers where n % 2 == 0 made it into the new list

names = ["Tommy Vercetti", "Lance", "Cortez", "Phil Cassidy"]
long_names = [name for name in names if len(name) > 6]
print(long_names)

Output ['Tommy Vercetti', 'Phil Cassidy']. Only names longer than 6 characters survived

We can transform and filter at the same time

names = ["tommy", "lance", "cortez"]
upper = [name.upper() for name in names]
print(upper)

Output ['TOMMY', 'LANCE', 'CORTEZ']

prices = [10, 25, 5, 40, 15, 30]
discounted = [price * 0.9 for price in prices if price > 20]
print(discounted)

Output [22.5, 36.0, 13.5, 27.0]. We took only prices above 20, then applied a 10% discount

When to use it? List comprehensions are great for simple transformations. If the logic gets complicated (nested ifs, multiple lines of processing), use a regular for loop instead. Readability is more important than being clever

scores = [85, 42, 91, 67, 38, 74, 95]
passed = [s for s in scores if s >= 50]
print(passed)

Clean. Easy to read. Perfect use case

Mission: Data Filter

You have a list of sensor readings named readings. Using list comprehension for each step:

  1. Create high: only the readings above 80
  2. Create doubled: every reading multiplied by 2
  3. Create passing: only the readings that are 50 or more
  4. Print all three lists, each on its own line

Output

[95, 88, 92]
[150, 60, 190, 176, 84, 184]
[75, 95, 88, 92]