Looping Over Lists

Why this matters

The point of a list is to do something with every item: add them up, find the biggest, show them to the user. A for loop does that, and it reads almost like English.

The idea

prices = [4, 9, 2]

for price in prices:
    print(price * 2)

for price in prices: runs the block once for each item, putting that item in price. No index, no range, no counter to mess up.

Python has shortcuts for common jobs:

print(sum(prices))   # 15
print(max(prices))   # 9
print(min(prices))   # 2

When you need the position as well as the item, use enumerate:

for position, price in enumerate(prices, start=1):
    print(f"{position}. {price}")   # 1. 4   2. 9   3. 2

Try it

Write a loop that prints every item of ["a", "b", "c"] in capitals. Then use enumerate to number them starting from 1.

Your turn

The starter reads some whole numbers from one line into a list called numbers (leave those lines alone: they use a loop you already know). Print three lines:

Total: 18
Largest: 8
Evens: 2

That is the sum, the biggest number, and how many of the numbers are even (for 5 3 8 2: total 18, largest 8, evens 2).

Recap

• for item in list: visits every item in order.
• sum, max, min and len summarise a list in one go.
• Use a counter or an accumulator when you need something the built-ins do not do, like counting evens.

Common mistakes

• Changing the list while looping over it, such as removing items. Loop over a copy or build a new list instead.
• Looping over len(numbers) by habit. for number in numbers: is simpler and safer.
• Resetting the counter inside the loop, so it never grows.