Lists & loops · step 13 of 22

For loops

10 minDo something once for every item — the pattern behind almost everything.

For every item

A for loop runs its block once per item in a sequence.

colors = ["red", "green", "blue"]
for color in colors:
    print(color)

color is the loop variable: it holds the current item on each trip around. Name it for one thing — for student in students reads far better than for x in students.

Counting with range()

for number in range(5):
    print(number)      # 0 1 2 3 4 — range stops before the end

for number in range(1, 4):
    print(number)      # 1 2 3 — start, then stop before 4

Building a total

This pattern shows up everywhere:

prices = [2.5, 3, 4.25]
total = 0
for price in prices:
    total = total + price
print(total)     # 9.75

Start the total at 0, add each item inside the loop, and print after the loop finishes. Indentation is what tells Python which lines are inside the loop.

Remember this

for runs the block once per item
range(5) counts 0 to 4
Indented lines under the for are the repeated ones

Quick quiz

Three questions. 3 correct answers and you have the idea — you can retry as often as you like.

1. What does range(3) produce?
2. How many times does `for x in [2, 4, 6]:` run its block?
3. Where does the line that adds a price to the total belong?

Answer all three to check them.

Now write it yourself

0/3 passed

Practice 1

The list `numbers` is ready. Print each number doubled, one per line.

practice_1.py

Given for you

numbers = [4, 8, 15, 16]

Practice 2

The list `prices` is ready. Add every price together and print the total.

practice_2.py

Given for you

prices = [2.5, 3, 4.25]

Practice 3

Print the numbers 1 to 5 — one per line — using range().

practice_3.py

Finish with two of three quiz answers correct and every exercise passing, and this lesson is marked complete.