Week Six

This week we learn about loops — a way to repeat code multiple times without writing the same lines over and over.

Presentation

Why Loops?

Sometimes you want your program to do the same thing many times. Instead of copying and pasting code, you use a loop.

# Without a loop — repetitive!
print("Hello!")
print("Hello!")
print("Hello!")
 
# With a loop — much cleaner!
for i in range(3):
    print("Hello!")

For Loops

A for loop repeats code for each item in a sequence.

Looping Over a List

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

Output:

red
blue
green

Using range()

range(n) creates a sequence of numbers from 0 to n - 1.

for i in range(5):
    print(i)

Output:

0
1
2
3
4

Looping with an Index

You can use range() with len() to loop through a list by index:

fruits = ["apple", "banana", "cherry"]
 
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

Output:

0: apple
1: banana
2: cherry

While Loops

A while loop keeps running as long as a condition is True.

count = 0
 
while count < 3:
    print(f"Count is {count}")
    count = count + 1

Output:

Count is 0
Count is 1
Count is 2

Looping and Lists Together

Loops are especially useful with lists:

scores = [85, 92, 78]
total = 0
 
for score in scores:
    total = total + score
 
print(f"Total: {total}")
print(f"Average: {total / len(scores)}")

Activity

Try writing a loop that prints your name 5 times, then one that prints every item in a list of your favorite foods!

foods = ["pizza", "sushi", "tacos"]
 
for food in foods:
    print(f"I love {food}!")