Loops

What are loops?

Loops let you repeat code without writing it multiple times. Instead of copying and pasting, you tell Python to repeat the code for you. Without loops:

print("Hello!")
print("Hello!")
print("Hello!")
print("Hello!")
print("Hello!")

With loops:

for i in range(5):
    print("Hello!")

Both do the same thing, but the loop is much cleaner!

For loops

The for loop is the most common loop in Python. Let’s start simple:

Repeat a specific number of times

# Print numbers 0 through 4
for i in range(5):
    print(i)

# Output:
# 0
# 1
# 2
# 3
# 4

Python starts counting at 0, not 1. This is called “zero-indexing”. So range(5) gives you 0, 1, 2, 3, 4 (five numbers total).

Count from different starting points

# Count from 1 to 5
for i in range(1, 6):
    print(i)
# Output: 1, 2, 3, 4, 5

# Count by 2s
for i in range(0, 10, 2):
    print(i)
# Output: 0, 2, 4, 6, 8

Loop through text

You can loop through each character in a string:

name = "Python"
for letter in name:
    print(letter)

# Output:
# P
# y
# t
# h
# o
# n

Loop through a list (preview)

We’ll learn more about lists later, but here’s a preview:

colors = ["red", "blue", "green"]
for color in colors:
    print(f"I like {color}")

# Output:
# I like red
# I like blue
# I like green

While loops

A while loop continues as long as a condition is true:

count = 0
while count < 5:
    print(f"Count is {count}")
    count = count + 1  # Increase count by 1

# Output:
# Count is 0
# Count is 1
# Count is 2
# Count is 3
# Count is 4

Always make sure your while loop will eventually stop! If you forget to update the variable, it will run forever.

Common mistakes

Forgetting the colon

# Wrong - missing colon
for i in range(5)
    print(i)

# Right - colon after the loop line
for i in range(5):
    print(i)

Wrong indentation

# Wrong - not indented
for i in range(3):
print(i)

# Right - indented inside the loop
for i in range(3):
    print(i)

Off-by-one errors

# Wrong - only goes to 4
for i in range(5):
    print(f"Item {i}")  # 0, 1, 2, 3, 4

# Right - if you want 1-5
for i in range(1, 6):
    print(f"Item {i}")  # 1, 2, 3, 4, 5

Data structures

Learn about lists, dictionaries, and more

Getting started

Data types

Working with data

Control flow

Data structures

What are loops?

For loops

Repeat a specific number of times

Count from different starting points

Loop through text

Loop through a list (preview)

While loops

Common mistakes

Data structures

​What are loops?

​For loops

​Repeat a specific number of times

​Count from different starting points

​Loop through text

​Loop through a list (preview)

​While loops

​Common mistakes

Data structures

What are loops?

For loops

Repeat a specific number of times

Count from different starting points

Loop through text

Loop through a list (preview)

While loops

Common mistakes