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.

Loop Control: break, continue, and else

Python provides keywords to change the normal flow of a loop:

1. break

The break statement exits the loop immediately, skipping any remaining iterations.

# Stop loop when we reach 5
for num in range(1, 10):
    if num == 5:
        break
    print(num)  # Prints: 1, 2, 3, 4

2. continue

The continue statement skips the rest of the current iteration and jumps directly to the next iteration.

# Skip the number 3
for num in range(1, 6):
    if num == 3:
        continue
    print(num)  # Prints: 1, 2, 4, 5

3. Loop else

A for or while loop can have an else block. The else block runs only if the loop completed normally without encountering a break.

# Finding a value in a list
numbers = [1, 2, 3, 4, 5]
search_for = 6

for num in numbers:
    if num == search_for:
        print("Found it!")
        break
else:
    print("Not found in the list.")  # This runs because the loop finished without breaking

Nested Loops & Breaking Outer Loops

A nested loop is a loop inside another loop.

# Multiplication table preview
for i in range(1, 4):       # Outer loop
    for j in range(1, 4):   # Inner loop
        print(f"{i}x{j}={i*j}", end="  ")
    print()  # Newline after inner loop finishes

Breaking from Outer Loops

A break statement only exits the innermost loop containing it. To break out of the outer loop from inside the inner loop, you can use a flag variable:

# Stop everything when we find a target sum
target = 5
found = False

for i in range(1, 4):
    for j in range(1, 4):
        if i + j == target:
            print(f"Target found: {i} + {j} = {target}")
            found = True
            break  # Breaks inner loop
    if found:
        break  # Breaks outer loop

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

Generating Star Patterns with Nested Loops

A structured and general way to print shapes is using nested loops based on two simple building blocks: Increasing Triangles and Decreasing Triangles.

The outer loop represents the rows (i).
The inner loop(s) represent columns (j).
We use print(..., end="") to print characters side-by-side, and a final empty print() to start a new row.

Let’s look at the exact range logic for each shape:

1. Increasing Triangle (Left-Aligned)

To print an increasing triangle, the outer loop i goes from 1 to n, and the inner loop j goes from 1 to i (representing an increasing number of stars per row):

n = 4
for i in range(1, n + 1):         # i ranges from 1 to n
    for j in range(1, i + 1):     # j ranges from 1 to i
        print("*", end="")
    print()  # New row

# Output:
# *
# **
# ***
# ****

2. Decreasing Triangle (Inverted Left-Aligned)

To print a decreasing triangle, the outer loop i goes from 1 to n, and the inner loop j counts down from n to i with a negative step of -1 (using range(n, i - 1, -1)):

n = 4
for i in range(1, n + 1):         # i ranges from 1 to n
    for j in range(n, i - 1, -1): # j counts from n down to i (step -1)
        print("*", end="")
    print()  # New row

# Output:
# ****
# ***
# **
# *

3. Right-Aligned Triangle

A right-aligned triangle is simply a combination of:

A decreasing triangle of spaces (j from i to n - 1).
An increasing triangle of stars (k from 1 to i).

n = 4
for i in range(1, n + 1):
    # 1. Decreasing triangle of spaces
    for j in range(i, n):         # j ranges from i to n-1
        print(" ", end="")
    # 2. Increasing triangle of stars
    for k in range(1, i + 1):     # k ranges from 1 to i
        print("*", end="")
    print()  # New row

# Output:
#    *
#   **
#  ***
# ****

Practice & Exercises

To reinforce what you’ve learned in this section (For Loops, While Loops, Range, and Conditions), practice with these interactive notebooks:

Follow-Along Practice

Practice range configurations, looping over characters, lists, and while loop iterations.💻 VS Code | 🚀 Colab | 📥 Download

Practice Exercises

Test your looping capabilities with challenges on summing series, while loop countdowns, and vowel filtering.💻 VS Code | 🚀 Colab | 📥 Download

What’s next?

Data structures

Learn about lists, dictionaries, and more

Getting started

Data types

Working with data

Control flow

Data structures

Packing & Unpacking

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

Loop Control: break, continue, and else

1. break

2. continue

3. Loop else

Nested Loops & Breaking Outer Loops

Breaking from Outer Loops

Common mistakes

Generating Star Patterns with Nested Loops

1. Increasing Triangle (Left-Aligned)

2. Decreasing Triangle (Inverted Left-Aligned)

3. Right-Aligned Triangle

Practice & Exercises

Follow-Along Practice

Practice Exercises

What’s next?

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

​Loop Control: break, continue, and else

​1. break

​2. continue

​3. Loop else

​Nested Loops & Breaking Outer Loops

​Breaking from Outer Loops

​Common mistakes

​Generating Star Patterns with Nested Loops

​1. Increasing Triangle (Left-Aligned)

​2. Decreasing Triangle (Inverted Left-Aligned)

​3. Right-Aligned Triangle

​Practice & Exercises

Follow-Along Practice

Practice Exercises

​What’s next?

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

Loop Control: break, continue, and else

1. break

2. continue

3. Loop else

Nested Loops & Breaking Outer Loops

Breaking from Outer Loops

Common mistakes

Generating Star Patterns with Nested Loops

1. Increasing Triangle (Left-Aligned)

2. Decreasing Triangle (Inverted Left-Aligned)

3. Right-Aligned Triangle

Practice & Exercises

What’s next?