Loops

Loops allow you to execute a block of code repeatedly. Python provides two main types of loops: for loops and while loops.

For Loops

The for loop is used to iterate over a sequence (We will cover this later, think of it as any ordered set of items). It can be a list, tuple, string, or any other iterable_a object. The loop iterates over each item in the sequence and executes the code block for each item.

Basic Syntax

for item in sequence:
    # Code block to execute for each item

Examples of For Loops

# Iterating over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit) # This will first print apple, then banana, and finally cherry

You can also get individual characters within a string. For example:

# Iterating over a string
message = "Python"
for character in message:
    print(character)

The range() Function

The range() function is commonly used with for loops to generate a sequence of numbers, which can be useful for iterating a specific number of times.

# range(stop) - Generates numbers from 0 to stop-1
for i in range(5):
    print(i)  # Outputs: 0, 1, 2, 3, 4 with each on a new line
    

You can also provide a start and stop value, essentially, it will start at the first number and end at the last.

# range(start, stop) - Generates numbers from start to stop-1
for i in range(2, 6):
    print(i)  # Outputs: 2, 3, 4, 5

You can also provide a optional step value, which will increment the number by that amount. For example, if you want to count by 2s, you can do this:

# range(start, stop, step) - Generates numbers from start to stop-1 with the specified step
for i in range(1, 10, 2):
    print(i)  # Outputs: 1, 3, 5, 7, 9

Nested For Loops

You can also place one loop within another loop's block. Reminder: The inner loop will complete all of its operations before the outer loop can run again.

# Nested for loops to create a multiplication table
for i in range(1, 4):  # Outer loop
    for j in range(1, 4):  # Inner loop
        print(f"{i} x {j} = {i * j}")
    print("---")  # Separator between groups

While Loops

The while loop executes a block of code as long as a specified condition is True.

Basic Syntax

while condition:
    # Code block to execute while the condition is True

Examples of While Loops

# Basic while loop
count = 0
while count < 5:
    print(count)
    count += 1  # Important: increment the counter to avoid an infinite loop

You can also use a while loop to get user input until a certain condition is met:

password = ""
while password != "secret":
    password = input("Enter the password: ")
    if password != "secret":
        print("Incorrect password. Try again.")
print("Access granted!")

Infinite Loops

An infinite loop occurs when the condition in a while loop never becomes False. This can happen by mistake or intentionally:

# Intentional infinite loop (uncomment to run)
# while True:
#     print("This will run forever unless interrupted")
#     # Use Ctrl+C to interrupt the program

In practice, infinite loops are often used with a break statement to exit the loop based on a condition inside the loop:

# Infinite loop with a break statement
counter = 0
while True:
    print(counter)
    counter += 1
    if counter >= 5:
        break  # Exit the loop when counter reaches 5

Loop Control Statements

Python provides statements to control the flow of loops:

break Statement

The break statement immediately terminates the loop and transfers control to the statement following the loop. It can be used in both for and while loops.

# Using break in a for loop
for i in range(10):
    if i == 5:
        break  # Exit the loop when i is 5
    print(i)

print("Loop exited") 
# This will be printed after the loop exits
# notice the last value printed was 4, therefore
# the loop exited before printing 5

# Using break in a while loop
count = 0
while True:
    print(count)
    count += 1
    if count >= 5:
        break  # Exit the loop when count reaches 5

continue Statement

The continue statement skips the rest of the code inside the loop for the current iteration and moves to the next iteration:

# Using continue in a for loop
for i in range(10):
    # Check if the number is even:
    # the remainder of any even number is 0, 
    # therefore we can use the remainder operator (%) to check if the number is even
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)  # This only prints odd numbers

It can also be used in a while loop

# Using continue in a while loop
count = 0
while count < 10:
    count += 1
    if count % 2 == 0:
        continue  # Skip even numbers
    print(count)  # This only prints odd numbers

else Clause in Loops

Both for and while loops can have an optional else clause that executes when the loop completes normally (without encountering a break statement):

# for loop with else
for i in range(5):
    print(i)
else:
    print("Loop completed normally")

# for loop with else and break
for i in range(5):
    print(i)
    if i == 2:
        break
else:
    print("This won't execute because the loop was exited with break")

# while loop with else
count = 0
while count < 5:
    print(count)
    count += 1
else:
    print("Loop completed normally")

Common Loop Patterns

Enumerating Items

The enumerate() function adds a counter to an iterable and returns it as an enumerate object. This is useful when you need both the index and the value of items in a list. A index is a number that represents a current position in a sequence (they start start at 0). We will explain this further, you can always come back here.

# Using enumerate to get both index and value
fruits = ["apple", "banana", "cherry"]
for index, fruit in enumerate(fruits):
    print(f"Index {index}: {fruit}")

Best Practices

1. Choose the right loop: Use for loops when you know the number of iterations in advance or want to iterate over a sequence; use while loops when the iteration continues until a condition is met.

2. Avoid infinite loops: Make sure the condition in a while loop can eventually become False, or include a break statement.

3. Minimize work inside loops: Move calculations outside the loop if they don't change with each iteration.

# Bad practice: Doing unnecessary work inside the loop
numbers = [1, 2, 3, 4, 5]
total = 0
for num in numbers:
    total += num
    average = total / len(numbers)  # This is calculated unnecessarily in each iteration
    
# Good practice: Do the calculation once after the loop
numbers = [1, 2, 3, 4, 5]
total = 0
for num in numbers:
    total += num
average = total / len(numbers)  # Calculate once after the loop

4. Use appropriate control statements: Use break to exit a loop early, and continue to skip iterations when needed.

5. Consider readability: Sometimes a more verbose loop is more readable than a concise but complex one.

Loops are essential for automating repetitive tasks and processing collections of data. In the next lesson, we'll explore comprehensions, which provide a more concise way to create lists, dictionaries, and sets using loops.

Footnotes

• a - An iterable is an object that can be looped over, such as a list, tuple, string, or dictionary.