Iterating with For and While Loops

1. The while Loop: Repeating Code While a Condition is True

A while loop executes a block of code as long as a specified condition is true. Think of it like a security check: "while the door is locked, keep checking". The loop stops only when the condition becomes false.

Here is a flowchart that shows this logic visually:

In C, you use curly braces {} to define the loop's body. As we've seen before, Python uses colons : and indentation.

Now, let's see a while loop in action. This first video will walk you through the basic syntax and the importance of having a condition that eventually becomes false to avoid an infinite loop.

Controlling Loop Flow: break and continue

Sometimes you need to exit a loop early or skip the current iteration. Just like in C, Python uses the break and continue statements for this.

• break: Immediately stops and exits the loop.
• continue: Stops the current iteration and jumps to the beginning of the next one.

This next part of the video demonstrates how to use break to stop a loop when a certain condition is met and continue to skip an iteration.

timer = 10

while timer > 0:
    print(timer)
    timer -= 1  # This is the same as timer = timer - 1

print("Blast off!")

# Expected output:
# 10
# 9
# 8
# 7
# 6
# 5
# 4
# 3
# 2
# 1
# Blast off!

2. The for Loop: Iterating Over a Sequence

While while loops are great for repeating an action an unknown number of times, for loops are designed to iterate over every item in a sequence, like a list or a dictionary. This is where Python's for loop differs significantly from the classic C for loop.

In C, a for loop is typically a counter: for (int i = 0; i < 10; i++). In Python, it works more like a "for each" loop. You provide it a collection, and it gives you each item in that collection, one by one.

Let's say you have a list of products from your storekeeper job. With a for loop, you can easily process each one.

# A list of products to check in inventory
products = ["Milk", "Bread", "Butter", "Apples"]

# Iterate over each product in the list
for product in products:
    print(f"Checking stock for: {product}")

# Output:
# Checking stock for: Milk
# Checking stock for: Bread
# Checking stock for: Butter
# Checking stock for: Apples

This is extremely powerful for automation. You can use it to process a list of files, a list of emails, or rows in a spreadsheet.

This video will give you a solid overview of how for loops work with lists, strings, and the range() function.

Recreating a C-style Loop with range()

What if you do want to loop a specific number of times, like in C? You can use Python's built-in range() function. range(5) generates a sequence of numbers from 0 to 4.

Getting the Index and Value with enumerate()

Sometimes, when you're iterating over a list, you need both the item and its index. A common way to do this in C would be to use the loop counter i. In Python, there's a more elegant way: the enumerate() function. It gives you back both the index and the item in each iteration.

This is very useful for tasks like creating numbered lists.

tasks = ["Check for new emails", "Download attachments", "Process downloaded files"]

for index, task in enumerate(tasks):
    # We add 1 to the index because we want the list to start from 1, not 0
    task_number = index + 1
    print(f"Task {task_number}: {task}")

# Expected output:
# Task 1: Check for new emails
# Task 2: Download attachments
# Task 3: Process downloaded files

3. Nested Loops and A Practical Example

You can place a loop inside another loop. This is called a nested loop. It's useful for working with grid-like data or for combining items from two different lists. The inner loop will run to completion for each iteration of the outer loop.

Real-World Application: Inventory Management

Let's tie this all together with an example that relates to your experience as a storekeeper. Imagine you have an inventory saved in a text file, and you need to write a script to search for an item. This task requires opening a file and looping through its contents.

These examples show how for loops are the workhorse for file processing and data manipulation tasks—two of the most common activities in automation.

Conclusion

Excellent work today! You've learned how to make your programs perform repetitive actions, which is the key to automating almost any task.

Key Takeaways:

• while loops are for repeating a block of code as long as a condition is true. They are ideal when you don't know in advance how many times you need to loop.
• for loops are for iterating over every item in a sequence (like a list, dictionary, or string). They are perfect for processing collections of data.
• The range() function allows you to create for loops that run a specific number of times, similar to C's for loop.
• The enumerate() function provides a clean, "Pythonic" way to get both the index and the value when looping over a sequence.
• Loops are essential for tasks like processing files, working with lists of data, and building interactive programs.

Next Lesson:
You now know how to write scripts that use variables, conditional logic, and loops. As your scripts get more complex, you'll want a way to organize them. In our next lesson, we will learn how to define and call functions. This will allow you to group related code into reusable blocks, making your automation scripts much cleaner, more powerful, and easier to maintain.