Looping with 'for' and 'while'

1. The for Loop: Iterating Through a Sequence

The most common type of loop you'll encounter is the for loop. It is designed for what we call definite iteration—that is, looping through a collection of items where the number of items is known beforehand. Think of it as going through a checklist: you perform a task for each item on the list until you reach the end.

The core idea is to take a sequence (like a list or a string) and execute a block of code once for each element in that sequence.

Syntax and Basic Usage

As you saw in the video, the syntax for a for loop is both readable and elegant. It follows this structure:

for variable_name in sequence:



    # Code to be executed for each item
    # This block must be indented

Let's break this down:

• for: The keyword that starts the loop.
• variable_name: A temporary variable that holds the current item from the sequence in each iteration. You choose this name. A common convention is to use a singular noun (e.g., fruit) if the sequence is a plural noun (e.g., fruits).
• in: The keyword that separates the variable from the sequence.
• sequence: The collection of items to iterate over, such as a list or a string.
• :: The colon marks the end of the loop header.
• Indented block: The code that runs for each item. Indentation is how Python knows this code belongs inside the loop.

Here's a practical example using a list, which you're already familiar with:

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(f"Current fruit: {fruit}")

print("Loop finished!")

When you run this code, Python does the following:

1. In the first iteration, fruit is assigned the value "apple", and Current fruit: apple is printed.
2. In the second iteration, fruit is assigned the value "banana", and Current fruit: banana is printed.
3. In the third iteration, fruit is assigned the value "cherry", and Current fruit: cherry is printed.
4. After the last item, the loop finishes, and the final line, Loop finished!, is printed.

Iterating with range()

What if you don't have a list, but you just want to repeat an action a specific number of times? For this, Python provides the range() function. It generates a sequence of numbers that you can loop over.

The range() function can be used in a few ways:

• range(stop): Generates numbers from 0 up to (but not including) stop.
• range(start, stop): Generates numbers from start up to (but not including) stop.

Here's how you can use it to run a loop 5 times:

# Prints numbers 0, 1, 2, 3, 4
for i in range(5):
    print(f"Repeating action, round {i}")

The variable i (short for index) is commonly used when the number itself is what matters.

2. The while Loop: Iterating Based on a Condition

While for loops are for iterating over a known sequence, we sometimes need to loop an unknown number of times. We might want to repeat an action while a certain condition remains true. This is where the while loop, or indefinite iteration, comes in.

A while loop will continue to execute as long as its condition evaluates to True.

Syntax and Structure

The syntax is simple:

while condition:



    # Code to execute as long as condition is True
    # This block must be indented

A while loop typically requires three parts to function correctly and avoid running forever:

1. Initialization: A variable used in the condition must be initialized before the loop starts.
2. Condition: The expression that is checked before each iteration. If it's True, the loop runs. If False, the loop stops.
3. Update: Inside the loop, something must happen to eventually make the condition False. This is usually an update to the variable from the initialization step.

Let's look at a classic counter example:

# 1. Initialization
count = 0




# 2. Condition
while count < 3:
    print(f"Count is: {count}")



    # 3. Update
    count = count + 1  # or count += 1

print("While loop finished!")

This loop will print the count for 0, 1, and 2. When count becomes 3, the condition 3 < 3 is False, and the loop terminates.

The Danger of Infinite Loops

What would happen if we forgot the count = count + 1 line? The value of count would always be 0, the condition count < 3 would always be true, and the loop would run forever! This is called an infinite loop.

If you accidentally run code with an infinite loop, you can usually stop it by pressing Ctrl+C in your terminal.

3. Controlling Your Loops: break and continue

Sometimes you don't want a loop to run to completion. You might want to exit early or skip a specific iteration. Python provides two statements for this: break and continue.

• break: Immediately terminates the entire loop. The program continues with the first statement after the loop.
• continue: Immediately stops the current iteration and jumps to the beginning of the next one.

Example: Using break

break is useful when you're searching for something and can stop as soon as you find it.

numbers = [1, 5, -3, 8, 2]

for num in numbers:
    if num < 0:
        print("Negative number found. Stopping.")
        break  # Exit the loop immediately
    print(f"Processing number: {num}")

Output:

Processing number: 1
Processing number: 5
Negative number found. Stopping.

The loop stops as soon as it encounters -3 and never processes 8 or 2.

Example: Using continue

continue is useful for skipping over values you don't want to process.

numbers = [1, 5, -3, 8, 2]

for num in numbers:
    if num < 0:
        print("Skipping a negative number.")
        continue  # Skip to the next iteration
    print(f"Processing number: {num}")

Output:

Processing number: 1
Processing number: 5
Skipping a negative number.
Processing number: 8
Processing number: 2

The loop skips the processing step for -3 but continues on to process 8 and 2.

4. Choosing the Right Loop

Now that you know both loops, how do you decide which one to use?

Here's the rule of thumb:

• Use a for loop when you know how many times you need to iterate—for example, when looping over all items in a list, all characters in a string, or a fixed number of times using range(). Think: "For each item in this collection..."
• Use a while loop when you need to loop based on a condition that can change, and you don't know when it will become false. Common use cases include waiting for user input, processing data from a stream, or retrying an operation until it succeeds. Think: "While this condition is still true..."

Conclusion

In this lesson, we've unlocked one of the most powerful capabilities in programming: automation through iteration. You now have the tools to make your programs perform repetitive tasks efficiently.

Here are the key takeaways:

• Loops allow you to execute a block of code multiple times.
• for loops are used for definite iteration over a known sequence (like a list, string, or range).
• while loops are used for indefinite iteration, running as long as a condition is True. Be mindful of the three components (initialization, condition, update) to avoid infinite loops.
• The break statement exits a loop entirely, while the continue statement skips the current iteration and proceeds to the next.

In our previous lessons, we learned about variables, conditionals, and lists. Now, with loops, your ability to write programs that can process data and perform complex logic has grown immensely.

In our next lesson, we will learn how to organize our code into reusable blocks called functions. This will allow us to take the logic we've built inside our loops and package it up so we can call on it whenever we need it, making our code even more powerful and organized.