Iterating with Loops

The for Loop: Iterating Over a Sequence

The first and most common type of loop in Python is the for loop. Its purpose is to iterate over a sequence of items—like a list of numbers, the characters in a string, or a defined range—and execute a block of code for each item. You use a for loop when you know in advance the collection of items you need to process.

Let's start with a visual model of how a for loop operates.

Now, let's see how this is implemented in Python. We'll watch a video that visually explains the syntax and mechanics.

Generating Sequences with range()

Often, you don't have a pre-made list but simply want to run a loop a specific number of times. Manually creating a list like [1, 2, 3, ..., 100] would be tedious. Python's built-in range() function is the standard solution for this.

range() generates a sequence of numbers, making it perfect for use in for loops. It can be used in a few ways:

• range(stop): Generates numbers from 0 up to (but not including) stop. For example, range(5) gives you 0, 1, 2, 3, 4.
• range(start, stop): Generates numbers from start up to stop. For example, range(1, 6) gives you 1, 2, 3, 4, 5.
• range(start, stop, step): Generates numbers from start to stop, incrementing by step. For example, range(0, 10, 2) gives you 0, 2, 4, 6, 8.

Let's continue with the video to see range() in action.

The while Loop: Iterating While a Condition is True

The second type of loop is the while loop. Unlike a for loop, which runs for a pre-defined number of items, a while loop runs as long as a specified condition remains True. You use a while loop when you don't know how many iterations you'll need, but you know the condition that should stop the loop.

The structure of a while loop involves three key parts:

1. Initialization: A variable is set up before the loop begins.
2. Condition: The while statement checks this variable in a condition.
3. Update: Inside the loop, the variable is modified to ensure the condition will eventually become False, preventing an infinite loop.

Let's see how this works.

Controlling Loop Execution: break and continue

Sometimes you need more granular control over a loop's execution. Python provides two statements for this:

• break: Immediately terminates the loop, and the program continues execution at the first statement after the loop.
• continue: Immediately terminates the current iteration and proceeds to the next one.

These are typically used inside an if statement to handle special cases. For example, you might break out of a loop once you find the item you're searching for, or you might continue to skip over invalid data points.

Your Turn: Automating System Checks

Now it's time to apply what you've learned. The following exercises will challenge you to use both for and while loops, along with control statements, in a scenario inspired by your "Tony Stark" goal of building intelligent systems.

Exercise 1: for loop for diagnostics (10 minutes)

You are programming a drone that needs to run a pre-flight check on its four propellers.

1. Write a for loop that iterates four times using the range() function.
2. Inside the loop, print a message for each propeller, like: Running diagnostics on propeller #1..., Running diagnostics on propeller #2..., and so on.

Example Code Structure:

# The range should produce numbers 1, 2, 3, 4
for propeller_id in range(1, 5):
    print(f"Running diagnostics on propeller #{propeller_id}...")

Exercise 2: while loop for system pressurization (15 minutes)

You need to write code that monitors a system's pressure as it builds up.

1. Create a variable pressure_psi and initialize it to 0.
2. Write a while loop that continues as long as pressure_psi is less than 50.
3. Inside the loop:
   • Increment pressure_psi by 10 at the start of each iteration.
   • Check the pressure:
     • If pressure_psi is exactly 30, print Pressure at 30 PSI. Venting minor excess. and use continue to skip to the next iteration without printing the regular status.
     • If pressure_psi exceeds 40, print CRITICAL: Pressure over 40 PSI. Aborting. and use break to exit the loop immediately.
   • If neither of the special conditions is met, print the current pressure: System pressure: <pressure_psi> PSI.
4. After the loop finishes (either by reaching the condition or by breaking), print a final message: Pressurization sequence ended.

Example Output:

System pressure: 10 PSI
System pressure: 20 PSI
Pressure at 30 PSI. Venting minor excess.
System pressure: 40 PSI
CRITICAL: Pressure over 40 PSI. Aborting.
Pressurization sequence ended.

Take your time to work through these exercises. They are designed to directly test your understanding of for, while, range, break, and continue.

Conclusion

You have now added another essential tool to your programming arsenal: iteration. You can write code that performs tasks repeatedly, which is fundamental to automating processes and working with data at scale. You've distinguished between iterating over known sequences with for loops and iterating based on a condition with while loops.

Key Takeaways:

• for loops are used to iterate over a finite sequence of items. They are ideal when you know how many times you need to loop.
• while loops are used to iterate as long as a certain condition is true. They are used when the number of iterations is not known beforehand.
• The range() function is a powerful tool for generating number sequences for for loops.
• break provides an exit from a loop, while continue skips the current iteration.
• Care must be taken with while loops to avoid infinite loops by ensuring the loop's condition will eventually become false.

Preview of the Next Lesson:
In this lesson, we often looped over simple sequences like ranges of numbers or hard-coded lists. The true power of loops is unlocked when you combine them with dynamic data structures. In our next lesson, you will learn to create and manipulate lists. Lists are one of the most versatile and commonly used data types in Python, and understanding them is the next critical step toward building complex applications.