Week 3 Loops.md

![[Nijika_Ijichi_Think_Python.png]]

Introduction to Loops: Don't Repeat Yourself!

In programming, a core principle is DRY—Don't Repeat Yourself. If you find yourself writing the same line of code over and over, there's almost certainly a better, more efficient way. That better way is using loops.

Loops allow us to execute a block of code multiple times, saving us from tedious repetition and making our programs more powerful and dynamic.

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while Loops: Repeating Indefinitely

A while loop is the simplest form of a loop. It will repeat a block of code as long as a certain condition remains True.

Think of it as telling the computer: "While this is true, keep doing this."

Python

# A simple counter using a while loop
i = 0
while i < 3:
    print("meow")
    i = i + 1 # Increment the counter

Anatomy of the while Loop:

1. Initialization: We start with a "counter" variable, i, set to 0.

2. Condition: The loop checks while i < 3:. The first time, 0 < 3 is True, so the loop runs.

3. Body: The indented code inside the loop (print("meow") and i = i + 1) is executed.

4. Update: We increment i. It becomes 1. The loop goes back to step 2 and checks the condition again (1 < 3 is True... then 2 < 3 is True).

5. Termination: Once i becomes 3, the condition 3 < 3 is False, and the loop stops.

⚠️ Beware of Infinite Loops! If you forget to update the variable in your condition (like forgetting i = i + 1), the condition will never become false, and your program will be stuck forever! Use Ctrl+C to break out of the loop in VS Code

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for Loops: Iterating Over a Sequence

A for loop is designed for definite iteration—that is, looping a specific number of times. It is perfect for iterating over a sequence of items, like a list.

Think of it as telling the computer: "For each item in this collection, do this."

Lists (list)

Before we master for loops, we need a data structure to loop over. A list is an ordered collection of items, stored in a single variable. You create a list using square brackets [].

Python

Gacha_Games = ["Zenless Zone Zero", "Wuthering Waves", "Genshin Impact"]

# Accessing an item by its index (position)
print(Gacha_Games[0]) # Output: "Zenless Zone Zero"

Using for with a List

The for loop elegantly handles the initialization, condition check, and updating for you.

Python

Characters = ["Iuno", "Augusta", "Galbrena"]

# The variable 'student' will take on the value of each item in the list, one by one.
for Characters in Characters:
    print(Characters)

This is far cleaner and less error-prone than a while loop for this task.

The range() Function

What if you just want to do something a specific number of times, without a list? The range() function generates a sequence of numbers for you.

• range(3) produces the sequence 0, 1, 2.

• range(5) produces 0, 1, 2, 3, 4.

Python

# A more "Pythonic" way to meow three times
for _ in range(3):
    print("meow")

When you need a loop to run a certain number of times but you don't care about the counter variable itself, it's a common convention to use an underscore (_) as the variable name.

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Dictionaries (dict): Key-Value Pairs

What if a simple list isn't enough? A dictionary or dict stores data not in an ordered sequence, but as key-value pairs. It's like a real-world dictionary where you look up a word (the key) to find its definition (the value). You create dictionaries with curly braces {}.

Python

# Using a dictionary to store characters and their games
characters = {
    "Iuno": "WutheringWaves",
    "Zani": "WutheringWaves",
    "Cartethiya": "WutheringWaves",
    "Seed": "Zenless",
    "Evelynn": "Zenless"
}

# Access the value using its key
print(characters["Iuno"]) #Output WutheringWaves

Looping Over a Dictionary

When you loop over a dictionary, you are iterating over its keys.

Python

# This loop prints each character's name (the key)
for character in characters:
    # We use the key to look up the corresponding value
    print(character, characters[character], sep=", ")

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Nested Loops: Loops Inside Loops

You can put a loop inside another loop. This is called a nested loop, and it's perfect for problems that involve grids, matrices, or combinations of items.

Imagine you want to print a square of bricks (#).

Python

def main():
    print_square(3)

def print_square(size):
    # The outer loop handles the rows (height)
    for i in range(size):
        # The inner loop handles the columns (width)
        for j in range(size):
            # The 'end=""' prevents print() from starting a new line
            print("#", end="")
        # After the inner loop finishes, print a newline to start the next row
        print()

main()

How it works:

1. The outer loop starts (i = 0).

2. The inner loop runs to completion, printing ###.

3. print() creates a new line.

4. The outer loop runs again (i = 1), and the inner loop runs to completion again, printing another row of ###.

5. This repeats until the square is complete.

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Keywords for Loop Control

Sometimes you need to alter a loop's behavior mid-run.

• break: Immediately terminates the loop it's in.

• continue: Skips the rest of the current iteration and jumps to the next one.

Python

# An example of getting positive input from a user
while True: # This creates an intentional infinite loop
    n = int(input("What's n? "))
    if n > 0:
        break # If n is positive, exit the loop

# This code will only be reached after the loop is broken
for i in range(n):
    print("meow")

This "loop forever and break" pattern is a very common and useful way to handle user input validation.