day4_cheatsheet.md

🐍 Python Day 4 Cheat Sheet

Automating with Loops

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📝 Table of Contents

1. Core Definitions
2. Key Differences: for vs while
3. The for Loop
4. The while Loop
5. Loop Control Statements
6. Quick Reference
7. Pro Tips
8. Common Mistakes to Avoid

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🧠 Core Definitions

• Iteration: The process of repeating a set of instructions. Each time the instructions are executed is called one "iteration".
• for Loop: A loop that iterates over a sequence (like a list, dictionary, tuple, or string) or other iterable objects. It runs once for each item in the sequence.
• while Loop: A loop that continues to execute as long as a certain condition remains True.
• break: A keyword that immediately terminates the current loop, and the program continues at the next statement after the loop.
• continue: A keyword that skips the rest of the code inside the current iteration of the loop and proceeds to the next iteration.

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📊 Key Differences: for vs while

Feature	for Loop	while Loop
Best For	Iterating over a known sequence	Looping as long as a condition is true
Typical Use	"For each item in this list..."	"Keep doing this until..."
Structure	for item in sequence:	while condition:
Infinite Loop Risk	Low (stops when sequence ends)	High (if condition never becomes false)

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🗂️ The for Loop

Looping Over a List

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like to eat {fruit}s.")

Looping with range()

# Loop 5 times (0 to 4)
for i in range(5):
    print(f"Loop number {i}")

# Loop from 2 up to (but not including) 6
for i in range(2, 6):
    print(f"Number: {i}")

Drawing with Text

# Drawing a 5x5 square
size = 5
for i in range(size):
    # For the first or last row, print a full line
    if i == 0 or i == size - 1:
        print("*" * size)
    # For middle rows, print a star, spaces, and a star
    else:
        print("*" + " " * (size - 2) + "*")

Looping Over a Dictionary

player_stats = {"name": "Alex", "level": 5}

# Using .items() to get key and value directly
for key, value in player_stats.items():
    print(f"{key}: {value}")

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🔄 The while Loop

Basic Counter Loop

# Prints numbers 1 through 5
count = 1
while count <= 5:
    print(count)
    count = count + 1 # Crucial! This prevents an infinite loop.

Creative Patterns

# Drawing a growing triangle
lines = 1
while lines <= 7:
    print("*" * lines)
    lines += 1 # Shortcut for lines = lines + 1

Interactive Loop (e.g., a Menu)

command = ""
while command.lower() != "exit":
    command = input("Enter command (or 'exit'): ")
    print(f"Executing: {command}")

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🛑 Loop Control Statements

break (Exit the Loop)

Use break to exit a loop immediately, regardless of the loop's condition.

# Find the first person named "Bob" and then stop.
names = ["Alice", "Charlie", "Bob", "David"]
for name in names:
    print(f"Checking {name}...")
    if name == "Bob":
        print("Found Bob!")
        break # Exit the loop now

continue (Skip to Next Iteration)

Use continue to skip the current iteration and move to the next one.

# Print all numbers from 0 to 9, but skip 5.
for i in range(10):
    if i == 5:
        continue # Skip the rest of this iteration
    print(i)

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⚡ Quick Reference

for Loop Patterns

# Over a list
for item in my_list:
    # do something with item

# For a specific number of times
for i in range(10):
    # do something 10 times

# With index and item
for index, item in enumerate(my_list):
    print(f"Index {index}: {item}")

while Loop Patterns

# Counter-based
i = 0
while i < 10:
    # do something
    i += 1 # Shortcut for i = i + 1

# User-input based (infinite loop with a break condition)
while True:
    response = input("Continue? (y/n): ")
    if response.lower() == 'n':
        break # Exit condition

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💡 Pro Tips

1. for loops are for for each: If you can phrase your problem as "for each item in my collection...", a for loop is almost always the right choice. It's safer and often more readable.

2. while loops are for until: If you can phrase your problem as "keep doing this until some condition changes...", a while loop is what you need. Perfect for games, menus, and waiting for events.

3. Combine Loops and if: The real power comes when you put if statements inside your loops to make decisions on each iteration.

4. Use enumerate() for Indexes: When you need both the index and the item from a list, enumerate() is cleaner than managing your own counter.

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🎯 Common Mistakes to Avoid

❌ Creating an Infinite while Loop

# Wrong: The condition `i < 5` never becomes false!
i = 0
while i < 5:
    print("This will never stop!")
# You forgot to increment i!

✅ Always Update the Condition Variable

# Correct
i = 0
while i < 5:
    print(i)
    i += 1 # The variable is updated, so the loop will end.

❌ Forgetting to Convert input() to a Number

# Wrong: 'guess' will be a string, so guess != 7 will always be true!
secret_number = 7
guess = ""
while guess != secret_number:
    guess = input("Guess the number: ") # guess is "7", not 7

✅ Convert Input for Comparison

# Correct
secret_number = 7
guess = 0
while guess != secret_number:
    guess = int(input("Guess the number: ")) # Converts to integer

❌ Modifying a List While Looping Over It

# This can cause unexpected behavior and skipped items.
numbers = [1, 2, 3, 4]
for number in numbers:
    if number % 2 == 0:
        numbers.remove(number) # DANGEROUS!

✅ Loop Over a Copy

# Correct: Loop over a copy to safely modify the original
for number in numbers[:]: # The [:] creates a copy
    if number % 2 == 0:
        numbers.remove(number)

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Happy coding! 🐍✨