🔁 Lesson 3.2: Loops — for and while

# Python: for + range()
# range(1, 6) produces 1, 2, 3, 4, 5  (start inclusive, stop exclusive)
for i in range(1, 6):
    print(i)

# range() with one argument starts at 0:
# range(5) → 0, 1, 2, 3, 4

# range() with three arguments adds a step:
# range(0, 10, 2) → 0, 2, 4, 6, 8  (count by 2s)
# range(10, 0, -1) → 10, 9, 8, ... 1  (count backwards)

// JavaScript: classic three-part for loop
// for (initialize; condition; update)
for (let i = 1; i <= 5; i++) {
    console.log(i);
}

// Breaking it down:
// let i = 1    → runs once before the loop starts
// i <= 5       → checked before each iteration
// i++          → runs after each iteration

// Count by 2s:
// for (let i = 0; i < 10; i += 2) { ... }

// Count backwards:
// for (let i = 10; i >= 1; i--) { ... }

// C#: same three-part syntax as JavaScript
for (int i = 1; i <= 5; i++)
{
    Console.WriteLine(i);
}

// The pattern is identical to JavaScript:
// for (initialize; condition; update)

// Count by 2s:
// for (int i = 0; i < 10; i += 2) { ... }

// Count backwards:
// for (int i = 10; i >= 1; i--) { ... }

# Python: for...in (same keyword, no range needed)
fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(f"I like {fruit}!")

# Output:
# I like apple!
# I like banana!
# I like cherry!

# Need the index too? Use enumerate():
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")
# 0: apple
# 1: banana
# 2: cherry

// JavaScript: for...of (not for...in!)
let fruits = ["apple", "banana", "cherry"];

for (let fruit of fruits) {
    console.log(`I like ${fruit}!`);
}

// Need the index too? Use forEach() or entries():
fruits.forEach((fruit, index) => {
    console.log(`${index}: ${fruit}`);
});

// ⚠️ WARNING: for...in loops over KEYS (indices), not values
// for (let i in fruits) → "0", "1", "2" (strings, not items)
// Always use for...of for arrays!

// C#: foreach (dedicated keyword)
string[] fruits = { "apple", "banana", "cherry" };

foreach (string fruit in fruits)
{
    Console.WriteLine($"I like {fruit}!");
}

// Need the index too? Use a regular for loop:
for (int i = 0; i < fruits.Length; i++)
{
    Console.WriteLine($"{i}: {fruits[i]}");
}

// Or use LINQ (more advanced):
// foreach (var (fruit, index) in fruits.Select((f, i) => (f, i)))
// {
//     Console.WriteLine($"{index}: {fruit}");
// }

# Python: while + condition + colon
count = 0

while count < 5:
    print(count)
    count += 1    # Don't forget this! (infinite loop otherwise)

# Output: 0, 1, 2, 3, 4

// JavaScript: while + parentheses + braces
let count = 0;

while (count < 5) {
    console.log(count);
    count++;      // Don't forget this!
}

// Output: 0, 1, 2, 3, 4

// C#: same structure as JavaScript
int count = 0;

while (count < 5)
{
    Console.WriteLine(count);
    count++;      // Don't forget this!
}

// Output: 0, 1, 2, 3, 4

# Keep asking until the user enters a valid number
while True:
    user_input = input("Enter a number between 1 and 10: ")

    if user_input.isdigit():
        number = int(user_input)
        if 1 <= number <= 10:
            break   # Valid! Exit the loop

    print("Invalid input. Try again.")

print(f"You entered: {number}")

// Browser: prompt returns a string (or null)
let number;

while (true) {
    let userInput = prompt("Enter a number between 1 and 10:");

    number = parseInt(userInput);
    if (!isNaN(number) && number >= 1 && number <= 10) {
        break;   // Valid! Exit the loop
    }

    console.log("Invalid input. Try again.");
}

console.log(`You entered: ${number}`);

int number;

while (true)
{
    Console.Write("Enter a number between 1 and 10: ");
    string userInput = Console.ReadLine();

    if (int.TryParse(userInput, out number) && number >= 1 && number <= 10)
    {
        break;   // Valid! Exit the loop
    }

    Console.WriteLine("Invalid input. Try again.");
}

Console.WriteLine($"You entered: {number}");

# Find the first negative number in a list
numbers = [4, 7, 2, -3, 8, 1]

for num in numbers:
    if num < 0:
        print(f"Found negative number: {num}")
        break       # Stop searching — we found one
    print(f"Checked {num} — positive")

# Output:
# Checked 4 — positive
# Checked 7 — positive
# Checked 2 — positive
# Found negative number: -3

let numbers = [4, 7, 2, -3, 8, 1];

for (let num of numbers) {
    if (num < 0) {
        console.log(`Found negative number: ${num}`);
        break;
    }
    console.log(`Checked ${num} — positive`);
}

int[] numbers = { 4, 7, 2, -3, 8, 1 };

foreach (int num in numbers)
{
    if (num < 0)
    {
        Console.WriteLine($"Found negative number: {num}");
        break;
    }
    Console.WriteLine($"Checked {num} — positive");
}

# Print only odd numbers from 1 to 10
for i in range(1, 11):
    if i % 2 == 0:
        continue   # Skip even numbers
    print(i)

# Output: 1, 3, 5, 7, 9

// Print only odd numbers from 1 to 10
for (let i = 1; i <= 10; i++) {
    if (i % 2 === 0) {
        continue;   // Skip even numbers
    }
    console.log(i);
}

// Print only odd numbers from 1 to 10
for (int i = 1; i <= 10; i++)
{
    if (i % 2 == 0)
    {
        continue;   // Skip even numbers
    }
    Console.WriteLine(i);
}

# Print a 3×3 coordinate grid
for row in range(3):
    for col in range(3):
        print(f"({row},{col})", end="  ")
    print()   # New line after each row

# Output:
# (0,0)  (0,1)  (0,2)
# (1,0)  (1,1)  (1,2)
# (2,0)  (2,1)  (2,2)

// Print a 3×3 coordinate grid
for (let row = 0; row < 3; row++) {
    let line = "";
    for (let col = 0; col < 3; col++) {
        line += `(${row},${col})  `;
    }
    console.log(line.trim());
}

// Print a 3×3 coordinate grid
for (int row = 0; row < 3; row++)
{
    for (int col = 0; col < 3; col++)
    {
        Console.Write($"({row},{col})  ");
    }
    Console.WriteLine();   // New line after each row
}