====== Control Structures and Functions in Lua ======


===== 1. Conditional Statements =====
Conditional statements allow your program to make decisions based on certain conditions. In Lua, you use if, elseif, and else to implement conditional logic.

=== If Statement ===
The if statement evaluates a condition and executes a block of code if the condition is true.
**Code Example:**
<code>
local devLevel = 18

if devLevel >= 18 then
    print("WOW! REALLY GREAT")
end
</code>

**Explanation:**

  * if devLevel >= 18 then: This line checks if the variable age is greater than or equal to 18.
  * print("WOW! REALLY GREAT"): If the condition (devLevel >= 18) is true, then this line will execute and print "WOW! REALLY GREAT"
=== Elseif and Else Statements ===

You can use elseif to check additional conditions and else to provide a default action if none of the conditions are true.

**Code Example:**
<code>
local temperature = 25

if temperature > 30 then
    print("It's hot outside!")
elseif temperature > 20 then
    print("It's warm outside.")
else
    print("It's cool outside.")
end
</code>

**Exercise:**

  * Write an if statement to check if a variable number is positive.
  * Extend the if statement with elseif and else to cover different ranges of values.
  * Modify the variables and test different conditions to see the output.


**Example Exercise Code:**
<code LUA>
local number = 10

if number > 0 then
    print("The number is positive.")
elseif number < 0 then
    print("The number is negative.")
else
    print("The number is zero.")
end
</code>

===== 2. Loops =====
Loops are used to repeat a block of code multiple times. Lua supports while, for, and repeat...until loops.

=== While Loop ===


The while loop repeats a block of code while a condition is true.

**Code Example:**
<code LUA>
local i = 1
while i <= 5 do
    print("i = " .. i)
    i = i + 1
end
</code>

**Explanation:**

  * **local i = 1**: Initializes a variable i with a value of 1.
  * **while i <= 5 do**: This line sets up the condition to execute the loop as long as i is less than or equal to 5.
  * **print("i = " .. i):** Prints the current value of i.
  * **i = i + 1:** Increments i by 1 in each iteration to avoid an infinite loop.


=== For Loop ===


The for loop iterates over a range of values specified by the user.

**Code Example:**

  for i = 1, 5 do
      print("i = " .. i)
  end
**Explanation:**

  * **for i = 1, 5 do:** This line initializes i to 1 and iterates up to 5.
  * **print("i = " .. i):** Prints the current value of i in each iteration.


=== Repeat...Until Loop ===


The repeat...until loop executes a block of code at least once and continues to repeat until a condition becomes true.

**Code Example:**

  local i = 1
  repeat
      print("i = " .. i)
      i = i + 1
  until i > 5
**Explanation:**

  * **repeat:** Starts the loop block.
  * **print("i = " .. i):** Prints the current value of i.
  * **i = i + 1:** Increments i by 1 in each iteration.
  * **until i > 5:** Ends the loop when i becomes greater than 5.

**Exercise:**

  * Write a while loop to count from 10 to 1 and print each number.
  * Use a for loop to print even numbers from 1 to 10.
  * Implement a repeat...until loop to prompt the user for input until they enter a valid number.
**Example Exercise Code:**
<code LUA>
  -- while loop
  local countdown = 10
  while countdown >= 1 do
      print(countdown)
      countdown = countdown - 1
  end

-- for loop
for i = 2, 10, 2 do
    print(i)
end

-- repeat...until loop
local userInput
repeat
    print("Enter a number: ")
    userInput = io.read("*n")  -- Read a number from input
until userInput ~= nil
print("You entered: " .. userInput)
</code>

===== 3. Functions =====

Functions are blocks of code that perform a specific task and can be reused throughout your program. They help in organizing code and making it more readable.

=== Defining Functions ===


You define a function in Lua using the function keyword followed by the function name and parameters.

**Code Example:**
<code LUA>
-- Function to calculate the square of a number
function square(num)
    return num * num
end

-- Calling the function
local result = square(5)
print("Square: " .. result)  -- prints: 25
</code>

**Explanation:**

  * function square(num): Defines a function named square that takes a parameter num.
  * return num * num: Computes the square of num and returns the result to the caller.

=== Parameters and Return Values ===


Functions can have parameters (input values) and return values (output values).

**Code Example:**
<code LUA>
-- Function with parameters and return value
function greet(name)
    return "Hello, " .. name .. "!"
end

-- Calling the function
local message = greet("Lua Learner")
print(message)  -- prints: Hello, Lua Learner!
</code>

**Exercise:**

  * Write a function that calculates the area of a rectangle given its length and width.
  * Create a function that converts Celsius to Fahrenheit.
  * Implement a function that checks if a number is prime and returns true or false.


**Example Exercise Code:**

<code LUA>
-- Function to calculate the area of a rectangle
function calculateArea(length, width)
    return length * width
end

-- Function to convert Celsius to Fahrenheit
function celsiusToFahrenheit(celsius)
    return celsius * 9 / 5 + 32
end

-- Function to check if a number is prime
function isPrime(number)
    if number <= 1 then
        return false
    elseif number == 2 then
        return true
    else
        for i = 2, math.sqrt(number) do
            if number % i == 0 then
                return false
            end
        end
        return true
    end
end

-- Example usage
local area = calculateArea(5, 10)
print("Area of rectangle: " .. area)

local fahrenheitTemp = celsiusToFahrenheit(25)
print("Temperature in Fahrenheit: " .. fahrenheitTemp)

local num = 17
if isPrime(num) then
    print(num .. " is a prime number.")
else
    print(num .. " is not a prime number.")
end
</code>