Conditions and Logic

Programs become useful when they can make decisions. In Lua, this is done using conditions and logical expressions. A condition is an expression that evaluates to either true or false, and the program can choose what to do based on that result.

The if Statement

The most basic decision structure in Lua is the if statement.

if condition then
    -- code runs if condition is true
end

If the condition evaluates to true, the code inside the block is executed. If it evaluates to false, the block is skipped.

Example:

local score = 80

if score > 50 then
    print("You passed!")
end

Comparison Operators

Lua provides comparison operators for building conditions:

• == equal to
• ~= not equal to
• < less than
• <= less than or equal

• greater than

• = greater than or equal

Example:

local age = 18

if age >= 18 then
    print("Access granted")
end

Strings can also be compared:

if name == "Lua" then
    print("Hello, Lua!")
end

elseif and else

To handle multiple possible cases, Lua uses elseif and else.

local score = 72

if score >= 90 then
    print("Grade A")
elseif score >= 75 then
    print("Grade B")
elseif score >= 60 then
    print("Grade C")
else
    print("Failed")
end

Lua checks conditions from top to bottom and executes the first block that evaluates to true.

Boolean Values

Booleans represent logical truth values:

true
false

They are often stored in variables:

local is_admin = true

if is_admin then
    print("Admin access")
end

Logical Operators

Lua supports logical operators to combine or modify conditions:

• and — true if both sides are true
• or — true if at least one side is true
• not — inverts a boolean value

Example using and:

if age >= 18 and has_id then
    print("Entry allowed")
end

Example using or:

if is_admin or is_moderator then
    print("Permission granted")
end

Example using not:

if not game_over then
    print("Game is running")
end

Truthiness in Lua

Lua has a simple rule for truth values:

• false and nil are false
• everything else is true

This includes:

• 0
• "" (empty string)
• {} (empty table)

Example:

if 0 then
    print("This will run")
end

Understanding this rule is critical to avoiding logic errors.

Operator Precedence

Logical operators follow this order:

1. not
2. and
3. or

Example:

if not a and b then
    -- same as: (not a) and b
end

Parentheses should be used when in doubt:

if not (a and b) then
    print("Condition met")
end

Common Mistakes

Using = instead of ==:

-- wrong
if x = 5 then
end

-- correct
if x == 5 then
end

Forgetting that 0 is true:

if lives then
    -- runs even when lives == 0
end

Summary

Conditions allow Lua programs to make decisions. The if, elseif, and else statements control program flow based on boolean expressions. Comparison and logical operators are used to build conditions, and understanding Lua’s truthiness rules is essential for writing correct and readable logic.