With if-statements we can control whether certain code should be executed or not.

if true then
    print("Hello world") -- Will get printed
end

if false then
    print("Bye world") -- Will not get printed
end

true and false are a type of value called boolean. What is inside the if-statement is only executed if the boolean provided is true. The provided boolean is called the condition.

Because a boolean is a type of value, we can assign it to a variable.

local a = true

We can use the keyword else to decide what should happen when the condition is not met.

if false then
    print("Bye world!") -- Will not get printed
else
    print("Oh, still here!") -- Will get printed
end

And we can use the keyword elseif to chain multiple if-statements.

local a, b, c = false, false, true

if a then
    print("a?") -- Not printed
elseif b then
    print("b?") -- Not printed
elseif c then
    print("c!") -- Printed!
else
    print("Nothing...") -- Not printed
end

Expressions #

We can also assign a boolean by using an expression.

local a = 5 > 9

Here, the value of a becomes whether the expression 5 > 9 is true or false. The character > means higher than. 5 is not higher than 9, so a becomes false.

local a = 5 > 9 -- false

if a then
    print("5 is higher than 9") -- Not printed
end

To check whether two values are the same, we use two equal signs.

if 3 == 7 then
    print("3 is equal to 7") -- Not printed
end

a = 10 -- An assignment, a becomes 10.
a == 10 -- An expression, whether a equals 10.

We can combine > and < with == to create >= and <=.

print(4 > 4) -- false, 4 is not higher than 4.
print(4 < 4) -- false, 4 is not lower than 4.

print(4 >= 4) -- true, 4 is equal to or higher than 4.
print(4 <= 4) -- true, 4 is equal to or lower than 4.

And finally, we have ~= to check if two values are NOT equal to each other.

print(4 == 4) -- true, 4 is equal to four.
print(4 ~= 4) -- false, 4 IS equal to four.

We can use the keyword not to check if a value is not true.

local a = 5 == 8

if not a then
    print("5 is NOT equal to 8") -- Printed!
end

Once inside an if-statement, it doesn't matter what happens to the variable that was used as a condition. Besides, the condition is not the variable itself, but the value it holds.

local a = true

if a then
    a = false
    print("Hello world!") -- Printed!
end

if a == true then

and & or #

With the keyword and we can combine expressions to check if they are all true.

if 5 < 9 and 7 > 3 then
    print("Both expressions are true") -- Printed!
end

if 6 > 1 and 5 == 4 and 8 > 2 then
    print("All these expressions are true") -- Not printed
end

With the keyword or we can combine expressions to check if any of them are true.

if 1 > 8 or 7 > 3 then
    print("Any of these two expressions is true") -- Printed!
end

if 6 > 1 or 5 == 4 or 8 > 2 then
    print("Any of these expressions is true") -- Printed!
end

We can also combine and & or. We can use parantheses to group these expressions. The placement of parantheses can change whether an expression is true or false. Can you figure out why the first statement gets printed, but the second one does not?

if 8 < 4 and 7 == 3 or 2 < 6 then
    print("Printed!")
end

if b = 8 < 4 and (7 == 3 or 2 < 6) then
    print("Not printed...")
end

Truthy and falsy #

So far we have talked about booleans, true and false, but if-statements can accept other values as well. We call these values truthy.

if 15 then
    print("Fifteen!")
end

The only two values that if-statements see as false are false and nil. We call these values falsy.

if true then print(1) end -- Printed!
if false then print(2) end -- Not printed
if nil then print(3) end -- not printed
if 4 then print(4) end -- Printed!
if "hello" then print(5) end -- Printed!
-- Output: 1, 4, 5

A function is also a type of value, and is also truthy. Unless you call the function, and it returns false or nil.

local function hello()
    return false
end

if hello then -- The function by itself is truthy.
    print("hello is truthy!") -- Printed!
end

if hello() then -- The function call returns false.
    print("Not printed...")
else
    print("hello() is falsy!") -- Printed!
end

Nested statements #

We can put an if-statement inside another if-statement. We call these nested statements.

local number = 80

if number > 50 then
    if number > 75 then
        if number > 100 then
            print("Higher than 100!")
        else
            print("Lower than 100") -- Printed!
        end
    else
        print("Lower than 75")
    end
else
    print("Lower than 50")
end

Usage #

By using if-statements, we can prevent code from being executed. For example, if you have an ammo variable, we can use the expression ammo > 0 to check whether the player still has bullets left.

if ammo > 0 then
    shoot()
    ammo = ammo - 1
end

In the demo below, we use an if-statement to stop a rectangle from moving as it touches the edge.

local x = 0

function love.update()
    if x < 700 then
        x = x + 1
    end
end

function love.draw()
    love.graphics.rectangle("fill", x, 50, 100, 100)
end

And in this demo, the square only moves to the right when holding down the right arrow key. If not, it will move to the left, as long as the value of x is higher than 0.

local x = 0

function love.update()
    if love.keyboard.isDown("right") then
        x = x + 1
    elseif x > 0 then
        x = x - 1
    end
end

function love.draw()
    love.graphics.rectangle("fill", x, 50, 100, 100)
end