If and else

An if statement runs code only when a condition is true. With elseif and else it chooses between several paths.

if

local health = 4

if health < 6 then
  print("You are low on health!")
end

The shape is always if condition then ... end. The code between then and end runs only when the condition is truthy. Indent it by two or four spaces so you can see where the block starts and stops. Lua doesn’t care about the indentation, but readers do.

else

else gives the other path:

local is_day = false

if is_day then
  print("The sun is up.")
else
  print("Watch out for mobs.")
end

elseif

For more than two choices, chain them with elseif. Lua checks the conditions from the top and runs the first one that is true. The rest are skipped:

local light = 9

if light == 0 then
  print("pitch black")
elseif light < 8 then
  print("dark: mobs can spawn")
elseif light < 15 then
  print("lit")
else
  print("full light")
end

This prints lit. elseif is one word. Writing else if starts a new if inside the else, which needs its own end.

Conditions can be any value

Remember the rule from the last lesson: only false and nil are falsy. So you can test whether something exists simply by naming it:

local held_item = "Torch"

if held_item then
  print("Holding " .. held_item)  --> Holding Torch
end

Nesting

An if can sit inside another if:

local mob = "Creeper"
local distance = 2

if mob == "Creeper" then
  if distance < 3 then
    print("Run!")
  else
    print("Keep an eye on it.")
  end
end

Deep nesting gets hard to read. Often you can join the conditions with and, or use an early return inside a function (you will see this in the functions chapter).

Comparing against many values

Lua has no switch statement. Use an elseif chain, or, when you only map values to values, a table lookup (tables come later, in Dictionaries):

local damage = { wooden = 4, stone = 5, iron = 6, diamond = 7 }
print(damage["iron"])  --> 6

Try it

Write code that takes a local time = 13000 (Minecraft ticks, 0 to 23999) and prints which part of the day it is: day for 0 to 11999, sunset for 12000 to 12999, night for 13000 to 22999, and sunrise for the rest. Test it with a few values.

local time = 13000

if time < 12000 then
  print("day")
elseif time < 13000 then
  print("sunset")
elseif time < 23000 then
  print("night")
else
  print("sunrise")
end
--> night

Recap: if ... then ... elseif ... else ... end runs the first branch whose condition is truthy.