if Statements

local temperature = 18

if temperature > 25 then
  print("It is hot")
elseif temperature > 15 then
  print("It is pleasant")
else
  print("It is cold")
end

It is pleasant

• if condition then ... end — every block ends with end.
• elseif is one word.
• Parentheses around the condition are optional.
• Indentation is for readability only — Lua uses then/end to find the block.

Combining conditions

local function describe(age, member)
  if age < 0 or age > 130 then
    return "invalid age"
  elseif age >= 65 or member then
    return "discount"
  else
    return "full price"
  end
end

print(describe(70, false))
print(describe(30, true))
print(describe(30, false))
print(describe(-1, false))

discount
discount
full price
invalid age

Checking for nil

local config = {port = 8080}

if config.host == nil then
  print("no host set")
end

if config.port then
  print("port is " .. config.port)
end

no host set
port is 8080

if x then is true for any value except nil and false — including 0. Write if x ~= nil then when false is a legitimate value.

Common mistakes

• Forgetting then or end — Lua reports "'then' expected" or "'end' expected" with a line number.
• Writing else if (two words) — that opens a nested if, which needs its own end.

Practice

Given an age, print adult for 18 and over, teen for 13–17, and child otherwise.

local age = 15
if age >= 18 then
  print("adult")
elseif age >= 13 then
  print("teen")
else
  print("child")
end

teen