Booleans in

About Booleans

The boolean type in Lua has the two traditional boolean values: true and false. These values can be assigned to a variable, combined with logical operators (and, or, not) and used in conditional tests (e.g. in control structures).

local is_true = true
local is_false = false

true and true  --> true
true and false --> false
false or true  --> true
false or false --> false
not false      --> true
not true       --> false

-- prints 0
if is_true == true then
  print(0)
else
  print(1)
end

Conditional tests and the logical operators consider both the boolean false and nil as false and anything else as true.

1 and 2          --> 2
nil and 2        --> nil
false and 2      --> false
1 or 2           --> 1
false or 'hello' --> "hello"
nil or false     --> false

Both and and or uses short-circuit evaluation, which means they evaluate their second operand only when necessary. All operations are evaluated according to the operator precedence, where not is evaluated before and and or.

Given that only false and nil are considered false and everything else is true a common idiom in Lua is:

x = x or y

This is equivalent to:

if not x then
  x = y
end

It is pretty common to use this in functions, to assign a default value if a parameter has no value (is nil):

local function hello(name)
  name = name or "World"
  print("Hello " .. name)
end

hello()       --> "Hello World"
hello('Mars') --> "Hello Mars"