Operators Tutorial

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We'll use the = expression shorthand notation for this page. The values can easily be assigned to a variable, e.g.,

> x = 7
> print(x)
7
> = 7
7

Arithmetic expressions

Lua has the usual binary arithmetic operators.

> = 2+3, 5-12, 2*7, 7/8
5       -7      14      0.875
> = 5*(2-8.3)/77.7+99.1
98.694594594595

> = -(-10), -(10)
10      -10

> = 15%7, -4%3, 5.5%1
1       2       0.5

> = 7^2, 107^0, 2^8
49      1       256

Relational expressions

Relational operators are supplied which return the boolean values true or false.

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

Examples:

> = 1 == 1, 1 == 0
true    false
> = 1 ~= 1, 1 ~= 0
false   true
> = 3 < 7, 7 < 7, 8 < 7
true	false	false
> = 3 > 7, 7 > 7, 8 > 7
false	false	true
> = 3 <= 7, 7 <= 7, 8 <= 7
true    true    false
> = 3 >= 7, 7 >= 7, 8 >= 7
false   true    true

> = "abc" < "def"
true
> = "abc" > "def"
false
> = "abb" < "baa"
true
> = "abc" == "abc"
true
> = "abc" == "a".."bc"
true

> = {} == "table"
false
> = {} == {}  -- two different tables are created here
false
> t = {}
> t2 = t
> = t == t2   -- we're referencing the same table here
true

> = "10" == 10
false
> = tonumber("10") == 10
true

Logical operators

Lua provides the logical operators and, or and not. In Lua both nil and the boolean value false represent false in a logical expression. Anything that is not false (either nil or false) is true, including 0, which might be surprising coming from some other languages. There are more notes on the implications of this at the end of this page.

> = false==nil   -- although they are both considered false by logical operators, they're still different values
false
> = true==false, true~=false
false   true
> = 1==0
false

not

The keyword not inverts a logical expression value:

> = true, false, not true, not false
true    false   false   true
> = not nil       -- nil represents false
true
> = not not true  -- true is not not true!
true
> = not "foo"     -- anything not false or nil is true
false

and

The binary operator and does not necessarily return a boolean value true or false to the logical expression x and y. In some languages the and operator returns a boolean dependent on the two inputs. Rather in Lua, it returns the first argument (without even executing the second one) if its value is false or nil, and the second argument if the first argument is not false or nil. So, a boolean is only returned if the first argument is false or the second argument is a boolean. Aka minimal evaluation.

> = false and true  -- false is returned because it is the first argument
false
> = nil and true    -- as above
nil
> = nil and false
nil
> = nil and "hello", false and "hello"
nil     false
> = false and print("hello") -- the print function isn't evaluated, so "hello" isn't printed
false

> = true and false
false
> = true and true
true
> = 1 and "hello", "hello" and "there"
hello   there
> = true and nil
nil
> = true and print("hello") -- the print function is evaluated here, so "hello" is printed
hello
nil

or

The or binary operator also does not necessarily return a boolean value (see notes for and above). If the first argument is true it is returned, otherwise the second argument is returned. So, a boolean is only returned if the first argument is true or the second argument is a boolean.

> = true or false
true
> = true or nil
true
> = "hello" or "there", 1 or 0
hello   1
> = true or print("hello") -- the print function isn't evaluated, so "hello" isn't printed
true

> = false or true
true
> = nil or true
true
> = nil or "hello"
hello
> = false or print("hello") -- the print function is evaluated here, so "hello" is printed
hello

This can be a very useful property. For example, setting default values in a function:

> function foo(x)
>>  local value = x or "default"  -- if argument x is false or nil, value becomes "default"
>>  print(value, x)
>> end
>
> foo()       -- no arguments, so x is nil
default nil
> foo(1)
1       1
> foo(true)
true    true
> foo("hello")
hello   hello

Ternary operators

Even though Lua doesn't have a ternary operator (if/else expression), it's possible to create similar behavior with and and or:

value = condition and trueval or falseval;

value = (condition and trueval) or falseval;

This does not work in a case of "value = false and true or false".