Expressions Tutorial

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Expressions are evaluated in order to perform calculations which may assign values to variables or pass arguments to functions. Expressions are covered pretty well in section 2.5 of the Reference Manual.[1] Expressions are covered here for completeness and to offer more examples.

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
Unary negation:
> = -(-10), -(10)
10      -10
Modulo (division remainder):
> = 15%7, -4%3, 5.5%1
1       2       0.5
Power of:
> = 7^2, 107^0, 2^8
49      1       256

Relational expressions

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

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
These also work on strings and other types.
> = "abc" < "def"
true
> = "abc" > "def"
false
> = "abc" == "abc"
true
> = "abc" == "a".."bc"
true
Objects will not be equal if the types are different or refer to different objects.
> = {} == "table"
false
> = {} == {}  -- two different tables are created here
false
> t = {}
> t2 = t
> = t == t2   -- we're referencing the same table here
true
Coercion does not work here, the types must be converted explicitly. See NumbersTutorial and StringsTutorial for explanation of coercion.
> = "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. There are more notes on the implications of this at the end of this page.

> = false==nil   -- although they represent the same thing they are not equivalent
false
> = true==false, true~=false
false   true
> = 1==0
false
> = does_this_exist  -- test to see if variable "does_this_exist" exists. no, false.
nil

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 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.

> = 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
All of the above expressions return the first argument. All of the following expressions return the second argument, as the first is true.
> = true and false
false
> = true and true
true
> = 1 and "hello", "hello" and "there"
hello   there
> = true and nil
nil
As you can see the logical expressions are still evaluated correctly but we have some interesting behaviour because of the values returned.

or

The or binary operator also does not necessarily return a boolean value (see notes for and above). If the first argument is not false or nil 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
All of the above expressions return the first argument. All of the following expressions return the second argument, as the first is false or nil.
> = false or true
true
> = nil or true
true
> = nil or "hello"
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

Ternary operators [2] are a useful feature in C. e.g.

int value = x>3 ? 1 : 0;

This behaviour can be partially emulated in Lua using the logical operators and and or. The C form:

value = test ? x : y;
roughly translates to the following Lua:
value = test and x or y

E.g.

> print( 3>1 and 1 or 0 )
1
> print( 3<1 and 1 or 0 )
0
> print( 3<1 and "True" or "False" )
False
> print( 3>1 and true or "false" )
true

This can be used for short hand to fill a hash:

> t = {}
> t[1] = 12;
> t[2] = 13;
> for i=1, 3 do
>>	t[i] = (t[i] or 0) + 1
>end

> for k, v in pairs(t) do
>>	print(k, v);
> end
1	13
2	14
3	1

However, there is a caveat: This only works when the first return value is not nil or false.

> print( 3>1 and 1 or "False" )        -- works
1
> print( 3>1 and false or "oops" )     -- failed, should return false
oops
> print( 3>1 and nil or "oops" )       -- failed, should return nil
oops

Note on test expressions and nil

An important point to note is that the value 0 is not a false test condition in Lua. In some languages, for example C, a test of:

if (0)
  printf("true");
else
  printf("false");
would display "false". In Lua,
> if 0 then
>>  print("true")
>> else
>>  print("false")
>> end
true
prints "true"! You should use false, or nil in place of 0:
> if false then print("true") else print("false") end
false
> if nil then print("true") else print("false") end
false

Why?

The reason for this is historical. Lua did not support boolean types (i.e. true and false) before version 5.0. Previous to version 5.0 a value of nil represented false. Now, both nil and false will act as a false condition in a test expression. E.g.,

> if nil then print("true") else print("false") end
false
> if 1 then print("true") else print("false") end
true
> if 0 then print("true") else print("false") end
true
> if 1==2 then print("true") else print("false") end
false

Another point to note is that true and false are not numerical values, e.g., 1 and 0 as they are in some languages.

> = true, false
true    false
> = 1 + true
stdin:1: attempt to perform arithmetic on a boolean value
stack traceback:
        stdin:1: in main chunk
        [C]: ?

Also, nil is coerced into a boolean value when used with a logical operator:

> = not nil
true
> = not 1
false
> = not 0
false

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Last edited September 15, 2014 5:53 pm GMT (diff)