Lua - Variables

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A variable is nothing but a name given to a storage area that our programs can manipulate. It can hold different types of values including finctions and tables.

The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because Lua is case-sensitive. There are eight basic types of values in Lua:

In Lua, though we don't have variable data types, we have three types based on the scope of the variable.

  • Global variables: All variables are considered global unless explicitly declared as a local.

  • Local variables: When the type is specified as local for a variable then its scope is limited with the functions inside their scope.

  • Table fields: This is a special type of variable that can hold anything except nil including functions.

Variable Definition in Lua:

A variable definition means to tell the interpreter where and how much to create the storage for the variable. A variable definition have an optional type and contains a list of one or more variables of that type as follows:

type variable_list;

Here, type is optionally local or nor type specified making it global, and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here:

local    i, j
local    i
local    a,c

The line local i, j both declares and defines the variables i and j; which instructs the interpreter to create variables named i, j and limits the scope to be local.

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows:

type variable_list = value_list;

Some examples are:

local d , f = 5 ,10 --declaration of d and f as local variables. 
d , f = 5, 10;      --declaration of d and f as global variables. 
d, f = 10           --[[declaration of d and f as global variables. 
                        Here value of f is nil --]]

For definition without an initializer: variables with static storage duration are implicitly initialized with nil.

Variable Declaration in Lua:

As you can see in the above examples, assignments for multiples variables follows a variable_list and value_list format. In the above example local d , f = 5 ,10 we have d and f in variable_list and 5 and 10 in values list.

Value assigning in Lua takes place like first variable in variable_list with first value in value_list and so on. Hence value of d is 5 and value of f is 10.

Example

Try following example, where variables have been declared at the top, but they have been defined and initialized inside the main function:


-- Variable definition:
local a, b
-- Initialization
a = 10
b = 30

print("value of a:", a)

print("value of b:", b)

-- Swapping of variables
b, a = a, b
print("value of a:", a)

print("value of b:", b)

f = 70.0/3.0
print("value of f", f)

When the above code is build and executed, it produces the following result:

value of a:	10
value of b:	30
value of a:	30
value of b:	10
value of f	23.333333333333

Lvalues and Rvalues in Lua:

There are two kinds of expressions in Lua:

  1. lvalue : An expression that is an lvalue may appear as either the left-hand or right-hand side of an assignment.

  2. rvalue : An expression that is an rvalue may appear on the right- but not left-hand side of an assignment.

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and can not appear on the left-hand side. Following is a valid statement:

g = 20

But following is not a valid statement and would generate build-time error:

10 = 20

In Lua programming language, apart from the above types of assignment, it is possible to have multiple lvalues and rvalues in the same single statement. It is shown below.

g,l = 20,30

In the above statement, 20 is assigned to g and 30 is assigned to l.



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