Assignment
Lua allows multiple assignments. Therefore, the syntax for assignment defines a list of variables on the left side and a list of expressions on the right side.
The elements in both lists are separated by
commas:
stat ::= varlist `=´ explist
varlist ::= var {`,´ var}
explist ::= exp {`,´ exp}Expressions are discussed in Expressions.
Before the assignment, the list of values is adjusted to
the length of the list of variables. If there are more values than needed, the excess values are
thrown away. If there are fewer values than needed, the list is extended with as many nulls as
needed. If the list of expressions ends with a function call, then all values returned by that call
enter the list of values, before the adjustment (except when the call is enclosed in parentheses;
see Expressions). The assignment statement first evaluates all its expressions and only then are the
assignments performed. Thus the code cyclically permutes the values of x, y, and
z.
An
assignment to a global variable x = val is equivalent to the assignment _env.x = val, where _env is
the environment of the running function. (The _env variable is not defined in Lua. We use it here
only for explanatory purposes.)i = 3
i, a[i] = i+1, 20
sets a[3] to 20, without affecting a[4] because the i in a[i] is
evaluated (to 3) before it is assigned 4. Similarly, the line
x, y = y, x
exchanges the values of x and y, and
x, y, z = y, z, x