Programming in Lua : 4.1

Part I. The Language Chapter 4. Statements

4.1 - Assignment

Assignment is the basic means of changing the value of a variable or a table field:

    a = "hello" .. "world"
    t.n = t.n + 1

Lua allows multiple assignment, where a list of values is assigned to a list of variables in one step. Both lists have their elements separated by commas. For instance, in the assignment

    a, b = 10, 2*x

the variable a gets the value 10 and b gets 2*x.

In a multiple assignment, Lua first evaluates all values and only then executes the assignments. Therefore, we can use a multiple assignment to swap two values, as in

    x, y = y, x                -- swap `x' for `y'
    a[i], a[j] = a[j], a[i]    -- swap `a[i]' for `a[i]'

Lua always adjusts the number of values to the number of variables: When the list of values is shorter than the list of variables, the extra variables receive nil as their values; when the list of values is longer, the extra values are silently discarded:

    a, b, c = 0, 1
    print(a,b,c)           --> 0   1   nil
    a, b = a+1, b+1, b+2   -- value of b+2 is ignored
    print(a,b)             --> 1   2
    a, b, c = 0
    print(a,b,c)           --> 0   nil   nil

The last assignment in the above example shows a common mistake. To initialize a set of variables, you must provide a value for each one:

    a, b, c = 0, 0, 0
    print(a,b,c)           --> 0   0   0

Actually, most of the previous examples are somewhat artificial. I seldom use multiple assignment simply to write several assignments in one line. But often we really need multiple assignment. We already saw an example, to swap two values. A more frequent use is to collect multiple returns from function calls. As we will discuss in detail later, a function call can return multiple values. In such cases, a single expression can supply the values for several variables. For instance, in the assignment

    a, b = f()

f() returns two results: a gets the first and b gets the second.