Programming in Lua : 4.1

This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
The fourth edition targets Lua 5.3 and is available at Amazon and other bookstores.
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Programming in Lua

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[j]'

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.