Programming in Lua : 3.6

This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
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Programming in Lua

Part I. The Language Chapter 3. Expressions

3.6 – Table Constructors

Constructors are expressions that create and initialize tables. They are a distinctive feature of Lua and one of its most useful and versatile mechanisms.

The simplest constructor is the empty constructor, {}, which creates an empty table; we saw it before. Constructors also initialize arrays (called also sequences or lists). For instance, the statement

    days = {"Sunday", "Monday", "Tuesday", "Wednesday",
            "Thursday", "Friday", "Saturday"}

will initialize days[1] with the string "Sunday" (the first element has always index 1, not 0), days[2] with "Monday", and so on:

    print(days[4])  --> Wednesday

Constructors do not need to use only constant expressions. We can use any kind of expression for the value of each element. For instance, we can build a short sine table as

    tab = {sin(1), sin(2), sin(3), sin(4),
           sin(5), sin(6), sin(7), sin(8)}

To initialize a table to be used as a record, Lua offers the following syntax:

    a = {x=0, y=0}

which is equivalent to

    a = {}; a.x=0; a.y=0

No matter what constructor we use to create a table, we can always add and remove other fields of any type to it:

    w = {x=0, y=0, label="console"}
    x = {sin(0), sin(1), sin(2)}
    w[1] = "another field"
    x.f = w
    print(w["x"])   --> 0
    print(w[1])     --> another field
    print(x.f[1])   --> another field
    w.x = nil       -- remove field "x"

That is, all tables are created equal; constructors only affect their initialization.

Every time Lua evaluates a constructor, it creates and initializes a new table. Consequently, we can use tables to implement linked lists:

    list = nil
    for line in io.lines() do
      list = {next=list, value=line}
    end

This code reads lines from the standard input and stores them in a linked list, in reverse order. Each node in the list is a table with two fields: value, with the line contents, and next, with a reference to the next node. The following code prints the list contents:

    l = list
    while l do
      print(l.value)
      l = l.next
    end

(Because we implemented our list as a stack, the lines will be printed in reverse order.) Although instructive, we hardly use the above implementation in real Lua programs; lists are better implemented as arrays, as we will see in Chapter 11.

We can mix record-style and list-style initializations in the same constructor:

    polyline = {color="blue", thickness=2, npoints=4,
                 {x=0,   y=0},
                 {x=-10, y=0},
                 {x=-10, y=1},
                 {x=0,   y=1}
               }

The above example also illustrates how we can nest constructors to represent more complex data structures. Each of the elements polyline[1], ..., polyline[4] is a table representing a record:

    print(polyline[2].x)    --> -10

Those two constructor forms have their limitations. For instance, you cannot initialize fields with negative indices, or with string indices that are not proper identifiers. For such needs, there is another, more general, format. In this format, we explicitly write the index to be initialized as an expression, between square brackets:

    opnames = {["+"] = "add", ["-"] = "sub",
               ["*"] = "mul", ["/"] = "div"}
    
    i = 20; s = "-"
    a = {[i+0] = s, [i+1] = s..s, [i+2] = s..s..s}
    
    print(opnames[s])    --> sub
    print(a[22])         --> ---

That syntax is more cumbersome, but more flexible too: Both the list-style and the record-style forms are special cases of this more general one. The constructor

    {x=0, y=0}

is equivalent to

    {["x"]=0, ["y"]=0}

and the constructor

    {"red", "green", "blue"}

is equivalent to

    {[1]="red", [2]="green", [3]="blue"}

For those that really want their arrays starting at 0, it is not difficult to write the following:

    days = {[0]="Sunday", "Monday", "Tuesday", "Wednesday",
            "Thursday", "Friday", "Saturday"}

Now, the first value, "Sunday", is at index 0. That zero does not affect the other fields, but "Monday" naturally goes to index 1, because it is the first list value in the constructor; the other values follow it. Despite this facility, I do not recommend the use of arrays starting at 0 in Lua. Remember that most functions assume that arrays start at index 1, and therefore will not handle such arrays correctly.

You can always put a comma after the last entry. These trailing commas are optional, but are always valid:

    a = {[1]="red", [2]="green", [3]="blue",}

Such flexibility makes it easier to write programs that generate Lua tables, because they do not need to handle the last element as a special case.

Finally, you can always use a semicolon instead of a comma in a constructor. We usually reserve semicolons to delimit different sections in a constructor, for instance to separate its list part from its record part:

    {x=10, y=45; "one", "two", "three"}