What is the function of square brackets around table keys in lua?

Question

I came across tables that have square brackets around keys:

local commands_json =
{
    ["request"] = {
        ["application"] = PW_APPLICATION,
        ["push_token"] = deviceToken
    }
}

Can the square brackets be omitted?

Preadamite

Perhaps the documentation says it best: Each field of the form [exp1] = exp2 adds to the new table an entry with key exp1 and value exp2..... In other words, the key value is determined at run-time. – Erickericka

Answer 1

It's simply the long form of specifying keys in a table. You can put any value between the [] (except nil. And floating-point NaNs). Whereas without them, you can only use identifiers.

For example, this is a compile error, since "key name" isn't an identifier (due to the space):

tbl =
{
  key name = 5,
}

In contrast, the following example does work:

tbl =
{
  ["key name"] = 5,
}

The following example with quotes and without square brackets is a compile error:

tbl =
{
  "key name" = 5,
}

If Lua sees a naked value like this, it thinks you're trying to add to the array part of the table. That is, it confuses it with:

tbl =
{
  "key name",
}

Which creates a 1-element array, with tbl[1] equal to "key name". By using [], the compiler can easily tell that you meant for something to be a key rather than the value of an array element.

The long form also lets you distinguish between:

local name = "a name";

tbl =
{
  ["name"] = 5,
  [name] = 7,
}

The second part means to evaluate the expression name, the result of which will be the key. So this table has the keys "name" and "a name".

Frumpish

if you have a table with an odd key like ["key name"] ... then at that point youd no longer be able to retrieve the value using dot notation? you'd have to go table["key name"] ?? – Preadamite

Yes, that's correct. However, that's not an "odd key". For tables that are intended to mimic structs, . notation is important for ease-of-use. For tables that are more like associative data arrays, notation is irrelevant. – Frumpish

Just a nitpick, but nil isn't the only unusable index. – Allanadale

@warspyking: Well, Lua 5.1 only excepts nil; 5.2+ adds floating-point NaN to that list, for obvious reasons. – Frumpish

@Nicol I think it's worth mentioning NaN. And I can't seem to use NaN in 5.1 either :P – Allanadale

Answer 2

You cannot omit the brackets

> x = { 'a' = 1 }
stdin:1: '}' expected near '='

the correct code is

> x = { ['a'] = 1 }
> print(x['a'])
1

or

> x = { a = 1 }
> print(x['a'])
1

However, the second one has its limitations. What if you want to have a key called "-"?

> x = { - = 1 }
stdin:1: unexpected symbol near '='
> x = { '-' = 1 }
stdin:1: '}' expected near '='

again the correct way is to use brackets

> x = { ['-'] = 1 }
> print(x['-'])
1

Or you want to create a field of name which is contained in a variable called a?

> a = 'cat'
> x = { [a] = 1 } 
> print(x['cat'])
1

Brackets are used as a general form of key creation, they give you ability to put any hashable object as a key - not only strings.

Meli