Question

I’d like to create a custom contains method on Lua’s table data structure that would check for the existence of a key. Usage would look something like this:

mytable  = {}
table.insert(mytable, 'key1')
print(mytable.contains('key1'))

Thanks.

Answer 1

In Lua you cannot change ALL tables at once. You can do this with simpler types, like numbers, strings, functions, where you can modify their metatable and add a method to all strings, all functions, etc. This is already done in Lua 5.1 for strings, this is why you can do this:

local s = "<Hello world!>"
print(s:sub(2, -2)) -- Hello world!

Tables and userdata have metatables for each instance. If you want to create a table with a custom method already present, a simple table constructor will not do. However, using Lua’s syntax sugar, you can do something like this:

local mytable = T{}
mytable:insert('val1')
print(mytable:findvalue('val1'))

In order to achieve this, you have to write the following prior to using T:

local table_meta = { __index = table }
function T(t)
    -- returns the table passed as parameter or a new table
    -- with custom metatable already set to resolve methods in `table` 
    return setmetatable(t or {}, table_meta)
end

function table.findvalue(tab, val)
    for k,v in pairs(tab) do
        -- this will return the key under which the value is stored
        -- which can be used as a boolean expression to determine if
        -- the value is contained in the table
        if v == val then return k end
    end
    -- implicit return nil here, nothing is found
end

local t = T{key1='hello', key2='world'}
t:insert('foo')
t:insert('bar')
print(t:findvalue('world'), t:findvalue('bar'), t:findvalue('xxx'))
if not t:findvalue('xxx') then
    print('xxx is not there!')
end

--> key2    2
--> xxx is not there!