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Tables - Value Indexed Lists

<cat>Dev.Lua</cat>
<title>Tables - Value Indexed Lists</title>
##  What is a List? 

Value-indexed tables are a commonly-neglected technique in Lua:

```
local tbl = { [1] = "a", [2] = "b", [3] = "c" } -- this is a normal sequential table.

-- Checking if a value is in a normal table:
function table.HasValue( tbl, value ) -- O(n) worst-case complexity
	for _, v in ipairs( tbl ) do
		if v == value then
			return true
		end
	end

	return false
end

local list = { ["a"] = true, ["b"] = true, ["c"] = true } -- this is a value-indexed version of that table.

--Checking if a value is in a value-indexed table:
function ListHasValue( tbl, value ) -- O(1) worst-case complexity
	return tbl[ value ]
end
```

We'll call them Lists, for short. They're incredibly useful! The idea is that by using the actual value we intend to store in the table as the key itself, we can retrieve and remove it from the table much easier and faster. This has many uses:

* Good for when the order of data saved is unimportant and each value is unique within the table.
* Creating a table in which the same value can't exist more than once.
* Rapidly checking whether a value is stored in a table without the use of iteration (seen above).

##  Two-Dimensional Lists 

We can take it one step further with 2D Lists:

```
-- A 2D sequential table holding players' friends as {guy=player, friends={their friends}}
local friendTbl = {
	{
		guy = Entity(1),
		friends = {
			Entity(2),
			Entity(3)
		}
	},
	{
		guy = Entity(2),
		friends = {
			Entity(1)
		}
	},
	{
		guy = Entity(3),
		friends = {
			Entity(1)
		}
	}
}

function TwoPlayersAreFriends( ply1, ply2 )
	for _, v in ipairs( friendTbl ) do
		if v.guy == ply1 then
			for _, friend in ipairs( v.friends ) do
				if friend == ply2 then
					return true
				end
			end

			return false
		end
	end

	return false
end

--And this is a 2D list which holds the same data as above.
local friendList = {
	[Entity(1)] = {
		Entity(2) = true
		Entity(3) = true
	},
	[Entity(2)] = {
		Entity(1) = true
	},
	[Entity(3)] = {
		Entity(1) = true
	}
}

--Checking if a value is in a value-indexed table:
function TwoPlayersAreFriends( ply1, ply2 )
   return friendList[ ply1 ][ ply2 ]
end
```

Look at how much faster and simpler that is! If you wanted to, you could even replace `true` in the list with some kind of data. This makes the list into a dictionary.

So remember: **Any time you need to save distinct data in an unordered table, opt to use a List instead!**

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