Lua Table Comparisons Within Tables

Question

So I have a table that holds references to other tables like:

local a = newObject()
a.collection = {}
for i = 1, 100 do
    local b = newObject()
    a[#a + 1] = b
end 

Now if I want to see if a particular object is within "a" I have to use pairs like so:

local z = a.collection[ 99 ]
for i,j in pairs( a.collection ) do
    if j == z then
    return true
  end
end

The z object is in the 99th spot and I would have to wait for pairs to iterate all the way throughout the other 98 objects. This set up is making my program crawl. Is there a way to make some sort of key that isn't a string or a table to table comparison that is a one liner? Like:

if a.collection[{z}] then return true end

Thanks in advance!

Answer 1

Why are you storing the object in the value slot and not the key slot of the table?

local a = newObject()
a.collection = {}
for i = 1, 100 do
    local b = newObject()
    a.collection[b] = i
end 

to see if a particular object is within "a"

return a.collection[b]

If you need integer indexed access to the collection, store it both ways:

local a = newObject()
a.collection = {}
for i = 1, 100 do
    local b = newObject()
    a.collection[i] = b
    a.collection[b] = i
end 

Finding:

local z = a.collection[99]
if a.collection[z] then return true end

Answer 2

Don't know if it's faster or not, but maybe this helps:

Filling:

local a = {}
a.collection = {}
for i = 1, 100 do
    local b = {}
    a.collection[b] = true  -- Table / Object as index
end

Finding:

local z = a.collection[99]
if a.collection[z] then return true end

If that's not what you wanted to do you can break your whole array into smaller buckets and use a hash to keep track which object belongs to which bucket.

Answer 3

you might want to consider switching from using pairs() to using a regular for loop and indexing the table, pairs() seems to be slower on larger collections of tables.

for i=1, #a.collection do
    if a.collection[i] == z then
        return true
    end
end

i compared the speed of iterating through a collection of 1 million tables using both pairs() and table indexing, and the indexing was a little bit faster every time. try it yourself using os.clock() to profile your code.

i can't really think of a faster way of your solution other than using some kind of hashing function to set unique indexes into the a.collection table. however, doing this would make getting a specific table out a non-trivial task (you wouldn't just be able to do a.collection[99], you'd have to iterate through until you found one you wanted. but then you could easily test if the table was in a.collection by doing something like a.collection[hashFunc(z)] ~= nil...)