How to check if a table contains an element in Lua?

user_56761 0
Viewed 228731

Is there a method for checking if a table contains a value ? I have my own (naive) function, but I was wondering if something "official" exists for that ? Or something more efficient...

function table.contains(table, element)
  for _, value in pairs(table) do
    if value == element then
      return true
    end
  end
  return false
end

By the way, the main reason I'm using this functions is to use tables as sets, ie with no duplicate elements. Is there something else I could use ?

lua-table lua set unique contains
5 Answers

You can put the values as the table's keys. For example:

function addToSet(set, key)
    set[key] = true
end

function removeFromSet(set, key)
    set[key] = nil
end

function setContains(set, key)
    return set[key] ~= nil
end

There's a more fully-featured example here.

user_1892050

Given your representation, your function is as efficient as can be done. Of course, as noted by others (and as practiced in languages older than Lua), the solution to your real problem is to change representation. When you have tables and you want sets, you turn tables into sets by using the set element as the key and true as the value. +1 to interjay.

Norman_Ramsey_416610

I can't think of another way to compare values, but if you use the element of the set as the key, you can set the value to anything other than nil. Then you get fast lookups without having to search the entire table.

user_2130980 
-- in some helper module
function utils_Set(list)
    local set = {}
    for _, l in ipairs(list) do set[l] = true end
    return set
end

-- your table here
long_table = { "v1", "v2", "v1000"}

-- Consult some value
_set = utils_Set(long_table)
if _set["v1"] then print("yes!") end
user_70870370
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