Lua Array with Numbers find if number is in Array

Question

So i am having an Array in Lua, lets say:

local Array = {1, 8, 10, 15, 20, 30}

so now i want to know if a specific number is in the Array. So if did this:

    local number = 4    

    if Array[number] then
       print("Is in Array")
    else 
        print("Not in Array")
    end

I want to know if 4 is part of the array. It isent so expected output is Not in Array but Lua takes the number as the position of the entery, there is an entery on position 4 so the output is Is in Array

How can i check if a specific number is part of the array without indexing numbers as strings?

See also stackoverflow.com/questions/38282234/…

lhf
– lhf

2022-12-10 09:46:52 +00:00
Commented Dec 10, 2022 at 9:46 — lhf
– lhf, Commented Dec 10, 2022 at 9:46

Luatic · Accepted Answer · 2022-12-10 09:50:22Z

Your code doesn't do what you think it does. Let's walk through it:

local Array = {1, 8, 10, 15, 20, 30}

this is just syntactic sugar for

local Array = {[1] = 1, [2] = 8, [3] = 10, [4] = 15, [5] = 20, [6] = 30}

thus Array[4] is 15. This is how arrays work: You index an array using an index. An index is not to be confused with the value this gives you. In this example, 4 is the index and 15 is the value.

What you want isn't an array but rather a set. You can easily build a set in Lua by using the values as keys and using truthy values in a table:

local set = {[1] = true, [8] = true, [10] = true, [15] = true, [20] = true, [30] = true}

if you find this awkward to type, just write yourself an utility to turn an array into a set:

local function make_set(array)
    local set = {}
    for _, val in ipairs(array) do
        set[val] = true
    end
    return set
end
set = make_set{1, 8, 10, 15, 20, 30}

You can then use this set to efficiently check whether it contains a number:

if set[number] then
    print"is in set"
else
    print"not in set"
end

Side notes:

Alternatively to building a set, you could use a linear search. This would be less efficient if done multiple times and isn't even much more convenient to implement. Lua makes the use of a "set" as a special case of the table very easy.
Uppercase in Lua is usually reserved for "class-like" tables, although there is no "official" naming convention.

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