Impossible to find a table’s index name using same index

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#1

I want to find the index instead of a value by using the value’s index (tables).

The issue is when I print a table, it will only return its value, but I want the value’s index. In this case, I want to check if myTable["Keys"] is named "Keys".

--simple explanation
local myTable = {}
myTable["Keys"] = {"123"}
print(myTable["Keys"]) -- returns {"123"} instead of "Keys", "Keys" is what I want.

-- usage case
local keyTable = myTable["Keys"]
-- tricky part, you can’t check if keyTable is named "Keys", instead you get {123} from print(keyTable).
--[[
    --if there were some possible way to grab a table’s name
    table.setname(keyTable, "Keys")
    if table.getname(keyTable) == "Keys" then
        for i, v in pairs(keyTable) do
            --code
        end
    end
]

I have tried searching through the developer hub for table’s library, none of those functions can find the value’s index while utilizing the same index and value.

1 Like
#2

I’m confused

local myTable = {}
myTable["Keys"] = "123"

local function findKeyByValue(value)
    for i, v in pairs(myTable) do
        if v == value then
            return i
        end
    end
    return nil
end
print(findKeyByValue("123")) -- This?
1 Like
#3

No, I want to find "Keys" while specifically using myTables["Keys"].

1 Like
#4

What’s the use case for this? You could try adding some metadata to each array like this?:

local s = {
    ["Keys"] = {
        Name = "Keys",
        OtherInfo = 123,
        Id = 1
    },
    ["NotKeys"] = {
        Name = "NotKeys",
        OtherInfo = 456,
        Id = 2
    }
}

print(s["Keys"].Name) -- print's "Keys"
print(s["Keys"]) -- prints the table
2 Likes
#5

You can’t do that with vanilla tables, you’ll need to use a for loop

for Key, Value in pairs(Table) do
    print(Key .. ":" .. Value)
end

This also isn’t particularly useful to have, because you’d have to know what key you saved a value to anyways to actually save the value to a table.

EDIT 17/05/2024: I got a like notification on this post, and I want to point out 2 new features Luau has, String Interpolation and Generalized Iteration. You can now “interpolate” strings like so:

-- NOW:
local MyString = `{Name} = {Value}`

-- BEFORE:
local MyString = tostring(Name) .. " = " .. tostring(Value)

, and loops can be shortened to this:

-- NOW:
for Key, Value in Table do
end

-- BEFORE:
for Key, Value in pairs(Table) do
end

So, the original code can now be cleaned up to this:

for Key, Value in Table do
    print(`{Key}: {Value}`)
end

Hope this helps, time-traveller!

2 Likes
#6

This seems a little counter-intuitive, you’re attempting to locate a key which you already have.

#7 
local myTable = {}
myTable["Keys"] = "123"

local Key = "Keys"
if myTable[Key] then
	print(Key)
end

If you’re attempting to verify if a key/field is valid.

#8

Hashtables do NOT have an index. You can manually add a index, but if you really want to use numeric indices, you can just do so instead of string keys.

#9

To get the index of a value in a table you can use a for loop like this:

local function getValueIndex(tbl, value)
    for i, v in next, tbl do
        if v == value then
            return i
        end
    end
end

-----Usage-----
local tbl = {"Keys" = "123"}

local idx = getValueIndex(tbl, "123")
print(idx) -- Keys

Hope this helped!

1 Like