Tables Are Shared

With numbers and texts, = copies the value. After local b = a, changing b doesn't change a:

local a = 5
local b = a
b = b + 1
print(a, b)

5	6

Tables behave differently. A variable doesn't hold the table itself, only a reference to it: something like the address of the table in the computer's memory. That's the 0x… in what print(t) prints. local b = a copies the reference, not the table, so both variables lead to the same table:

local prices = {carrot = 5, potato = 8}
local joePrices = prices

joePrices.carrot = 6
print(prices.carrot)
print(prices == joePrices)

6
true

Grandpa Joe changed “his” price, and Emma's changed too, because there is only one table with two names. And == on tables asks whether it's the same table, not whether two tables have the same contents. Each {} creates a new table, so:

print({} == {})
print({carrot = 5} == {carrot = 5})

false
false

To compare the contents of two tables, compare their keys and values yourself, for example in a loop.

The same thing happens when you pass a table to a function. The parameter refers to the same table, so the function can change it, and the change is visible outside. That's often what you want; such a function doesn't even have to return anything:

local function harvest(barn, crop, amount)
    barn[crop] = (barn[crop] or 0) + amount
end

local barn = {}
harvest(barn, "carrot", 12)
harvest(barn, "carrot", 3)
print(barn.carrot)

15

When you want a separate table, you have to copy it: create a new table and copy every key and value into it with pairs:

local function copy(t)
    local result = {}
    for key, value in pairs(t) do
        result[key] = value
    end
    return result
end

local prices = {carrot = 5, potato = 8}
local joePrices = copy(prices)

joePrices.carrot = 6
print(prices.carrot, joePrices.carrot)
print(prices == joePrices)

5	6
false

The function works for lists too, because pairs also goes through the keys 1, 2, 3… But it copies only one level: when a value is itself a table, like the barn in farm.barn, the copy gets a reference to the same inner table.