Keys in Variables

The key in square brackets doesn't have to be written out. It can be in a variable, or come from a parameter or a loop:

local prices = {carrot = 5, potato = 8, strawberry = 12}

local crop = "potato"
print(prices[crop])

This prints:

8

Lua first looks into the variable crop, finds the text "potato" in it, and then reads the value with that key. This is how you write code for a key you don't know yet when you write the program: the crop a customer asks for, or each crop in a list.

And now the classic mistake. The dot is always followed by the key itself, never by a variable. prices.crop means prices["crop"]: the key is the text "crop", and there's no such key in the table:

local prices = {carrot = 5, potato = 8, strawberry = 12}

local crop = "potato"
print(prices.crop)
print(prices["crop"])
print(prices[crop])

This prints:

nil
nil
8

So: when you know the key as you write the code, use a dot (prices.potato). When the key is in a variable, use square brackets without quotes (prices[crop]). With quotes, it's just a text again: prices["crop"] is the same as prices.crop.

Keys in variables are most useful in functions and loops. This function works for any crop, even one you add to the table later:

local prices = {carrot = 5, potato = 8, pumpkin = 20}

local function priceOf(crop, amount)
    return prices[crop] * amount
end

print(priceOf("carrot", 3))
print(priceOf("pumpkin", 2))

This prints:

15
40

If you wrote prices.crop * amount in it by mistake, the program would end with attempt to perform arithmetic on a nil value (field 'crop'). The message names the key that Lua looked for, 'crop': that's the sign that you wrote the name of the variable instead of using its value. And when the key from a variable isn't in the table, the message has a question mark instead of a name, because Lua doesn't know where the key came from: priceOf("corn", 1) ends with attempt to perform arithmetic on a nil value (field '?').

One more detail: a key can also be a number, and a number is a different key from a text. t[1] and t["1"] are two different keys.