Records

So far, Emma's dictionaries have held many things of the same kind: crops and their counts, crops and their prices. A table can also describe one thing with several values. Such a table is often called a record, and its keys fields:

local carrot = {name = "Carrot", days = 3, seedPrice = 2, sellPrice = 5}

print(carrot.name .. " grows in " .. carrot.days .. " days.")
print("Profit: " .. (carrot.sellPrice - carrot.seedPrice) .. " coins")

This prints:

Carrot grows in 3 days.
Profit: 3 coins

It's the same kind of table as the barn, used in a different way. In the barn, the keys are the data: Emma can add any crop at any time. In a record, the keys are fixed and known in advance. Every crop has a name, days, a seedPrice and a sellPrice, and the program reads them with a dot. The values can be of different types:

local bella = {name = "Bella", kind = "cow", age = 4, milked = false}

bella.milked = true
print(bella.name, bella.kind, bella.age, bella.milked)

Bella	cow	4	true

Records go well with functions. Instead of passing the name, the days and the prices one by one, you pass one record, and the function reads the fields it needs:

local function profit(crop)
    return crop.sellPrice - crop.seedPrice
end

local carrot = {name = "Carrot", days = 3, seedPrice = 2, sellPrice = 5}
local potato = {name = "Potato", days = 4, seedPrice = 3, sellPrice = 8}
print(profit(carrot), profit(potato))

3	5

A function can also create a record and return it. When you need many records of the same shape, you write their fields only once, in the function:

local function newPlot(crop)
    return {crop = crop, day = 0, watered = false}
end

local plot = newPlot("pumpkin")
print(plot.crop, plot.day, plot.watered)

pumpkin	0	false

crop = crop may look strange. On the left of = is the key, the field crop. On the right is its value: the parameter crop, which holds "pumpkin" in this call. Each call returns a new table, so every plot has its own fields.

A typical mistake is a typo in a field's name. Lua doesn't check the names of fields, so carrot.sellprice (with a lowercase p) is simply nil, and computing with it ends with attempt to perform arithmetic on a nil value (field 'sellprice'). When you see field '…' in an error, compare the name in the message with the name in the table.