Keys and Values

In the lesson on lists, a table kept its items in order, under the numbers 1, 2, 3… That works for a shopping list. But Emma also wants to write down what's in her barn: 12 carrots, 30 potatoes and 4 pumpkins. A list {12, 30, 4} would hold the numbers, but nothing in it would say which number belongs to which crop.

A table can do more than a list. Instead of the numbers 1, 2, 3, it can use names:

-- Emma's barn
local barn = {carrot = 12, potato = 30, pumpkin = 4}

print(barn.carrot)
print(barn["potato"])

This prints:

12
30

A table used like this is called a dictionary. Just as you look up a word in a real dictionary to find what it means, you look up a key (carrot) to find its value (12). Inside the curly braces are key–value pairs, separated by commas: the key, = and the value. A value can be anything: a number, a text, true or false, even another table.

Lua has no special type for dictionaries. It's the same table you know from lists, and type(barn) is "table". A list is simply a table whose keys are 1, 2, 3…

There are two ways to read a value, and they mean exactly the same:

• barn.carrot: a dot and the name of the key;
• barn["carrot"]: the key as a text in square brackets, like an index in a list.

The dot is shorter, so you'll see it more often. But it works only for keys that follow the rules for variable names: letters, digits and _, not starting with a digit, and not a keyword such as end. A key with a space, such as sweet corn, must be written in square brackets and quotes, both in the curly braces and when you read it:

local barn = {carrot = 12, ["sweet corn"] = 9}

print(barn["sweet corn"])

This prints:

9

Without the brackets, Lua reads only sweet and doesn't know what to do with corn:{sweet corn = 9} ends with main.lua:1: '}' expected near 'corn'.

A table with more pairs is easier to read with each pair on its own line. A comma after the last pair is allowed, which is handy when you add another line later:

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

print(carrot.name .. " sells for " .. carrot.sellPrice .. " coins")

This prints:

Carrot sells for 5 coins

To see a whole dictionary, print(barn) won't help: as with lists, it prints only table: 0x…. And table.concat joins only the items of a list. You'll learn to print every pair later in this lesson.

Typical mistakes:

• A missing comma between two pairs: {carrot = 12 potato = 30} ends with main.lua:1: '}' expected near 'potato'.
• A colon instead of =, as some other languages write it: {carrot: 12} ends with main.lua:1: <name> expected near '12'.
• A key in quotes, but without the square brackets: {"carrot" = 12} ends with main.lua:1: '}' expected near '='.