Tables (part 2)

In the last section we saw how simple it was to make a list. Working with the list was a little tricky at first but hopefully not too bad. If we rewind back, we can remember that we created a table by assigning some keys values.

boxes = {
  [1] = "John Doe",
  [2] = "Amanda Parker",
  [3] = "Tyler Reese"
}

Think of it like post office boxes and we label each box with a unique number. Whenever we reference a postal box, we do so by referencing the number within the array (list) of boxes: boxes[2]. The label, or key, is ultimately arbitrary though. For making a list, we label things in an incremental order to make them easier to loop over and to give us a sense of linear sequence. Keys don't need to be numbers. They could just as well be strings:

coins = {
  ["half"] = "50 cents",
  ["quarter"] = "25 cents",
  ["dime"] = "10 cents",
  ["nickel"] = "5 cents",
  ["penny"] = "1 cent"
}

Which would be accessed just the same way:

print(coins["nickel"])

5 cents

This can be really useful for doing a lookup if we instead use a variable for the key. Try this one out:

coins = {
  ["half"] = "50 cents",
  ["quarter"] = "25 cents",
  ["dime"] = "10 cents",
  ["nickel"] = "5 cents",
  ["penny"] = "1 cent"
}

print("Which coin do you have?")
response = io.read()

print("Your coin is worth " .. coins[response] .. ".")

This isn't far off from how certain databases and digital services work. Items are stored in a unique key that can be referenced for getting a definition out of later. That's why this data structure is sometimes called a dictionary. Remember, we can add items to a table after it is defined:

coins["silver dollar"] = "1 dollar"

Another shortcut Lua gives us is we don't need to use the square braces or quotes when adding keys that are strings.

coins.nickel = "5 cents"

The limitation with doing this is the keys defined this way can't have spaces or special characters. They must be valid in the same way variable names are valid.

coins.silver dollar = "1 dollar"  -- INVALID
coins.silver_dollar = "1 dollar"  -- Valid
coins.100 = "1 dollar" -- INVALID

You can use variable names for keys when creating the table too:

color = "purple"
description = "the best color"
colors = {
  [color] = description
}

print(colors.purple)
print(colors[color]) -- prints the same thing

By convention, strings are typically used for dictionary-like tables while lists are numbers. Don't make the mistake of thinking these are the same:

list = {
  1 = "some item",
  ["1"] = "a unique item"
}

You could use other data types as keys, but you might find your results to be very unexpected:

crazy_list = {
  [true] = "works",
  [false] = "works",
  ["true"] = "not the same",
  ["false"] = "not the same"
}

print(crazy_list[true])
print(crazy_list[false])
print(crazy_list["true"])
print(crazy_list["false"])

crazy_key = {}
crazy_list = {
  [crazy_key] = "works"
}
print(crazy_list[crazy_key])

crazy_list = {
  [nil] = "doesn't work!"
}
print(crazy_list[nil])

Throws an error:

[string "crazy_list = {..."]:2: table index is nil

Values in a table can be any type of data, including functions:

cat = {
  color = "gray",
  smelly = true,
  make_sound = function()
    print("meyuow!")
  end
}

cat.make_sound()

Exercises

• Remember the early function we made that returned the animal sounds? Make a function with a table in it, where each key in the table is an animal name. Give each key a value equal to the sound the animal makes and return the animal sound. Try invoking the function and see if you get back the correct sound.