Adding and Removing Keys

A dictionary can change while the program runs. You change a value the same way as a variable, with =. And when you assign to a key that the table doesn't have yet, Lua adds it:

local barn = {carrot = 12, potato = 30}

barn.carrot = barn.carrot + 5
barn.eggs = 18
barn["sweet corn"] = 9

print(barn.carrot, barn.eggs, barn["sweet corn"])

This prints:

17	18	9

You can also start with an empty table, {}, and add the pairs one by one: local barn = {}, then barn.eggs = 18.

Each key is in a table only once. Assigning to a key that is already there replaces its value; it doesn't add a second pair with the same key.

When you read a key that isn't in the table, it isn't an error: you get nil, as with an index past the end of a list. The error comes only when you try to compute with that nil:

local barn = {carrot = 12, potato = 30}

print(barn.pumpkin)
print(barn.pumpkin + 1)

This prints:

nil
main.lua:4: attempt to perform arithmetic on a nil value (field 'pumpkin')

The message says field 'pumpkin': in error messages, Lua calls a key of a table a field. A typo ends the same way: barn.carot is nil too, with no warning, because Lua can't know that you meant carrot. Uppercase and lowercase letters matter as well: barn.Carrot is a different key.

When a missing key should count as zero, use or, as for default values in the lesson on conditions: barn.pumpkin or 0 is 0 when there are no pumpkins in the barn, and their number when there are.

To remove a key, assign nil to it: barn.potato = nil. For Lua, a key whose value is nil and a key that isn't in the table are the same thing, so after this line, the potatoes are gone from the table.

And one surprise: # doesn't count the pairs of a dictionary. It counts only the items of a list, the keys 1, 2, 3…:

local barn = {carrot = 12, potato = 30, pumpkin = 4}

print(#barn)

0

You'll see how to count the keys in the exercise on pairs.