nil: No Value

Sometimes a variable has no value. Lua has a special value for that, nil, which means “nothing”. A variable that you create without = and a value holds nil:

local strawberries
print(strawberries)
print("Strawberries:", strawberries)

This prints:

nil
Strawberries:	nil

nil comes in handy for things that aren't known yet or aren't there. You can also store it in a variable yourself, strawberries = nil, when the variable shouldn't hold anything anymore. But you can't calculate with nil, and you can't join it to a text:

local coins
print("Coins: " .. coins)

Lua reports:

main.lua:2: attempt to concatenate a nil value (local 'coins')

Read the message from the end. (local 'coins') is the variable: a local one, called coins. a nil value is what it held. And attempt to concatenate is what Lua tried to do with it: join it to a text (concatenate means join). With +, - and the other operators, the message says attempt to perform arithmetic on a nil value (local 'coins'): an attempt to calculate.

The most common source of nil is a typo in a name:

local coins = 120
print(cions)

This prints:

nil

No error! Lua didn't find a local variable called cions, so it looked for a global one. There's no such variable either, and a variable that doesn't exist has the value nil. print prints it without complaining. The error comes only when the program does something with the value:

local coins = 120
print("Coins: " .. cions)

Lua reports:

main.lua:2: attempt to concatenate a nil value (global 'cions')

The word in parentheses gives you a clue:

• local: the name is right, but the variable has no value yet. Look for the line where it should have got one.
• global: all your variables are local, so this one probably doesn't exist. Look for a typo or a wrong capital letter: Coins is a different name from coins.