Converting Values

A number can also hide in a text: "12" in quotes isn't the number 12, but a text made of the characters 1 and 2. Programs get numbers as texts all the time: what someone typed on the keyboard, a line from a file, a price on a note. And print shows both the same way, so you can't tell them apart by looking:

local eggs = 12
local note = "12"
print(eggs, note)
print(type(eggs), type(note))

This prints:

12	12
number	string

The function tonumber turns a text into a number. If the text isn't a number, it returns nil:

print(tonumber("12"))
print(tonumber("2.5"))
print(tonumber(" 7 "))
print(tonumber("12 eggs"))
print(tonumber("2,5"))

This prints:

12
2.5
7
nil
nil

Spaces around the number don't matter, but anything else in the text does, even a decimal comma. Because the result can be nil, it's a good idea to check it; you'll learn how in the lesson on conditions.

The function tostring works the other way round: it turns any value into a text. You need it when you want to join a boolean or nil to a text with ..:

local barnOpen = true
print("Barn open: " .. tostring(barnOpen))
print("Pumpkins: " .. tostring(nil))

This prints:

Barn open: true
Pumpkins: nil

Numbers don't need it: .. turns them into text by itself, as in "Coins: " .. 120.

Lua can also convert a text to a number by itself, when you calculate with it:

print("10" + 5)
print("10" * "2")
print("10" .. 5)

This prints:

15
20
105

The last line doesn't calculate: .. always joins, so "10" and 5 make the text 105. Don't rely on these conversions. Such a program works only as long as the text really is a number: "ten" + 5 stops with the error attempt to add a 'string' with a 'number'. And comparisons, which you'll learn in the next lesson, don't convert at all. With tonumber, the code clearly shows that a conversion happens, and you get nil instead of an error, which you can check.