Finding Text

Does Grandpa Joe's note say anything about rain? Where in a text does a word start? The method find searches a text for another text. It gives two numbers: the position where the found text starts and the position where it ends. When the text isn't there, it gives nil:

local note = "Hugo wants 12 pumpkins"
print(note:find("pumpkins", 1, true))
print(note:find("carrots", 1, true))

This prints:

15	22
nil

What are the 1 and the true? The number says where to start searching: from the first character. true says that you're looking for exactly the text you wrote. Without it, find takes the text as a pattern, in which some characters have a special meaning. You'll learn patterns later in this lesson; for now, one example. In a pattern, a dot means “any character”, so find(".") finds the very first character, whatever it is:

local price = "Price: 3.50"
print(price:find(".", 1, true))
print(price:find("."))

This prints:

9	9
1	1

So when you look for an exact text, always add 1, true. With letters only, such as "pumpkins", find would work without them too, but with a period, a parenthesis or a percent sign, it wouldn't.

In a condition, nil counts as false and any number as true. So you can put find straight into an if, to check whether a text contains another one:

local weather = "Cloudy, rain in the evening"
if weather:find("rain", 1, true) then
    print("Bring the hens inside")
end

This prints:

Bring the hens inside

find tells capital and small letters apart: in "Rain all day", it doesn't find "rain". When the case doesn't matter, search the text in small letters: weather:lower():find("rain", 1, true).

The positions that find gives fit sub. This is how you cut a text in two at a colon:

local entry = "Bella: 18 liters"
local colon = entry:find(":", 1, true)
print(entry:sub(1, colon - 1))
print(entry:sub(colon + 2))

This prints:

Bella
18 liters

When you store the result of find in one variable, you get the first number, the start; the second one is dropped. For both, write two variables, local first, last = entry:find(":", 1, true), as with any function that returns two values.

Typical mistake: using the position without checking it. When the text has no colon, colon is nil, and colon - 1 ends the program:

local entry = "Bella 18 liters"
local colon = entry:find(":", 1, true)
print(entry:sub(1, colon - 1))

main.lua:3: attempt to perform arithmetic on a nil value (local 'colon')

When you aren't sure the text is there, check the result with an if first.