12. Strings

Strings are the values you print most. Lua has a small, handy toolkit for them: gluing, measuring, changing case, repeating, and formatting numbers into them. This chapter covers the ones you use daily.

Joining strings with `..`

You have seen .. (two dots) already. It is the concatenation operator: it returns one new string with the second stuck onto the end of the first:

local first = "Hello, "
local second = "world!"
print(first .. second)   -- Hello, world!

The original variables are untouched — .. builds a new string. You can glue text without variables too:

print("Level " .. 7 .. " complete")   -- Level 7 complete

A number on either side of .. becomes text automatically. Booleans and nil do not — wrap them in tostring() first.

Length with `#`

The # operator returns a string's length in bytes (the same as its character count for plain English text):

print(#"hello")        -- 5
print(#"")             -- 0
local greeting = "Welcome"
print(#greeting)       -- 7

Use # whenever you need a string's length — for centring text, checking input length, or building borders.

Open exercises/12/01-length.lua. Add a line that prints the length of your own name.

Changing case

Lua's string library lives in the table string. Two simple calls are string.upper(s) and string.lower(s). Both return a new string and leave the original alone:

local name = "Keiko"
print(string.upper(name))   -- KEIKO
print(string.lower(name))   -- keiko
print(name)                  -- Keiko  (still the original)

Repeating a string

string.rep(s, n) returns the string s repeated n times:

print(string.rep("-", 20))      -- --------------------
print(string.rep("ab", 3))      -- ababab
print(string.rep("=", 4) .. " TITLE " .. string.rep("=", 4))
-- ==== TITLE ====

It is the right tool for drawing borders or padding text.

Formatting with `string.format`

When you need numbers or text plugged into specific spots, string.format beats a long chain of ... The first argument is a template with placeholders starting with %; the rest fill them in order:

local name = "Keiko"
local level = 7
local hp = 95
print(string.format("%s (Lv %d) HP %d", name, level, hp))
-- Keiko (Lv 7) HP 95

The placeholders you need today:

Placeholder	What it expects	Example output
`%s`	a string	`Keiko`
`%d`	a whole number	`7`
`%.2f`	a decimal with two digits after the point	`3.14`

There are many more, but those three cover most everyday uses. The full list is in the Lua reference manual under string.format.

Open exercises/12/02-format.lua. Change the format string so the output reads Keiko has 95 HP at level 7.

Escape sequences

Some characters are awkward in a string because they collide with Lua's punctuation. The fix is an escape sequence — a backslash \ followed by a letter, which Lua replaces with the special character:

Sequence	What it becomes
`\n`	a newline (move to next line)
`\t`	a tab
`\"`	a literal double quote
`\'`	a literal single quote
`\\`	a literal backslash

Example:

print("Line 1\nLine 2\nLine 3")
-- Line 1
-- Line 2
-- Line 3

print("Name\tLevel\tHP")
-- Name    Level   HP

print("She said \"hi\" and waved.")
-- She said "hi" and waved.

Multi-line strings

When a string spans many lines with lots of quotes, escape sequences get noisy. Lua offers another way: [[ to open and ]] to close. Anything between them is the string, including newlines and quotes:

local poem = [[
Roses are red,
Violets are blue,
"Pick a number,"
said the program to you.
]]

print(poem)

Once created, multi-line strings are just regular strings — a friendlier way to write them.

You met [[ ]] before, in --[[ ]] for multi-line comments. Same idea — Lua reads everything between the brackets as one block. The -- in front is what turns a multi-line string into a multi-line comment.

Homework

Problem 1 — Loud and quiet

Open exercises/12/homework/01-loud-and-quiet.lua. A starter variable holds your name. Print it three times: as-is, in upper case, then in lower case.

Problem 2 — Stat line

Open exercises/12/homework/02-stat-line.lua. Three variables are declared at the top: a string name, a number level, a number hp. Using string.format, print one line in this exact shape:

Keiko  Lv 7   HP 95

The labels (Lv, HP) and the spacing between fields can be literal spaces inside the format string.

Problem 3 — One-line three-line poem

Open exercises/12/homework/03-three-line-poem.lua. Print a three line poem using exactly one print call. The line breaks must come from \n inside the string. Add a -- comment after the print call explaining why one print can produce three lines.

Challenge — Title block

Open exercises/12/homework/04-title-block.lua. A variable holds a title (any string you like). Print a block like this, where the top and bottom dash rows match the title's length plus two dashes on each side:

----- INVENTORY -----
Inside text goes here
----- INVENTORY -----

Use string.rep and # to size the borders without counting by hand. Changing the title should change the border length automatically.

Stuck or finished? Open the homework solutions page.