Strings Tutorial

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Quotes

Strings are introduced in section 2.1 of the Reference Manual [1]. Strings can be defined using single quotes, double quotes, or double square brackets.

> = "hello"
hello
> = 'hello'
hello
> = [[hello]]
hello

Why so many ways to make a string? It allows you to enclose one type of quotes in the other. e.g.,

> = 'hello "Lua user"'
hello "Lua user"
> = "Its [[content]] hasn't got a substring."
Its [[content]] hasn't got a substring.
> = [[Let's have more "strings" please.]]
Let's have more "strings" please.

Double bracketed strings also have a few other special properties, discussed below.

Escape sequences

Lua can also handle C-like escape sequences. There are more details in the Reference Manual, section 2.1 [1].

> = "hello \"Lua user\""
hello "Lua user"
> = 'hello\nNew line\tTab'
hello
New line        Tab

Escape sequences are not recognized when using double brackets, so:

> = [[hello\nNew line\tTab]]

hello\nNew line\tTab

Multiline quotes

Double square brackets can be used to enclose literal strings which traverse several lines. e.g.,

> = [[Multiple lines of text
>> can be enclosed in double square
>> brackets.]]
Multiple lines of text
can be enclosed in double square
brackets.

Nesting quotes

Double square brackets allow nesting, but they require one or more = inserted in the outer-most brackets to distinguish them. It doesn't matter how many = are inserted, as long as the number is the same in the beginning and ending brackets.

> = [[one [[two]] one]]        -- bad
stdin:1: nesting of [[...]] is deprecated near '['
> = [=[one [[two]] one]=]      -- ok
one [[two]] one
> = [===[one [[two]] one]===]  -- ok too
one [[two]] one
> = [=[one [ [==[ one]=]       -- ok. nothing special about the inner content.
one [ [==[ one

Concatenation

Strings can be joined together using the concatenation operator "..". e.g.,

> = "hello" .. " Lua user"
hello Lua user
> who = "Lua user"
> = "hello "..who
hello Lua user
Numbers can be concatenated to strings. In this case they are coerced into strings and then concatenated. You can read more about coercion below.
> = "Green bottles: "..10
Green bottles: 10
> = type("Green bottles: "..10)
string

Doing a large number of concatenation operations may be slow because each concatenation may allocate a new string in memory. The following three examples give the same result except the first may be much slower:

-- slow
local s = ''
for i=1,10000 do s = s .. math.random() .. ',' end
io.stdout:write(s)

-- fast
for i=1,10000 do io.stdout:write(tostring(math.random()), ',') end

-- fast, but uses more memory
local t = {}
for i=1,10000 do t[i] = tostring(math.random()) end
io.stdout:write(table.concat(t,','), ',') 

The string library

Lua supplies a range of useful functions for processing and manipulating strings in its standard library. More details are supplied in the StringLibraryTutorial. Below are a few examples of usage of the string library.

> = string.byte("ABCDE", 2) -- return the ASCII value of the second character
66
> = string.char(65,66,67,68,69) -- return a string constructed from ASCII values
ABCDE
> = string.find("hello Lua user", "Lua") -- find substring "Lua"
7       9
> = string.find("hello Lua user", "l+") -- find one or more occurrences of "l"
3       4
> = string.format("%.7f", math.pi) -- format a number
3.1415927
> = string.format("%8s", "Lua") -- format a string
     Lua

Coercion

Lua performs automatic conversion of numbers to strings and vice versa where it is appropriate. This is called coercion.

> = "This is Lua version " .. 5.1 .. " we are using."
This is Lua version 5.1 we are using.
> = "Pi = " .. math.pi
Pi = 3.1415926535898
> = "Pi = " .. 3.1415927
Pi = 3.1415927
As shown above, during coercion, we do not have full control over the formatting of the conversion. To format the number as a string as we would like we can use the string.format() function. e.g.,
> = string.format("%.3f", 5.1)
5.100
> = "Lua version " .. string.format("%.1f", 5.1)
Lua version 5.1
This is explicit conversion using a function to convert the number, rather than coercion. You can read more about coercion of numbers in the NumbersTutorial.
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Last edited October 22, 2013 7:53 am GMT (diff)