Strings in Lua

11 October 2021 - 3 mins read time
Tags: Lua

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Strings in Lua

String in Lua is a sequence of bytes.
We can store any binary data into strings
UTF-8 and UTF-16 encoding are supported in Lua

Strings in Lua are immutable, which means we cannot change the string after creating, but we can copy it and resign it to some other or same variable

String literal

a="Hello world"

Length of string

a="Hello world"
print(#a) --> 11

# operators return the size in bytes. Because each char in string is a byte long, we can use # to get the number of chars in string.

Note getting number of chars using # works fine in the case of ASCII strings.

Concatenating two strings

.. operator concatenate two strings.

a = "hello" .. " world" --> hello world

We can concatenate a string and number

a = "today is " .. 11 .. " Oct " .. 2021
b = "3/2 is = " .. 1.5

Writing long strings

To write long strings we use [[ ]] operator

alongMessage = [[
program main
implicit none
write(*,*) "hello world"
end program main
]]

Some times, when we are using Lua code as string, then we may face a problem if Lua code contains comments using --[[ --]] . Therefore we use [=[ and ]=] operator for writing strings.

You can use any number of = symbol in between [[ and ]] , however, the opening and closing brackets should have the same number of equal sign.

alongMessage = [==[
program main
implicit none
write(*,*) "hello world"
end program main
]==]

Converting numbers to string

tostring() function can convert a number into a string.

tonumber() function can convert a string into a number.

Standard string library

method	interface	comment	use
string.len()	string.len(aString)	returns length of string	string.len(“hello”)
string.rep()	string.rep(s,n)	returns a string after repeating s, n times	string.rep(“0”,5)
string.reverse()	string.reverse(s)	reverse a string	string.rev(“hello”)
string.lower()	string.lower(s)	lower case version of string s	string.lower(“AAA”)
string.upper()	string.upper(s)	uppercase version of string s	string.upper(“aaa”)
string.sub()	string.sub(s,i,j)	extracts a piece of the string s, from the i-th to the j-th character	string.sub(“hello world”, 1, -1)
string.char()	string.char(s)	converts an ascii code s to ascii char	string.char(96)
string.byte()	string.byte(s)	converts ascii char to ascii code	string.byte(“A”)

string.byte("hello world", 1, -1) will convert all char in the string to ascii code and return a list of integers.

string.byte("hello world", 1, -1)
--> 104	101	108	108	111	32	119	111	114	108	100

There is another interesting string function called, string.format() for formatting strings and converting numbers to strings.

string.format("day=%d, month=%d, year=%d", 11, 9, 2021)
--> day=11, month=9, year=2021

This function follows the rules of printf function of C language.

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