Lua for the impatient

A cheat sheet to get started quickly in Lua

Comments begin with --

Can have multiple lvalues and rvalues in assignment, e.g.

a, b = 5, 10 -- assigns 5 and 10 to a and b respectively

Values have data types, but variables don't. Variables have types global, local, and table fields

Zero and the empty string are truthy, not falsy

By default, all variables will point to nil

Has a type() function that returns a string naming the type

lvalues and rvalues are just like in C

Operators #

arithmetic operators are just like in C

~= means not-equal, other relational operators are just like C

logical operators are named not, and, and or

Other operators #

.. concatenates two strings

# returns the length of a string

Loops #

Loops include while, for, repeat…until

while #

while(condition)
do
    statement(s) -- executes as long as condition is true
end

for #

for i = 10,1,-1
do
   print(i) -- counts from 10 down to 1
end

There is a break statement like in C

Conditionals #

if #

if( a < 20 )
then
   print("a is less than 20" );
end

if…else #

if( a < 20 )
then
   print("a is less than 20" );
else
   print("a is not less than 20");
end

Functions #

Mostly like C. A function can be declared local, otherwise it will default to global

Formal parameters are named without types

Returning multiple values is possible, by using a comma separated list after a return statement

function max(num1, num2)
   if (num1 > num2) then
      result = num1;
   else
      result = num2;
   end

   return result;
end

Because functions are types, they can be passed as parameters

myprint = function(param)
   print("This is my print function -   ##",param,"##")
end

function add(num1,num2,functionPrint)
   result = num1 + num2
   functionPrint(result)
end

add(2,5,myprint)

Varargs #

Varargs can be done like this:

function average(...)
   result = 0
   local arg = {...}
   for i,v in ipairs(arg) do
      result = result + v
   end
   return result/#arg
end

print("The average is",average(10,5,3,4,5,6))

Strings #

Strings can be quoted by 'single quotes', "double quotes" or [[pairs of double square brackets]]

The usual escape sequences work, as well as \[ and \]

Manipulation #


string.upper(arg)	To upper case
string.lower(arg)	To lower case
string.gsub(main,find,replace)	returns a new string, replacing find with replace
string.find(main,find,start,end)	returns the start and end index of find. start and end args are optional
string.reverse(arg)	gra
string.format(…)
string.char(arg)	It's ASCII 😒
string.byte(arg, index)	returns the character code for the first, or optionally index'th char
string.len(arg)	another way to get the length
string.rep(string,n)	repeats string n times

Arrays #

Indexing starts at 1 when you assign like this 🙃…

array = {"Apple", "Orange"}

print(array(0))
print(array(1))

prints

nil
Apple

… but you can assign to zero or even negative numbered elements

array[-5] = "Pear" -- valid

Multidimensional arrays are achived either by using

arrays of arrays
single dimensional array with index arithmetic

Iterators #

Generic `for` iterator #

array = {"Apple", "Orange"}
for key,value in ipairs(array)
do
    print(key, value)
end

Stateless iterators #

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