Lua.md

Lua Programming

-- defines a factorial function
function fact (n)
    if n == 0 then
        return 1
    else
        return n * fact(n - 1)
    end
end
print("enter a number:")
a = io.read("*n") -- reads a number
print(fact(a))

• each piece of code that Lua executes, such as a file or a single line in interactive mode is called chunk.

• Lua is used also as a data-description language, chunks with several megabytes are not uncommon.

• call the function os.exit(), from the Operating System library to exit interactive mode.

• In older versions (< 5.3), we need to precede expression with an equal sign to print it.

a = 15
= a^2  --> 225.0

lua -i prog

• Start interactive session after running a given chunk.

• Another way to run chunks is with the function dofile, which immediately executes a file.

--- lib1.lua
function norm (x, y)
    return math.sqrt(x^2 + y^2)
end

function twice (x)
    return 2.0 * x
end

> dofile("lib1.lua")  --load library
> n = norm(3.4, 1.0)

• Comment starts anywhere with two consecutive hyphens (--). Long comments with two hyphens followed by two opening square brackets and run until the first occurance of two consecutive closing square brackets.

--[[multi-line long comment ]]

• Lua is a dynamically-typed language.

• userdata are used to represent new types created by an application program or a library written in C; for instance

• Booleans do not hold a monopoly of condition values: in Lua, any value can represent a condition.

• Lua considers both zero and the empty string as true in conditional tests. Only Boolean false and nil are considered as false and anything else as true.

• logical operator and, or and not.

• The result of the and operator is its first operand if that operand is false; otherwise, the result is its second operand.

• The result of the or operator is its first operand if it is not false; otherwise, the result is its second operand.

• A useful Lua idiom is x = x or v, which is equivalent to if not x then x = v end. sets x to a default value v when x is not set (provided that x is not set to false).

• ((a and b) or c) or simply (a and b or c) is equivalent to the C expression a ? b : c, provided that b is not false.

• The not operator always gives a Boolean value

• The -e option allows us to enter code directly into the command line, like here:

% lua -e "print(math.sin(12))"

• A script can retrieve its arguments through the predefined global variable arg. A script can also retrieve its arguments through a vararg expression. In the main body of a script, the expression ... (three dots) results in the arguments to the script.