How do I get left and right directions from a direction?

I need to get left and right directions (in global) based on a direction. How do I do it?

Do you mean find the direction to the left and right of where you're currently heading? Are you using north/south/east/west as directions or (+/-)1 for x and y (or more dimensions)?

Assuming this is probably a turtle thing, you'll be dealing with north, south, east and west. This should work:

local directions = {"north", "east", "south", "west"}
local direction = "east"

local function getRight() -- returns the direction to the right of the current direction (defined by the 'direction' variable)
	for i = 1, #directions do
		if directions[i] == direction then
			return directions[i % 4 + 1]
		end
	end
end
local function getLeft() -- returns the direction to the left of the current direction (defined by the 'direction' variable)
	for i = 1, #directions do
		if directions[i] == direction then
			return directions[i == 1 and 4 or i - 1]
		end
	end
end

I meant a vector. I am trying to calculate positions for turtles based on single point and direction. Thank you for that code, I will definitely use that too.

EDIT: I'm just going to use that and transform the directions to vectors.

With vectors, you can do this:

local function rotateAnticlockwise( x, y ) -- also known as left 90 degrees
	return -y, x
end
local function rotateClockwise( x, y ) -- also known as right 90 degrees
	return y, -x
end

Edit: just to clarify, positive Y would be forward and positive X would be right.