Efficient way to return something by chance?

Hello there! This is not too difficult/complex to do. The simplest method I could think of, was 1.) get the length of the dictionary, 2.) randomly choose a number in that length, 3.) loop until you reach that random number. Because the iterator, pairs, is arbitrary already, this should leave you with a pretty random selection. You cannot just get the length of a dictionary with the # operator in Lua. You would have to create your own method. I recently posted three different methods discussed here. Ok, so now to the example:

local function dictLengthFinder(tbl) -- using one of the three methods
	local length = 0
	for i,v in pairs(tbl) do
		length = length + 1
	end
	return length
end
local function getRan(dict)
	local dictLength = dictLengthFinder(dict)
	local ranIndex = math.random(dictLength)
	local counter = 1
	for i,v in pairs(dict) do
		if counter ~= ranIndex then
			counter = counter + 1
		else
			return v 
			--[[
				I don't know what you want here. 
				I will just return the value. 
				You could also return i if you wanted.
			--]]
		end
	end
end
local dict = {} -- some dictionary
local ranItem = getRan(dict)

I hope this helps. Have a great day scripting!

Edit:

For the value to determine the probability would be rather inefficient, and there are probably better ways you could structure your code in order to remove the need for this. I will, however, walk you through how to accomplish this. 1.) create a table named chanceTbl (for this example) 2.) loop through your dictionary and store the index value amount of times in the chanceTbl 3.) randomly select an index in the chanceTbl and return the value, which would be the index in your dictionary. Here is an example:

local function getRan(dict)
	local chanceTbl = {}
	for i,v in pairs(dict) do
		for num = 1, v do
			table.insert(chanceTbl, i)
		end
	end
	local ranIndex = math.random(#chanceTbl)
	--[[
		we are able to get the length of chanceTbl with # because all the 	
		indices are in numeric order because we used table.insert
	--]]
	local ranItem = chanceTbl[ranIndex] 
	--[[
		I used two different variables so that you could 
		clearly see what is going on. Note that you could 
		do this in one variable if you just defined the variable like this:
		local ranIndex = chanceTbl[math.random(#chanceTbl)] 
	--]]
	return ranItem
	--[[
		In fact, you could just return the whole statement:
		return chanceTbl[math.random(#chanceTbl)]	
	--]]
end
local dict = {} -- some dictionary
local ranItem = getRan(dict)

Good luck!