how does an advanced for loop actually work?

In your question, you are trying to combine two different types of for loops in one. The Programming in Lua book says that, "A numeric for has the following syntax:

for var=exp1,exp2,exp3 do
	something
end

" In the explanation, var stands for variable. This form defines var to be the number you want to start at. The second number, exp2, is how far you want the loop to go. The exp3 is optional and it is the amount your variable increases by every loop. The default value for exp3 is one. For example,

for i = 1, 5 do
	print(i)
end
-- output --> 1, 2, 3, 4, 5
for i = 4, 7 do
	print(i)
end
-- output --> 4, 5, 6, 7
for i = 2, 4, .5 do -- using exp3
	print(i)
end
-- output --> 2, 2.5, 3, 3.5, 4

You can even loop backwards with a negative value in the exp3 spot. The thing is, you will need to remember to use a larger value in the exp1 spot than the exp2 spot or else you will get an error. The reason it will throw an error is that you cannot count from something like 1 to 20 by -1. You can, however, count from 20 to 1 by -1. Here is an example:

for i = 3, 1, -1 do
	print(i)
end
-- output --> 3, 2, 1
for i = 3, 1, -.5 do -- using a decimal negative number
	print(i)
end
-- output --> 3, 2.5, 2, 1.5, 1
for i = 1, 5, -1 do -- would not work because you cannot count up to five by -1
	print(i)
end

The next kind of for loop is the generic for loop. The book says, "The generic for loop allows you to traverse all values returned by an iterator function." ipairs and pairs would be examples of iterator functions. This allows you to loop through dictionaries and arrays (or tables as they are called in Lua). An example would be

local array = {"a", "b", "c"}
for i,v in pairs(array) do
	print(i, v)
end
-- output --> 1, a; 2, b; 3, c

Generally, these letters are used because i stands for index and v stands for value. If you don't want to call them that you don't have to. I recommend using names that make sense based on the situation you are using it in. Often in Lua it is convention to use _ as i if you are not going to use it. For example

local array = {"a", "b", "c"}
for _,v in pairs(array) do
	print(v)
end
-- output --> a, b, c 

This also works with dictionaries as shown in this example:

local dictionary = {
	["box"] = "wood"
	["jug"] = "plastic"
	["spoon"] = "silver"
}
for item,material in pairs(dictionary) do -- using names that make sense in the situation
	print(item, material)
end
-- output --> box, wood; jug, plastic; spoon, silver

ipairs works similar to pairs, but stops at a nil value in an array. Here is a simple example:

local array = {"happy", nil, "bob"}
for _,v in ipairs(array) do
	print(v)
end
-- output --> happy
-- the loop stops at the nil value in the array
local dictionary = {
	["box"] = "wood"
	["jug"] = "plastic"
	["spoon"] = "silver"
	["hmm"] = nil
	["key"] = "iron"
}
for item,material in ipairs(dictionary) do
	print(item, material)
end
-- output -- > box, wood; jug, plastic; spoon, silver
-- the loop stops at the nil value in the dictionary as well
local array = {"happy", "good", "rocket"}
for _,v in ipairs(array) do
	print(v)
end
-- output -- > happy, good, rocket
-- it works the same as pairs when there are no nil values

I hope this helps and have a great day scripting!

Links for reference:

• Numeric for

• Generic for

• Iterators

• Generic for loop semantics

• For loops This one is from the Developer wiki